I

is a vowel which represents two very different sounds in different languages. In this country it denotes a rapid pronunciation of the diphthong ai. In French, Italian, and many other tongues, its sound is identical with that of the English e. In the series of the vowels established by the experiments of Mr. Willis [ALPHABET], i, as denoting the latter sound, lies at one of the two extremes. It is pronounced with the lips retracted so as to shorten the vocal tube, whereas the same organs are protruded to produce the sound represented at the other extremity by u. The various forms which have been used to represent the letter i may be seen in the article already quoted, cols. 240, 241. The character there given as used by the Phoenicians and early Greeks is somewhat complicated, and differs widely from the single stroke into which it eventually degenerated. In this last state it was the simplest of all the alphabetical characters, and was therefore well adapted to be the symbol of a small quantity. In this sense the terms a jod and an iota are still retained, jod being

the Hebrew, iota the Greek name for the character.

The letter is interchangeable as follows:

1. With the diphthongs ai, oi, ei. This may be seen most distinctly in the Latin language, where alais, requairo, pueroi, puerois, nulloius, deico, &c., were corrupted into alis, requiro, pueri, pueris, nullius, dico. In the same language, when one i was followed by another i, it was not uncommon to denote them by a single long i, as tibicen, Chius, alius (gen.), inscitia, for tibiicen, Chiius, aliius, insciitia. In such cases it was a common practice to give greater length to the letter, thus,

CHIVS.

2. The short i was interchangeable with nearly all the short vowels, more particularly in the penult syllables of polysyllabic words, which are very indistinctly pronounced. Thus the Greek mechănē is in Latin machina. In the same manner the Nomad races of North Africa are called by the Greeks Nomades, by the Romans Numidae. Again, aveμos and animus are kindred words. Bonitas must have been originally bonotas, and would have been written in Greek with a termination-orns. Lastly, in a large number of words a short u degenerated into an i: as maximus, decimus, recupero, maritumus, scribimus (compare sumus), into maximus, decimus, recipero, maritimus, scribimus. Even Cicero wrote all these words with a u, though our editions give an i.

3. A short i before n or m is not unfrequently in French changed into ai or a. Thus the Gallic town Inculisma is the origin of the name Angoulême: vincere is in French vaincre, &c.

4. In the same language the vowel i is changed into oi very commonly, as sitis, soif; mi, moi; fides, foi; Ligeris, Loire, &c., and this though the i in Latin be short.

5. I is often inserted in French or Spanish words before the vowel e: miel, bien, vient, &c., from the Latin mel, bene, venit.

6. The vowel i is often inserted after the vowels a, o, and u in the French language, particularly when a contraction has taken place, as aimer, connoître, reduire, from amare, cognoscere, reducere.

7. When the vowel in the Latin language has a vowel after it, and is preceded by one of the consonants p, b; t, d; c, g; the derived languages have often a sibilant in the place of the former consonant. Thus sapiam is in French sache: rabies, rage; ratio, raison; medius, in Italian mezzo (compare the Greek Meσos). The double sound of c and g in our own language appears to have originated in this

way.

8. A similar change occurs even in other cases, as simia, Fr. singe; vindemia, vendange; lineus, linge.

IAMBICS, a species of verse composed of a succession of iambi (~~), or equivalent feet, was freely used both by Greek and Latin poets. According to Aristotle ('De Poetic.'), the iambic measure was first employed in satirical poems, called iambi, which appear to have been represented or acted; since Plato ('De Rep.,' vii. 17) forbids boys to be spectators of iambi and comedies. The iambic is the most common metre in the Greek tragic poets. We are informed by Aristotle (De Poetic.') that "originally the trochaic tetrameter was made use of, as better suited to the satyric and saltatorial genius of the poem at that time; but when the dialogue was formed, nature itself pointed out the proper metre; for the iambic is of all metres the most colloquial, as appears evidently from this fact, that our common conversation frequently falls into iambic verse, seldom into hexameter, and only when we depart from the usual melody of speech." (Twining's 'Transl.,' part i., c. 4.)

In the following table a list is given of the feet which may be admitted in the iambic metre in the Greek tragic poets, which is

The anapast in proper names is also introduced in every place of the verse except the last, with this general restriction,-that the anapest should be contained in one word. The comic trimeter admits the same feet as the tragic, and also a dactyl in the fifth place, and an anapast in common words in every place but the last. [CESURA], which usually occurs in the middle of the third or the Much of the beauty of the iambic trimeter depends upon the casura

middle of the fourth foot; as, for example :

οἱ μὲν θέλοντες | εκβαλεῖν ἕδρας Κρόνον.
ἱκτηρίοις κλάδοισιν | ἐξεστεμμένοι.

One of these cæsuras may be considered as generally necessary; the cæsura in the middle of the third foot is much more common than in the middle of the fourth. There is also frequently a cæsura in the middle of the second or the middle of the fifth foot. When a line is divided in the middle of a verse with the elision of a short vowel, or of the little words dè, μè, σè, yè, tè, that division is called by prosodians the quasi-casura; as, for example:

γυναιξὶ παρθένοις τ' | ἀπόβλεπτος μέτα.

For an account of the other iambic metres employed by the Greek and Latin poets, see Hermann, 'Elementa Doctrine Metrica.' In English poetry the iambic metre is very common; as, for example:

"On Lin'den, whe'n the su'n was lo'w,
All bloo'dless la'y th' untro'dden sno'w,
And dar'k as win'ter wa's the flo'w," &c.

ICE. In several preceding articles of the present division of this Encyclopædia, reference has been made to the article now commenced for an account of an important fact among the properties of ice, which, from the investigation it has received at the hands of several of the most eminent living men of science, has been elevated almost to the position of a principle in physics. This is the property of REGELATION, by which liquefied ice remaining in contact with ice still solid, returns

itself to the solid state.

In the year 1850, Professor Faraday invited attention, in a scientific point of view, to the fact that two pieces of moist ice, when placed in contact, will unite together, even when the surrounding temperature is such as to keep them in a thawing state. He showed experimentally that when two pieces of ice at 32° Fahr., with moistened surfaces, were placed in contact, they became cemented together by the freezing of the film of water between them. When the ice was below 32°, and therefore dry, no adhesion took place between the pieces; and he referred, in illustration of this point, to the well-known experiment of making a snowball. In frosty weather the dry particles of ice will scarcely cohere, but when the snow is in a thawing condition, it may be squeezed into a hard compact mass. He attributed this phenomenon to a property which he supposed ice to possess, of tending to solidify water in contact with it, and of tending more strongly to solidify a film or a particle of water, when the water has ice in contact with it on both sides, than when it has ice on only one side.

To these Professor Tyndall afterwards added the following illustrative facts. "On one of the warmest days of last July [1856], when the thermometer stood at upwards of 80° Fahr. in the shade, and above 100° in the sun, a pile of ice-blocks" being observed in a shop-window, the observer" thought it interesting to examine whether the pieces were united at their places of contact. Laying hold of the topmost block, the whole heap, consisting of several large lumps, was lifted bodily out of its vessel. Even at this high temperature the pieces were frozen together at the places of contact, though the ice all round these places had been melted away, leaving the lumps in some cases united by slender cylinders of the substance. A similar experiment may be made in water as hot as the hands can bear; two pieces of ice will freeze together, and sometimes continue so frozen in the hot water until, as in the case above mentioned, the melting of the ice

around the points of contact leaves the pieces united by slender columns of the substance." 'Phil. Trans.,' 1857, p. 329.

6

Mr. Faraday, in his more recently published Experimental Researches in Chemistry and Physics' (being essentially a republication in a collective form of his papers on subjects belonging to those sciences, first published in the Philosophical Transactions, and in several scientific journals), adheres to his original mode of accounting for the phenomenon he had observed, adopting the name Regelation, applied to it by Professor Tyndall. While alluding to certain views of Professor Forbes, which will presently be stated, as possibly being admissible as correct, and to an explanation offered by Professor James Thomson as being probably true in principle, and possibly having a correct bearing on the phenomena of regelation, he considers that the principle originally assumed by himself may after all be the sole cause of the effect. The principle he has in view, he then states as being, when more distinctly expressed, the following:-"In all uniform bodies, possessing cohesion, that is, being either in the liquid or the solid state, particles which are surrounded by other particles having the like state with themselves tend to preserve that state, even though subject to variations of temperature, either of elevation or depression, which if the particles were not so surrounded, would cause them instantly to change their condition." Referring to water in illustration, he says that it may be cooled many degrees below 32° Fahr., and still retain its liquid state; yet that if a piece of the same chemical substance-ice, at a higher temperature, be introduced, the cold water freezes and becomes warm. He points out that it is certainly not the change of temperature which causes the freezing, for the ice introduced is warmer than the water; and he says he assumes that it is the difference in the condition of cohesion existing on the different sides of the changing particles which sets them free and causes the change. Exemplifying, in another direction, the principle he is propounding, he refers to the fact that water may be exalted to the temperature of 270° Fahr., at the ordinary pressure of the atmosphere, and yet remain water, but that the introduction of the smallest particle of air or steam will cause it to explode, and at the same time to fall in temperature. He further alludes to numerous other substances, such as acetic acid, sulphur, phosphorus, alcohol, sulphuric acid, ether, and camphine, which manifest like phenomena at their freezing or boiling points to those referred to as occurring with the substance of water, ice, and steam; and he adverts to the observed fact, that the contact of extraneous substances with the particles of a fluid usually sets these particles free to change their state, in consequence, he says, of the cohesion between them and the fluid being imperfect; and he instances that glass will permit water to boil in contact with it at 212° Fahr., or by preparation can be made so that water will remain in contact with it at 270° Fahr., without going off into steam, but that an ordinary piece of glass will set the water off at once to freeze. Professor Faraday afterwards comes to a point in his reasoning which he admits may be considered as an assumption. It is, "that many particles in a given state exert a greater sum of their peculiar cohesive force upon a given particle of the like substance in another state than few can do; and that as a consequence a water particle with ice on one side and water on the other, is not so apt to become solid as with ice on both sides; also that a particle of ice at the surface of a mass [of ice] in water is not so apt to remain ice as when, being within the mass, there is ice on all sides, temperature remaining the same."

fracture, change of position of the fractured parts, and regelation of those parts in their new position; the term regelation being now first given to the fact, the scientific importance of which had been originally pointed out by Faraday.* Professor James Thomson, Queen's College, Belfast, whose first express contribution to the subject we now have to record, conceiving Professor Tyndall's theory of the viscosity of glacier ice to be wrong, made public a theory of his own involving a different view of the nature of the phenomenon of regelation. This, as sketched in outline by himself, is as follows:-If to a mass of ice at its meltingpoint, pressures tending to change its form be applied, there will be a continual succession of pressures applied to particular parts-liquefaction occurring in those parts through the lowering of the meltingpoint by pressure-(experimentally demonstrated in 1850 by Professor William Thomson, of Glasgow, his brother)-evolution of the cold by which the so melted portions had been held in the frozen state,— dispersion of the water so produced in such directions as will afford relief to the pressure, and recongelation, by the cold previously evolved, of the water on its being relieved from this pressure: and the cycle of operations will then begin again; for the parts recongealed, after having been melted, must in their turn, through the yielding of other parts, receive pressures from the applied forces, thereby to be again liquefied and to proceed through successive operations as before.+ Professor James D. Forbes adopts the view of Persoz, that the dissolution of ice is a gradual, not a sudden, process, and so far resembles the tardy liquefaction of fatty bodies, or of the metals, which in melting pass through intermediate stages of softness or viscosity. He thinks that ice must essentially be colder than water in contact with it; that between the ice and the water there is a film varying in local temperature from side to side, which may be called plastic ice, or viscid water; and that through this film heat must be constantly passing from the water to the ice, and the ice must be wasting away, though the water be what is called ice-cold. On this, Professor J. Thomson thus comments:-"There is a manifest difficulty in conceiving the possibility of the state of things here described; and I cannot help thinking that Professor Forbes has been himself in some degree sensible of the difficulty; for in a note of later date by a few months than the paper itself [in which the view had been given], he amends the expression of his idea by a statement to the effect that, if a small quantity of water be inclosed in a cavity in ice, it will undergo a gradual regelation [§],—that is, that the ice will in this case be gradually increased instead of wasted." In reference to the first case, Professor J. Thomson asks, "What becomes of the cold of the ice, supposing there to be no communication with external objects, by which heat might be added to or taken from the water and ice jointly considered? Does it go into the water and produce viscidity beyond the limit of the assumed thin film of viscid water at the surface of the ice? Precisely a corresponding question may be put relatively to the second case,-that of the large quantity of ice inclosing a small quantity of water, in which the reverse process is assumed to occur. Next, let an intermediate case be considered, that of a medium quantity of ice, and in which no heat nor cold, practically speaking, is communicated to the water or the ice from surrounding objects. This, it is to be observed, is no mere theoretical case, but a perfectly feasible one. The result, evidently, if the previously described theories be correct, ought to be that the mixture of ice and water ought to pass into the state of uniform viscidity. Professor Forbes' own words distinctly deny the permanence of the water and ice in contact in their two separate states; for he says, 'Bodies of different temperatures cannot continue so without interaction. The water must give off heat to the ice; but it spends it in

The terms fracture and regelation, Professor J. Thomson remarks, then came to be the brief expression of Professor Tyndall's idea of the plasticity of ice. But the former, whose views we are about to give in the text, observes given by, Professor Tyndall as denoting the second, or mending stage in his on the nomenclature of the process, "I suppose the term regelation has been theory of fracture and regelation.' Congelation would seem to me the more proper word to use after fracture, as regelation implies previous melting. If my theory of melting by pressure and freezing again by relief of pressure be admitted, then the term regelation will come to be quite suitable for a part of the process of the union of the two pieces of ice, though not for the whole, which then ought to be designated as the process of melting and regelation." Proceedings of the Royal Society' (vol. x.) for Nov. 24, 1859, p. 154, note. It may be remarked on this, however, that while the latter phrase may be required in discussing the phenomena of glaciers, as the word re-gelation itself

The view of the subject involved in the statement of Professors Faraday and Tyndall, given above (before the former had enunciated his own views in the more extended form), was adopted by the latter physicist as the basis of a theory by which he proposed to explain the viscosity or plasticity of ice previously known to be the quality in glaciers [GLACIERS, in NAT. HIST. DIV.], in virtue of which their motion down their valleys is produced by gravitation; but which he described as being not true viscosity, but, in brief, as the result of

6

in designating simply the physical process to which it was originally applied.

In explanation of the simplest case of regelation, Professor J. Thomson expressed himself in the following manner in a communication to the British Association, in 1857:-"The two pieces of ice (at 32°) on being pressed together at the point of contact, will, at that place, in virtue of the pressure, be in part liquefied and reduced in temperature, and the cold evolved in their liquefaction will cause some of the liquid film intervening between the two masses to freeze."

Sir J. F. W. Herschel, when he terms regclation "a sort of welding

(HAIL), appears to concur with this view.

[This use of the term regelation we conceive to be at once inaccurate, and tending to ambiguity. The water in this case need not have been frozen before; and to call its solidification, by the effect of the contiguous ice, regelation, is erroneously to extend the application of that term to all cases in which water, however originally resulting, is frozen by the contact of ice.]

an insignificant thaw at the surface, which, therefore, wastes even though the water be what is called ice-cold.' Now, the conclusion arrived at, namely, that a quantity of viscid water could be produced in the manner described, is, I am satisfied, quite contrary to all experience. No person has ever, by any peculiar application of heat to, or withdrawal of heat from, a quantity of water, rendered it visibly and tangibly viscid. We even know that water may be cooled much below the ordinary freezing point and yet remain fluid." Professor Forbes regards Mr. Faraday's fact of regelation as being one which receives its proper explanation through his theory described above; and, in confirmation of the supposition that ice has a tendency to solidify a film of water in contact with it, and in opposition to the theory given by Professor J. Thomson, that the regelation is a consequence of the lowering of the melting-point in parts pressed together, he adduces an experiment made by himself. He states that mere contact without pressure is sufficient to produce the union of two pieces of moist ice, and then describes as follows his experiment by which he supposes that this is proved:-"Two slabs of ice, having their corresponding surfaces ground tolerably flat, were suspended in an inhabited room upon a horizontal glass rod passing through two holes in the plates of ice, so that the plane of the plates was vertical. Contact of the even surfaces was obtained by means of two very weak pieces of watch-spring. In an hour and a half the cohesion was so complete that, when violently broken in pieces, many portions of the plates (which had each a surface of twenty or more square inches) continued united; in fact, it appeared as complete as in another experiment, where similar surfaces were pressed together by weights." He concludes that the effect of pressure in assisting "regelation" is principally or solely due to the larger surfaces of contact obtained by the moulding of the surfaces to one another.

Professor J. Thomson has himself repeated this experiment, and has found the results described by Professor Forbes to be fully verified. It was not even necessary to apply the weak pieces of watch-spring, as he found that the pieces of ice, on being merely suspended on the glass rod in contact, would unite themselves strongly in a few hours. This fact Professor Thomson explains by the capillary forces of the film of interposed water, as follows:-Firstly, the film of water between the two slabs-being held up against gravity by the capillary tension or contractile force of its free upper surface, and being distended besides, against the atmospheric pressure, by the same contractile force of its free surface round its whole perimeter, except for a very small space at bottom, from which water trickles away, or is on the point of trickling away-exists under a pressure which, though increasing from above downwards, is everywhere, except at that little space at bottom, less than the atmospheric pressure. Hence the two slabs are urged towards one another by the excess of the external atmospheric pressure above the internal water pressure, and are thus pressed against one another at their places of contact by a force quite notable in its amount. If, for instance, between the two slabs there be a film of water of such size and form as might be represented by a film one inch square, with its upper and lower edges horizontal, and with water trickling from its lower edge, it is easy to show that the slabs will be pressed together by a force equal to the weight of half a cubic inch of water. But so small a film as this would form itself, even if the two surfaces of the ice were only very imperfectly fitted to one other. If, again, by better fitting, a film be produced of such size and form as may be represented by a square film with its sides 4 inches each, the slabs will be urged together by a force equal to the weight of half a cube of water, of which the side is 4 inches; that is, the weight of 32 cubic inches of water, or 1.15 pound, which is a very considerable force. Secondly, the film of water existing, as it does, under less than atmospheric pressure, has its freezing-point raised in virtue of the reduced pressure. Much more, he adds, will it freeze in virtue of cold given out in the melting by pressure of the ice at the points of contact, where, from the first two causes stated above, the two slabs are urged against one another.

The different explanations and interpretations which have been enunciated of the facts of regelation have been stated in this article partly in the words of their authors, and partly in those of Professor James Thomson, by whom the subject has been most recently treated. References to some of the original memoirs will be found in his paper in the 'Proceedings of the Royal Society' (vol. x.), for Nov. 24, 1859, pp. 152-160; and others will be indicated in the sequel of this article.

It is manifest that the subject of regelation is one of great importance in physics and in the history of nature, being connected with that of the mutual relations of the different states of aggregation in which the same species of ponderable matter-the same substance, chemically speaking-can exist, and also with those "hidden and unseen motions," to use the language of Boyle, by which the molecular condition of such matter is perpetually changing. Some bodies, the equilibrium of the proximate elements of which is very unstable, present, in the solid state, phenomena, seemingly at least, analogous to those presented by water, and others in their alternation between the liquid and the solid states. Of these, glass is an example, and the now well-known fact of the incorporation into one mass of two or more plates of (plate-) glass, the polished surfaces of which have been placed in close contact with each other, presents a curious parallel to the incorporation into

ARTS AND SCI. DIV. VOL. IV.

one of several slabs or other separate portions of ice by regelation, as taking place in the experiments described in this article, and to determine in what manner these two subjects are related to each other would appear to deserve careful investigation. The principal facts, so far as glass is concerned, with the bearings on molecular philosophy they appeared to possess before the phenomena of regelation had been scientifically considered, will be found in the abstracts of two lectures on that substance delivered by Mr. Brayley before the Pharmaceutical Society of London in 1845, published in the 'Pharmaceutical Journal' (vol. v.) for August and October of that year.

In a paper on the Physical Properties of Ice, Professor Tyndall has shown that when a sunbeam traverses a mass of ice, the latter melts at innumerable points in the track of the beam, and that each portion melted assumes the form, not of a globule, but of a flower of six petals. The planes in which these flowers are formed are independent of the shape of the mass, and of the direction of the beam through it; they are always formed parallel to the surface of freezing. This, he observes is a natural consequence of the manner in which the particles of ice are set together by the crystallizing force. By the slow abstraction of heat from water in the process of freezing, its particles build themselves into these little stars, and by the introduction of heat into a mass so built, the architecture is taken down in a reverse order. In watching the formation of artificial ice, by the machine of Mr. Harrison (noticed under FREEZING-APPARATUS), Professor Tyndall has seen little solid stars formed, which were the exact counterparts of the little liquid stars formed by melting. Phil. Trans.' 1858, pp. 211-227; Ib. 1859, pp. 298, 299. Appended to the former paper is a letter from Professor Faraday on the irregular fusibility of ice.

Another important part of the history of ice is its production on the bed of rivers, when it receives the name of Ground-ice, Bottom-ice, and Ground-gru; the Glace-du-fond of the French, and the Grund-eis of the Germans. It is generally imagined that rivers freeze only at the surface; this however is not the fact, ice being frequently formed at the bottom of running water. Thus, according to the late Rev. Dr. Farquharson, F.R.S., the phenomenon is so common, and so well known in certain parts of Aberdeenshire, that the inhabitants have given it the name of Ground-gru, a name which that gentleman has adopted in his paper on the subject in the 'Philosophical Transactions' for 1835, p. 329. Gru is the name by which the people of Aberdeenshire designate snow saturated with or swimming in water; and as the ice formed at the bottom of rivers very nearly resembles that in appearance, a better name than Ground-gru could hardly be given, though it would be more precise to call it subaqueous ice, in contradistinction to that found at the surface, and because the term ground-ice, which this formation has also received, has been sometimes given to the ice occasionally met with at certain depths in the ground in northern countries.

Common, however, as may be the phenomenon of subaqueous ice, and although it has been noticed at various times, it has but lately attracted the serious attention of observers. Ireland, in his Picturesque Views of the River Thames,' published in 1792, 2 vols. 8vo., mentions the ground-ice of that river, and on the subject quotes Dr. Plott, who says, "The watermen frequently meet the ice-meers, or cakes of ice, in their rise, and sometimes in the underside enclosing stones and gravel brought up by them ab imo.”

M. Arago published an interesting paper on the subject in the 'Annuaire du Bureau des Longitudes' for 1833, in which he mentions the following observations made on ground-ice :-In the Thames, by Hales, in 1730; in the river Déome, department of Ardèche, France, by Desmarets, in 1780; in the Elbe, by M. Braun, in 1788; in the Teine, Herefordshire, by Mr. T. A. Knight, in 1816; in the Canal de la Birze, near Bâle, by M. Mérian, in 1823; in the Aar, at Soleure, by M. Hugi, in 1827 and 1829; in the Rhine, at Strasburg, by Professor Fargeau, in 1829; and in the Seine, by M. Duhamel, in 1830. More lately still, Colonel Jackson, in a paper on the congelation of the Neva, published in the 5th volume of the 'Journal of the Royal Geographical Society,' mentions the formation of ground-gru at the bottom of that river; and in the 6th volume of the same journal there is a paper expressly on the ice formed at the bottom of the Siberian rivers. The Rev. Mr. Eisdale has, in the Edinburgh New Philosophical Journal,' vol. xvii., p. 167, a paper on ground-ice; and, finally, Dr. Farquharson, as already mentioned, published his observations on the ground-gru of the rivers Don and Leochal, in Aberdeenshire.

Almost all who have written on ground-gru have endeavoured to account for its formation, though no explanation yet given is perfectly satisfactory. Dr. Farquharson, whose paper contains an original investigation of the subject, says it is the result of radiation, and endeavours to substantiate his reasoning upon the principles of the formation of dew. It was remarked in this article, as originally published, that he seems to forget that Dr. Wells maintains expressly that wind and shade are alike obstacles to radiation; and that consequently a body of moving water so deep as to be impervious to light, and particularly when covered, as in the case of the Ñeva, with a sheet of ice three feet thick, and as much more of snow, must present an insurmountable obstacle to the radiation of heat from the bottom of the river. This objection, however, is unsound, and is removed, as in many other instances of the supposed insufficiency of the principle of 3 a

radiation to account for depression of temperature, by an adequate consideration of the entire series of the phenomena concerned. It is true that the thick sheet of ice and its covering of snow will present an obstacle to the direct radiation of heat into space from the bed of the river, though they themselves will have such radiation above. But the bed of the river, at a comparatively high temperature, will suffer refrigeration by the radiation of its heat to the ice on the surface of the river, and may thus (and if there be time enough must necessarily thus) be cooled eventually to the freezing point, and so effect the formation of the ground-ice. A familiar and readily intelligible illus tration of this will be found in an observation of Dr. Joseph D. Hooker, which occurs in the invaluable collection of physical facts presented by his Himalayan Journals,' and already cited in the article DEw. When in the narrow valleys of East Nepal, in the month of November, and at the elevation of 8000 feet, the nights were so brilliant, and the radiation from the earth and bodies upon it consequently so powerful, that the upper blanket of his bed became coated with dew, from the rapid abstraction of heat by its radiation to the tarpaulin of his tent, itself frozen by its own radiation to the sky. The direct radiation of the blanket to the sky was prevented by the tarpaulin, but this did not prevent the conversion of the aqueous vapour in contact with the blanket into dew. In this case, the frozen tarpaulin corresponded in its action to the surface-ice of the river, the blanket to the river-bed, and the dew upon it to the ground-ice. Had the exposure been longer continued, or the temperature of the whole system of radiating bodies been lower, hoar-frost instead of dew would have been formed, and the parallel would then have been perfect, according to Dr. Farquharson's view of the origin of ground-ice.

Mr. Eisdale thinks ground-ice is the result of frozen spiculæ from the atmosphere, analogous to hoar-frost, falling into the river, and there forming nuclei, around which the water freezes at the bottom; but this is quite inadmissible. M. Arago's explanation in part, and the very simple fact that water, when at 32° of Fahr., if at rest, or in very slow motion (which is the case at the bottom of rivers), will freeze, seem among the most natural ways of accounting for the formation of ground-gru. M. Arago attributes the formation to three circumstances-1st, the inversion, by the motion of the current, of the hydrostatic order, by which the water at the surface cooled by the colder air, and which at all points of the temperature of water under 39° Fahr. would, in still water, continue to float on the surface, is mixed with the warmer water below; and thus the whole body of water to the bottom is cooled alike by a mechanical action of the stream; 2nd, the aptitude to the formation of crystals of ice on the stones and asperities of the bottom in the water wholly cooled to 32°, similar to the readiness with which crystals form on pointed and rough bodies in a saturated saline solution; 3rd, the existence of a less impediment to the formation of crystals in the slower motion of the water at the bottom than in the more rapid one near or at the surface. But, as has been said, no explanation yet given is quite satisfactory, and the phenomenon yet remains to be studied under all the variety of circumstances which may attend it. A knowledge of the temperature of the water at different depths is most essential to a just appreciation of the real cause of the phenomenon.

Ground-gru differs materially from surface-ice. Dr. Farquharson describes it as having "the aspect of the aggregated masses of snow, as they are seen floating in rivers during a heavy snow-shower; but on taking it out of the water, it is found to be of a much firmer consistence than these: it is a cavernous mass of various sized, but all small, pieces or crystals of ice, adhering together in an apparently irregular manner by their sides, or angles, or points, promiscuously; the adhesion varies according to circumstances." This corresponds precisely with what is stated by Col. Jackson to have been observed by him in the Neva at St. Petersburg. Dr. Farquharson says, that when it begins to form at the bottom, it aggregates in forms somewhat resembling little hearts of cauliflower. Mr. Weitz, author of the paper in the Journal of the Geographical Society' on the ground-gru of the Siberian rivers, says that which he noticed at the bottom of the Kann (an affluent of the Jenisseï), 40 versts from Krasnojarsk, was of a greenish tinge, and resembled patches of the confervoidea. From these facts we conclude that though the appearances of the ground-gru may vary with circumstances, it is in all cases essentially different from the solid compact sheets of surface-ice.

[DEW; FREEZING; HAIL; HOAR-FROST; METEOROLOGY; SNOW; WATER.]

ICEBERG. [SEA.]

ICE-HOUSES AND ICE-TRADE. Considering ice as an article of commerce, one of the most important points connected with it is the adoption of means for preventing the substance from melting away in hot weather. Ice-houses are expressly constructed to this end. Such structures are not only useful for preserving ice which is to be applied to the cooling of liquors, or to the preparation of articles of confectionary, but also as affording the most ready if not the most effectual means known for keeping meat, fish, game, vegetables, and fruit sweet and fresh in hot weather. Although these important conveniences are rarely to be found among the buildings of an English farm, they are frequent in those of North America, and might be advantageously introduced in this country, especially upon such farms as are connected with inns.

One of the simplest modes of preserving ice consists in enveloping it in a great quantity of straw, above the surface of the ground, in such a position that moisture, which is even more injurious than heat, may drain off freely. For this purpose the ground should be raised in the form of a flattened cone, upon which should be laid a stratum of faggots. Straw is laid upon the faggots to the thickness of a foot or more, and the ice is piled upon it in a compact conical mass, the larger the better. Over the ice is laid first about a foot thickness of straw; then faggot-wood to a further thickness of two feet, the interstices of which have the effect of keeping a stratum of confined air round about the pile of ice; and, finally, two or three feet of straw arranged as a thatch. An underground ice-house may be simply a large cellar, with hollow or double walls, floor, roof, and doors, and furnished with a trapped drain to allow the escape of such water as may be produced by a partial thaw, without admitting any air. Such ice-houses are usually formed in the shape of an inverted cone, which is considered the most advantageous because it keeps the ice more compactly together than any other form, and because, in case of any thaw taking place, the remaining ice will naturally slip down, so as to keep the mass solid. In all cases it is well to interpose a layer of straw, reeds, or chaff (the last named is preferred to straw in Italy, where it is used for packing ice for travelling) between the walls and the ice; and by the use of faggots as well as straw any perfectly dry cellar in a suitable situation may be used as an ice-house. In some situations a sufficient degree of hollowness in the walls may be produced by the adoption of the plan of building with bricks on edge, or by some similar contrivance. One mode of building hollow walls which may be thus applied consists in the use of half bricks divided longitudinally, as stretchers, leaving a space equal to the full width of a brick between them. Hollow floors for ice-houses may be constructed in various ways, with bricks on edge and tiles or flags. Whatever be the construction of the ice-house itself, there should be no opening by which it can communicate with the external air excepting through the entrance passage, which is usually at least two or three yards long, and furnished with two, three, four, or more doors, of which not more than one must be opened at a time. Where the difficulty of excluding external temperature is very great, treble walls, roofs, and floors may be used; and the entrance-passage may be made crooked, with a door at every turn.

Loudon gives a ground-plan and section of a complete ice-house of approved construction, of the inverted conical shape, with an arched roof, which it is proposed to cover with two or three feet of earth, or more in hot climates, over which he suggests the propriety of training ivy, for the sake of excluding solar heat. In this design a small pump is shown communicating with a well in the drain of the ice-house, for the purpose of raising the thaw-water for drinking or other use. Ure describes a similar structure, but with solid walls and a conical roof of timber, which may be simply thatched, or covered with brickwork and thatched, and which should have a gutter round it to collect and conduct to a distance all rain that falls upon it. In Gordon's plan the excavation is made considerably larger than the ice-house, which consists of a framework of strong timbers, roughly boarded outside, and lined with straw set on end and confined by laths nailed to the timbers. The conical roof is thatched with straw or heath, and the space between the outer boarding and the surface of the excavation is filled with heath, brushwood, or fir-tops, and neatly thatched or turfed over. In some situations simple excavations in calcareous soils, with a long circuitous passage by way of approach, are used instead of more regular icehouses.

In filling an ice-house, the ice should be broken with mallets to a coarse powder, and well rammed down as it is thrown into the icewell; its upper surface being kept of a concave shape, and a little water being occasionally added to fill up all interstices, and to facilitate the congelation of the whole into a solid mass. A better method is to sprinkle the ice with water saturated with salt, at the rate of a pound of salt to a gallon of water. This salt and water may be applied by a common watering-pot upon the surface of the ice at intervals of two feet from bottom to top of the mass, an extra quantity being poured on when the filling is completed. By this means the ice becomes so firmly compacted as to need the force of a pickaxe to break it up, even in the heat of summer. Snow is occasionally preserved in a similar manner to ice, it being carefully compressed into a solid mass. In Portugal and some other countries, when the snow has been collected in a deep gulf, some grass or green sods, covered with dung from the sheep-pens, is thrown over it; and under this covering the snow is so well preserved that it may be taken up and transported to a considerable distance throughout the summer.

An içe-box, or sort of portable ice-house, is occasionally used. It consists of an inner and outer casing, six inches apart, the interval between which is filled with burnt cork reduced to powder, this being found to possess higher non-conducting properties than the charcoal of wood. The lid is double, and is filled with the same substance; and it is made perfectly air-tight by means of projecting ledges, which, when shut, dip into a gutter filled with water. Ice may be preserved for several weeks in such a box, in which also bottles, dishes, &c., may be placed. Similar to this contrivance is the American ice-safe, introduced a few years ago into this country.

The French, in 1859, constructed an extensive range of ice-houses in the Bois de Boulogne, between the Auteuil railway and the fortifi

cations of Paris. During the hard frost of December in that year, 250 carts were employed for 10 days in conveying ice to these buildings.

A remarkable traffic has sprung up in America, namely, the transport of ice to various parts of the world. In the East Indies the artificial formation of ice has been long carried on, as the only means of cooling beverages and food. The ground near Hoogly, about 40 miles from Calcutta, is formed into shallow troughs; into these troughs, on a layer of straw, are placed pans of porous earthenware. Shortly before midnight in the winter months, and when the wind happens to be blowing from the north-west, a little water is poured into each vessel or pan; and if all the circumstances are favourable, a film of ice is found in each pan on the following morning, which ice is collected and stored with the utmost care. The selling price of this ice at Calcutta was formerly about sixpence per pound; but the Calcutta inhabitants were surprised by the arrival, in 1833, of a ship from the United States, laden entirely with ice, which was offered for sale at three-pence per pound, and was understood to yield a good profit, even after paying all the expenses of a long voyage. Since then the price has been much lowered, and ice has become a regular article of shipment from America. The idea of this trade had occurred to a Boston merchant, Mr. Tudor, twenty years earlier; but it was only by much patience and perseverance that he overcame the various difficulties.

The trade is now chiefly in the hands of the Wenham Lake Company. This company purchased a lake of pure water, and the surrounding land, at Wenham, about eighteen miles from Boston; they built storehouses, and formed a railway from Wenham to Boston. The lake is very deep, and is supplied solely by springs, which issue from its bed. During winter the ice which forms on it is very thick, clear, and compact. When the ice is about a foot thick, a number of men, horses, and machines are set to work. The ice is first swept scrupulously clean; an ice-plane is drawn over it, to cut away a layer of loose or imperfect ice; an ice-plough is drawn over it, to cut a groove across the lake; and other machines are successively employed, until the ice is removed from the lake, in solid blocks weighing from one to two cwts. each. They take two acres of lake surface into operation at one time; this will yield, at the average thickness, about two thousand tons of ice; and forty men, assisted by twelve horses, will cut and stow four hundred tons of this in a day. The company's store-house, near the lake, is built of wood, and has double walls two feet apart: the intervening space being filled with sawdust; twenty thousand tons of ice can be stored in this building at one time. The company convey the ice to Boston on their own railway, and thence transmit it to various parts of the world. Large store-houses have been formed in many parts of the United States, as well as in London and Liverpool. So many are the establishments now engaged in this trade, and so important has it become, that the ice-farms of the states of Massachusetts and New York are reputed to be equal in commercial value to the rice farms of Georgia. Boston is the great storehouse, containing sometimes as much as 300,000 tons of ice in store at once. About 10,000 persons altogether are supposed to be concerned in and with the ice-trade of America, and about 6,000,000 dollars of invested capital.

A curious project was started a few years ago, for sending a steamboat to Newfoundland, to tow home an iceberg; an iceberg of 10,000 tons would, it was conceived, pay the expenses and yield a good profit. There have been many projects for producing ice artificially-by rarefaction, by evaporation, by the contact of freezing mixtures, and by other means; some of which are described in the articles FREEZING; FREEZING APPARATUS; FREEZING MIXTURES.

ness is removed. (Pereira.) When this is removed, the starchy matter differs little from wheat-flour in nutritive properties, though Olasson asserts that a soup prepared with it is twice as nutritious as one made with flour. (Sparmann, Voyage,' iii. p. 129, note.) Certain it is that the inhabitants of Norway, Lapland, and above all, of Iceland, use it extensively as an alimentary substance, the latter regarding it as the gift of "a bountiful Providence, which sends them bread out of the very stones." Dr. Henderson (' Tour in Iceland') says that a porridge made of this lichen-flour is to a foreigner not only the most wholesome, but the most palatable, of all the articles of Icelandic diet. It is submitted to no other preparation than repeated steepings in cold water, drying, and powdering; after which it is either made into cakes or boiled in milk. Unless it be steeped, it is both offensively bitter, and also to many persons purgative; hence it has been called lichen catharticus. (Borrichius, Act. Hafnien.,' 1671, p. 126.) But cattle turned out to browse on it in spring, though at first purged, ultimately become fat. (Boerhaave.) Owing to its intensely bitter taste, as it had not been previously steeped, Sir John Franklin, even when pressed by hunger, could not use it, though the tripe de roche suited well. (First Journey to Shores of Polar Sea,' 4to., p. 413, 414.)

The excellence of Iceland moss depends upon its freshness and freedom from accidental impurities, which should be carefully reinoved before it is used. In its natural state, that is, while still containing the bitter principle, it is tonic, stomachic, febrifuge, demulcent, and nutritious. It has acquired a high reputation, not merely as an article of diet, but as a medicinal agent in consumption and chronic diarrhoeas, and dysenteries devoid of inflammatory states of the intestines. To obtain benefit from it, the use of it must be persisted in for a long time. This constitutes at once a difficulty in the employment of it, and casts a doubt on the exact nature of the cases in which it is said to have proved serviceable. The unpleasantness of the bitter it contains renders it unpalatable to most persons, and also its heating qualities unfit it for those who have either much general fever, or a state of sub-acute inflammation of the stomach, a very frequent condition in genuine phthisis pulmonalis. Hence there is every reason for suspecting that in the instances where it has been used for a long time and proved beneficial, the disease was chronic bronchitis, in which bitters and demulcents are extremely useful. To disguise the disagreeable flavour many expedients have been had recourse to, such as uniting it with chocolate or cocoa, and flavouring it with orange-flower water, &c. (A full account of these may be found in Hufeland's ' Journal,' August, 1824, p. 126, from the pen of Dr. Oppert. Many formulæ may be found in Geïger, 'Pharmacopoeia Universalis.') The only officinal form in Britain is the decoction, which is frequently made the vehicle of medicinal agents. Cetrarin has been given in a separate form as a succedaneum for cinchona bark, and, like many other very bitter articles, is of considerable efficiency in agues. Many substitutes for Iceland moss have been proposed; one of the best of which is the Carrageen or Irish moss (Fucus crispus). This, when the brackish taste is lessened by repeated steepings in cold water, forms an excellent jelly, much relished by consumptive patients, and much cheaper than any other. The Sticta pulmonacea, or lung-wort, is of unquestionable efficacy in some cases of asthma. But none are so palatable as the Ceylon moss (Fucus amylaceus). This can be procured not only from Ceylon, but abundantly from the east coast of Bengal. In the form of jelly, soup, lozenges, or other mode of preparation, it not only agrees better, but is more relished than any jelly, either animal or vegetable. It is to be hoped that it will become a regular article of commerce.

All the Iceland moss imported into Britain is not used for medicinal purposes; much is employed in baking ship-biscuits, as those into the composition of which it enters are said not to be attacked by worms, or suffer much from sea-water. In Saxony, in time of scarcity, it is advantageously added to wheaten flour. In some countries it is employed in brewings.

ICHNOGRAPHY (from xvos and ypaon), a representation of the groundwork of a building. The ichnography of a building is, in fact, what is more commonly called the plan, or ground-plan: as the ortho

ICHTHIN. An albuminous principle extracted from the yolk of the eggs of cartilaginous fishes, such as the ray fish. It has the appearance of white transparent soft grains, insoluble in alcohol, water, and ether. Hydrochloric acid dissolves it without violet coloration, which distinguishes ichthin from albumen. It contains:

Carbon
Hydrogen
Nitrogen

Phosphorus (?)

ICHTHIDIN. [ICHTHULIN.]

50.9

6.7

14.7 1.9

ICHTHULIN. An albuminoid substance found along with ichthidin in the roe of certain species of fish. It is precipitated by the addition of water to the expressed fluid of the roe. Ichthulin when first precipitated is viscous like gluten, but it afterwards becomes pulverulent. It contains :

Carbon Hydrogen Nitrogen Sulphur Phosphorus