Athy, must have been much longer than the Greeks reAtlantic. ported it. But as no vestiges are now discoverable of so magnificent a work, the whole story has been called in question:

Perforatus Athos, et quicquid Græcia mendax
Audet in historia.

We must not here omit the daring proposal of Stasicrates, an engineer in the service of Alexander, who offered to convert the whole mountain into a statue of that prince. The enormous figure, which must have been in a sitting posture, was to hold a city in its left hand, containing 10,000 inhabitants, and in the right, an immense basin, whence the collected torrents of the mountain should issue in a mighty river. But the project was thought to be too extravagant, even by Alexander.

Mount Athos is now peopled by a numerous horde of Greek monks, denominated Caloyers, who are of the order of St Basil. These devotees, who amount to 6000, fare very hardly, abstaining entirely from flesh, and subsisting chiefly on pickled olives. They were at one time distinguished for their learning, at least for possessing several valuable manuscripts, and for their numerous copies of the Scriptures, to transscribing which, they applied themselves with much laudable assiduity. Though now extremely illiterate, so much so that they can scarcely read or write, they have the merit of being sober, peaceable, and industrious: and these qualities have procured for them the good opinion of the Turks, who afford them protection and sustenance. They have twentyfour monasteries situated on different parts of the mountain, raised in stories to a great height, and surrounded with walls; and these buildings, interspersed with churches, hermitages, and some fortifications, on which are mounted some pieces of artillery, give an extraordinary appearance to this lofty eminence, and present, to the eye of the traveller, as he approaches the scene, a most picturesque object, and a pleasing specimen of manual industry. Mount Athos is now called Hagiosoros, or Monte Santo, from the reputed sanctity of its inhabitants. It is in 40° 10′ N. Lat. 24° 45' E. Long.

See Herodot. 1. vi. c. 44. ; l. vii. c. 21. &c. Plut. in vitâ Alexand. Elian. de Anim. 1. xiii. c. 20. Lucan. 1. ii. v. 672. Plin. 1. iv. c. 10. Strab. Epit. 1. vii. Mela de Sit. Cellarii Geog. Belon Observ.

1. i. c. 25. (E)

ATHY, a town in the county of Kildare, in Ireland, situated on the river Barrow, which is navigable to the sea. A branch of the great canal from Ďublin to the Shannon joins the Barrow at Athy. The exports from the adjacent country to Dublin amount annually to £20,000, and consist of corn, coals, flour, butter, and potatoes. Number of houses 550. Population 3300. W. Long. 7° 1'. N. Lat. 52° 9'. of Ireland, and

a

Anthologia Hibernica, vol. i. (1)

ATLANTIC OCEAN, the name of that immense tract of sea which separates the western shores of Europe and Africa from the eastern shores of North and South America. An account of Mr Kirwan's theory of the formation of the Atlantic may be seen

VOL. III. PART L.

in the Transactions of the Royal Irish Academy, vol. Atlantis. vi. P. 228. (w)

ATLANTIS, an island mentioned in Plato's Timæus, as situated beyond the Pillars of Hercules, and surpassing in extent Asia and Africa taken together. Many consider the whole account as an idle fable, not deserving of the least attention. Perizonius, and others, consider it as a proof that the ancients had some obscure knowledge of America: whilst others suppose, that it refers to an immense island or continent formerly existing in the place now occupied by the Atlantic ocean. We shall lay before our readers an abstract of the original account as given in the Timæus, and then shall advert more particularly to the several opinions entertained respecting it. Critias, one of the speakers, professes to have heard the account from his grandfather, who received it from Solon, and this latter learned it from the Egyptian priests, when he studied under them in Egypt. The sum of their accounts is this: that the vast island of Atlantis was situated near the straits of Gades: that it was governed by a race of mighty conquerors, who subdued all Africa as far as Egypt, and all Europe as far as the Tuscan sea. In succeeding ages, however, owing to prodigious earthquakes and inundations, the Atlantic island was suddenly ab. sorbed into the bosom of the ocean, which for many ages afterwards could not be navigated, on account of the numerous rocks and shelves with which it abounded.

There is so much of the marvellous and improbable in this account, that but a moderate share of incredulity is necessary to make us reject the whole as a fable. The information comes to us in a very roundabout way, and from a very suspicious quarter; and it would not be very safe to receive as authentic history, the ipse dixit of an Egyptian priest.

There are circumstances, however, which have induced some to think that this Atlantic island is no other than the continent of America. Ammianus Marcellinus affirms, that the account recorded by Plato is no fable. Crantor also, Plato's first interpreter, considers it as a true history. It is admitted that there is an error in the account, as to the proximity of the island to the Straits of Gibraltar: for Diodorus Siculus informs us, that the Phoenicians in early times, sailing beyond the Pillars of Hercules, were carried by storms and tempests far to the west, till they fell in with a vast island, having navigable rivers and a fruitful soil. It is thought that the Atlantis of Plato, and this island mentioned by Diodorus, can be nothing else than the continent of America; and that the account of the submersion of this vast island arose from the circumstance of its becoming, in course of time, entirely unknown to the

ancients.

But many naturalists, among whom are Buffon and Whitehurst, have thought it probable, that such an island or continent as that described by Plato actually existed, and that the Canary islands, the Azores, and Teneriffe, are nothing else than the summits of mountains belonging to such an island or continent submerged, and the fragments of an antediluvian world, consumed and shattered by earthquakes and volcanic

G

eruptions. Whitehurst is of opinion, that the Atlantic island of Plato, was probably the portion of land which, stretching from the north of Ireland, extended to the Azores, and from the Azores to the continent of America. He thinks that the Giant's Causeway, and the abrupt cliffs which environ part of the Atlantic ocean, are a sufficient demonstration, that some violent disruption of the earth has taken place in that quarter at some remote period of the world. We shall let him speak for himself on this subject.

These circumstances render it necessary to observe, that whosoever attentively views and considers these romantic rocks, together with the exterior appearances of that mountainous cliff, will, I presume, soon discover sufficient cause to conclude, that the erater, from whence that melted matter flowed, together with an immense tract of land towards the north, have been absolutely sunk and swallowed up into the earth, at some remote period of time, and beeame the bottom of the Atlantic ocean: A period indeed much beyond the reach of any historical monument, or even of tradition itself. But though it does not appear, that any human testimony, or record, has been handed down to us concerning such a tremendous event, yet the history of that fatal catastrophe is faithfully recorded in the book of nature, and in language and characters equally intelligible to all nations, and therefore will not admit of a misinterpretation; I mean the range of lofty abrupt cliffs which environs a part of the Atlantic ocean.

These are characters which cannot mislead, or divert our attention from the true cause thereof; and we may further add, as a collateral testimony, that subterraneous fires have frequently burst open the bottom of that ocean in various parts, and have formed new islands of considerable magnitude; whence it is evident that the same cause still exists, and produces similar effects. I say, the consideration of such disasters, together with that of the causes still subsisting under the bottom of that immense ocean, almost persuade me to conclude, that Ireland was originally a part of the island Atlantis, which, according to Plato's Timæus, was totally swallowed up by a prodigious earthquake, in the space of one day and night, with all its inhabitants, and a numerous host of warlike people, who had subdued a considerable part of the known world.” See the Timæus of Plato. Un. Hist. vol. xviii. p. 250. Buffon's Nat. Hist. vol. ix. p. 162. Whitehurst's Inquiry, p. 258. Maurice's Hist. of Hindostan, vol. i. p. 538. (g)

ATLAS, a chain of mountains in the north-west of Africa, called in the Arabic Jibbel Attils, or the Mountains of Snow. This chain of mountains is inhabited by the various tribes of Berebbers, and extends from (Jibbel d'Zatute) Ape's Hill on the Mediterranean to Shtuka and Ait Bamaran in Lower Suse, passing at the distance of 30 miles to the east of Morocco, where they are of an immense height, and covered with eternal snow. This part of the range appears in a clear day like a saddle when seen from Mogodor, a distance of 140 miles, and it is visible at sea to vessels several leagues off the coast. These mountains, though extremely cold in winter, are salubrious and pleasant. The vallies are well cultivated,

and the mountains having the advantage of various climates, abound in excellent fruits, and extensive forests. The contrast between their snowy summits and the rich verdure below, gradually decaying as it approaches the limit of congelation, has a very singular and picturesque appearance. In the part of the great chain which passes by Morocco to the east there are excellent mines of copper, and the branches which traverse the district of Suse produce silver, copper, iron, lead, and sulphur of saltpetre. They have also mines of gold mixed with antimony and lead ore. According to the Moors, there are many quarries of marble granite, and other valuable rocks in this extensive range. The Berebbers, who inhabit the upper regions of Atlas, live from November to February inclusive in excavations in the mountains. See Pliny, lib. v. cap. 1; Strabo, lib. xvii; Shaw's Travels in Barbary, p. 5; Lempriere's Journey to Morocco, p. 75; Chenier's Present State of Morocco, vol. i. p. 13; Pinkerton's Geography, vol. iii. p. 815; but particularly Jackson's Account of the Empire of Marocco, 1809, p. 10. (0)

ATLAS, the name of that joint or vertebra of the neck which is nearest the head. See ANATOMY. (w) ATMOMETER, ATMIDOMETER, or ATMEDOMETER, from arμos, vapour, and μergov, a measure, the name given to an instrument for measuring evaporation. An ingenious instrument of this kind has been described by the celebrated Professor Richman, in the Nov. Comment. Petropol. vol. ii. p. 121. (w) ATMOSPHERE, that invisible elastic fluid which surrounds the earth, and encloses it on all sides. It received its name from the Greeks, in consequence of the vapours which are continually mixing with it. The ancients considered it as one of the four elements of which all things were composed, and some of them seem to have thought that it enters as a constituent principle into other bodies, or at least that air and other bodies are mutually convertible into each other. (Lucret. lib. v. 274.) No experiments on its nature could well be made by the ancients, as they were unprovided with every instrument fitted for such investigations, and unacquainted with the principles upon which their construction depended. But it has occupied a great deal of the attention of modern philosophers, and has given birth to some of the most brilliant discoveries that grace the annals of science. Its weight was first ascertained by Galileo, and applied by Torricelli to explain the rise of water in pumps, and of mercury in barometrical tubes, and by Paschal to the mensuration of the height of moun tains. Its elasticity was accurately determined by Boyle, who may be considered as in some measure the founder of the science of pneumatics. Halley and Newton explained the effects produced on it by moisture. Hooke, Newton, Boyle, Derham, pointed out its relation to light, to sound, and to electricity. Its effect upon combustibles and animals was investigated by Boyle, Hooke, Mayow, Hales, Priestley, Scheele, and Lavoisier. Its constituents were detected and measured by the experiments of Priestley, Scheele, Lavoisier, and Cavendish. The effect of heat on it was determined by Shuckburgh, Dalton, and Gay Lussac. But it would be an endless task to enumerate all the philosophers who have distin

Atma guished themselves by their investigations of the atsphere. mosphere, a list which would include almost all the celebrated names of the last century.

Expansion by heat.

From the experiments of Sir George Shuck. burgh Evelyn, (Phil. Trans. 1777 and 1798,) made with a degree of precision and patient industry, which perhaps have never been surpassed, it appears, that at the temperature of 60°, when the barometer stands at 30 inches, the specific gravity of atmospherical air is 0.001208, that of water being 1.000, or its weight is to that of water as 1 to 828. Hence 100 cubic inches of it under that pressure and at that temperature weigh 30.5 grains: For a cubic inch of pure water at that temperature weighs 252.506 grains, according to experiments of Shuckburgh corrected by Mr Fletcher. (Nicholson's Journal, iv. 35.) The result of the experiments of Lefevre Gineau, who was employed by the French government to ascertain the weight of water, in order to fix their standard of weights, was somewhat different. According to him, a cubic inch of The difwater at 60° weighs 252.72 grains troy. ference may be partly owing to some small error in the allowance of the expansion of water from 40°, the temperature at which his experiments were made, to 60°. At any rate, the known precision, and the excellent apparatus of Sir George Shuckburgh, entitle his result to the preference. Hawksbee's experiments make air 850 times lighter than water, the barometer being at 29.7, and Dr Halley supposed it about 800. But neither of these numbers is to be put in competition with the result of Sir George Shuckburgh given above. The air when weighed, is supposed to be in its usual state of dryness; when very moist, its specific gravity is diminished. An exact knowledge of the weight of a given bulk of air is of great importance, because it enables us with much facility to ascertain the weight of all other aërial bodies: for it is easy to determine the relative weight of any elastic fluid to that of air.

When heat is applied to atmospherical air, its bulk increases; while cold, on the other hand, diminishes its bulk. As this change in bulk is very considerable, it affects very much the accuracy of all experiments on it. It has therefore been an object with philosophers to determine the precise amount and rate of the change in bulk produced upon air by heat. M. De Luc, Sir George Shuckburgh Evelyn, General Roy, Mr Dalton, and Mr Gay Lussac, are the gentlemen to whom we are indebted for the solution of this problem. In examining the dilatability of air by heat, it is necessary that no water be in contact with it. For as heat converts water into vapour, this vapour mixing with the air, would destroy the accuracy of the results, and make the dilatation appear much greater than it really is. According to the experiments of De Luc, air at the temperature of 55° when heated 1° of Fahrenheit's thermometer, expands part; according to Shuckburgh, the expansion is according to General Roy, it is; according to Dalton, it is; and according to Gay Lussac, As Dalton and Gay Lussac were at pains to exclude moisture, we may consider their experiments as more accurate than those that preceded them. As to the rate of expansion, General Roy found it a slowly diminishing ratio from 32° to 212°.

Mr Dalton found the same thing. But he considers this diminution as apparent only, and not real, and owing to the expansion of mercury not being equable. According to him, water, mercury, and all liquids, expand as the square of the temperature, reckoning from the freezing point of the respective liquid. Ac cording to this notion, the expansion of air, (and indeed of all permanently elastic fluids,) is in geometrical progression to equal increments of temperature. The following Table exhibits the rate of expansion of air from 32° to 212°, according to Mr Dalton: Degrees of Fahrenheit.

Bulk of Air.

Degrees of Dalton's Thermometer.

62

72

82

92

102

1173.1

122

130.4 141.1 152 163.2

199.2 212. 359.1 539.8 754.7 1000 1285

1351.8

1376

The reader will observe, that the expansion of air in the second column of the Table constitutes a geometrical progression, the ratio of which is 1.0179. The third column exhibits the corresponding degrees of a Fahrenheit's thermometer graduated, according to Mr Dalton's notion of the expansion of mercury, according to the square of the temperature. This notion of Dalton must be allowed to be very ingenious. Unfortunately we are not in possession of any mode of ascertaining how far it is correct. It is only supported by analogical reasoning, and cannot well be otherwise in the present state of our knowledge of heat.

Atme

sphere.

Atmospherical air was long considered as a sim- Composi ple elementary body. But it is now known to con- tion of the sist of at least four distinct substances, namely, oxygen, atmosphere azote, carbonic acid, and aqueous vapour. The first

two substances must be considered as its essential constituents, and constitute in fact almost the whole of it. The other two are variable in their proportion, and exist only in minute quantities, which it is difficult to appreciate.

The first knowledge of the composition of the at- History of mosphere must have been after the period of the dis- the discocovery of oxygen gas by Dr Priestley in 1774. La- very. voisier, indeed, in his posthumous works, appears to insinuate a knowledge of it in 1772. But this claim cannot be admitted, as he gives no hint of any such

knowledge in his volume of essays published after sphere. that period, and as he was entirely unacquainted with oxygen gas when Priestley shewed him the way to prepare it at Paris, about the end of 1774. It is very probable that Lavoisier became acquainted with the composition of atmospherical air not very long after that period; though some years elapsed before he made it known to the public. Whether he preceded Scheele in his knowledge of this important fact, we do not exactly know. But there is no doubt whatever, that Scheele's investigations were carried on without any assistance from abroad, and that it was in consequence of the publication of his Treatise on Air and Fire, that the chemical world became acquainted with the nature and composition of atmospherical air. This important work was printed at Upsal in 1777, with an introduction by Bergmann, and translated into English by Dr Foster in 1780. The experiments of Priestley indeed would have warranted the conclusions respecting the composition of atmospherical air drawn by Scheele; but those of Dr Priestley were different and more complicated. In Scheele's first experiments, he estimated the bulk of oxygen gas in air at 30 per cent. But in the year 1779, he published a set of experiments continued for a whole year, in order to ascertain whether the bulk of oxyen in air be constant, or varies with the season of the year. He found it in general remarkably constant, and amounting to 27 per cent. The smallest bulk was 24, and the greatest observed was 30 per cent. Dr Priestley had made similar experiments, and had estimated the bulk of the oxygen at 4th of the air, or 20 per cent. Mr Lavoisier's experiments, which were very numerous and varied, almost coincided with those of Scheele. He considered air as composed of 27 parts by bulk of oxygen, and 73 of azote. Mr Cavendish's experiments were published in the Philosophical Transactions for 1783. He proved decisively, that the proportion of the azote and oxygen in the atmosphere does not vary; and by a very careful analysis, concluded, that 100 parts of air in bulk are composed of

Bulk of

oxygen and azotic gas.

79.16 azote

20.84 oxygen 100.00

The

This opinion was not at first acceded to by chemists, misled by the previous conclusions of Scheele and Lavoisier; and it was not till towards the commencement of the 19th century, that the true proportion of these constituents was generally known. experiments of Berthollet in Egypt and in Paris, seem to have led the way to it. These were almost immediately confirmed by those of Davy, Beddoes, and many other chemists. At present it is universally admitted, that atmospheric air never varies in its composition; that it is the same in all places, and in all seasons; and that it consists in bulk of

of the most remarkable substances in nature, and highly worthy of the investigation of the chemist. Dr Priestley, its original discoverer, gave it the name of dephlogisticated air, Scheele called it empyreal air, Lavoisier called it at first highly respirable air, then vital air, and at last oxygen gas, because he considered it as the acidifying principle. It possesses the mechanical properties of common air; combustibles burn in it with great brilliancy; and animals can breath it much longer than the same quantity of common air. If the specific gravity of common ajr be reckoned 1.000, that of oxygen gas, according to the experiments of Kirwan and Lavoisier, is 1.103; according to Davy, 1.127; according to Fourcroy, Vauquelin, and Seguin, 1.087; and according to Allen and Pepys, 1.090. These results do not differ much from each other, except that of Mr Davy. His oxygen was obtained from the black oxide of manganese, and might perhaps contain a little carbonic acid gas. If we exclude his, the average of the other three is 1.093. This may be considered as near the truth as can well be attained. Rating its specific gravity at 1.093, 100 cubic inches of it, at the temperature of 60° when the barometer stands at 30 inches, will weigh 334 grains troy.

Atmo

sphere.

Azotic gas, the other constituent of atmosphe- Properties rical air, is chiefly recognised by its negative qualities. of azote. It possesses the mechanical properties of air; it does not support combustion; and no animal can breath it without death. It constitutes the base of nitric acid, and is one of the constituents of ammonia. There is reason to consider it as a compound body, but hitherto chemists have not been able to ascertain its constituents; though several extraordinary phenomena, observed during the decomposition of ammonia by Davy and Berzelius, cannot well be accounted for, without supposing hydrogen to be a constituent of it. It has been supposed a compound of hydrogen and oxygen; but several circumstances militate against this opinion. Mr Davy has been for some time occupied incessantly in attempts to ascertain its composition, but hitherto without success. Till the discovery be made, some of the most interesting parts of chemistry remain involved in impenetrable obscurity. The specific gravity of azotic gas, according to Kirwan, is 0.985, that of air being 1.000; while, according to Lavoisier and Davy, it is 0.978. This last estimate we are disposed to consider as most correct. If so, 100 cubic inches of it, at the temperature of 60° when the barometer stands at 30 inches, weigh 29.83 grains troy.

Reckoning the specific gravity of oxygen gas Relative 1.093, and that of azotic gas 0.978, and supposing weight of atmospherical air to be composed of 79 parts of azote the conand 21 oxygen by bulk, it follows, that 100 parts of it in weight are composed of

77.43 azote 22.57 oxygen

100.00

stituents.

Though it has been ascertained, that these two constituents of air never vary in their proportions, yet Eudiomeas the methods of analysing air are very useful in all ter. chemical investigations of gaseous bodies, and have led to many discoveries of importance, it will be proper to give an account of them here. They consist

Atmo

in the application of various substances to a given sphere. bulk of air, which have the property of absorbing and removing the oxygen, but which do not act upon the azote. The diminution of bulk gives the quantity of oxygen ; the residue that of azote. The apparatus contrived for these experiments, received the name of eudiometers, because they were considered at first as measurers of the goodness of air. For it was supposed that the proportion of oxygen was variable, and that the salubrious or noxious qualities of the air depended upon that proportion. Ingenhousz thought he discovered, that the atmosphere above the sea contained more oxygen than that above the land; hence he accounted for the supposed salubrity of the sea air, which has been highly extolled from the, remotest times.

Nitrous gas

ter.

Improved.

The first eudiometer was applied, in consequence of Dr Priestley's discovery, that nitrous gas absorbs the oxygen from common air. When nitrous gas comes in contact with oxygen gas, they immediately combine and form nitric acid; and if the mixture be standing over water, the acid is immediately absorbed by the liquid. Hence the bulk of a mixture of nitrous gas and common air immediately diminishes, and the diminution is proportional to the quantity of oxygen in the air, supposing all other circumstances the same, and of course measures that quantity. Dr Priestley's method was, to let up into a gradua ted tube 100 measures of air, and then to add 100 measures of nitrous gas. The mixture became yellow, and its bulk diminished. He denoted the good, ness of the air by the residual gas. Thus if 114 parts remained, he said that the goodness of the air, by the test of nitrous gas, was 114; of course the smaller the residue, the greater was the goodness of the air. This method did not ascertain the absolute quantity of oxygen. It was soon observed, that even when the air operated upon was absolutely the same, the residue was liable to considerable variation from apparently triffing circumstances. Thus, for example, if the tube was agitated during the mixture, it was observed that the residue was always much less than if no agitation was applied. If the tube was narrow, the residue was always more considerable than if the tube was wide. The purity of the water, too, over which the experiment was made, had considerable influence. Mr Cavendish observed, that if the water was in such a state, that it frothed when agitated as if it had contained soap, then the residue was always less than it otherwise would be.

The apparatus was much improved by Fontana, who regulated the size of the tube and the manner of mixing the gases; hence the instrument is usually known by the name of Fontana's eudiometer. This eudiometer was employed by Ingenhousz, and the variations which he found in the compositions of the atmosphere, were obviously owing to the errors to which it was liable. Mr Cavendish first pointed out the precautions necessary, in order to ensure accuracy when this eudiometer was employed. But before nitrous gas could be used with advantage in the analysis of air, it was necessary to ascertain the proportion of it which combined with a given bulk of oxygen gas. This was first undertaken by Mr Dalton, (Phil. Mag. xxiii. 351.) According to him,

21 parts of oxygen gas are capable of uniting either with 36 measures of nitrous gas, or with 2× 36=72 measures. Both of these compounds are soluble in water. If the tube in which the mixture is made be wide, and if agitation be employed, the two gases come at once in contact, so that the oxygen combines with a maximum of nitrous gas. If the tube be narrow, and if no agitation be employed, the oxygen gas combines with a minimum of nitrous gas. In tubes of intermediate bore, the proportion of nitrous gas which combines with the oxygen, is intermediate between 36 and 72. Hence his rule is to employ a tube of so small a bore, that water can just be poured easily out of it; to put up into this tube the quantity of air to be examined, and then to let up a quantity of nitrous gas equal to about half the bulk of the air; to allow this mixture to remain two or three minutes without any agitation, and then to observe the diminution of bulk. This diminution is to be multiplied by }, or 0.368. The product is the bulk of the oxygen gas contained in the air examined. Suppose we employ 100 measures of air, and let up 50 measures of nitrous gas, and that the diminution of bulk amounts to 57; then 57×0.368= 20.976, or very nearly 21. This indicates, that the 100 measures of air contain 21 measures of oxygen gas. We have tried the method of Dalton very frequently, and have found that when the tube is sufficiently narrow, and the experiment carefully made, the mean error cannot be rated higher than 1 per cent. When the gas examined contains much more oxygen than common air, and above all, when it is almost pure oxygen, the error is greater ; so great, indeed, that the method cannot be depended on.

But though Dalton's method is correct, as far as the analysis of atmospherical air is concerned, there can be little doubt that the proportions which he has assigned as the limits in which oxygen gas and nitrous gas combine are incorrect. We have made many trials to ascertain these limits, but never could obtain the proportions given by Mr Dalton. Mr Gay Lussac has lately turned his attention to this subject, and has given a very simple and satisfactory account of the proportions in which the two gases combine, (Memoires D'Arcueil, tom. ii. 233.) In a paper which he published on the combination of gaseous bodies, he shewed, that in all cases they unite either in equal bulks, or one part in bulk of one with two or three parts of another, and in no intermediate proportions. This opinion was obviously founded on a very ingenious hypothesis of Mr Dalton, relative to the way in which substances combine. This led him to examine the combination of oxygen gas and nitrous gas. The result was, that 100 parts of oxygen gas unite either with 200 or with 300 parts of nitrous gas. The first compound constitutes nitric acid, the second nitrous acid. His method of analysing air, founded upon this discovery, is to let up 100 measures of air into a wide vessel, and then to add 100 measures of nitrous gas. In about a minute the absorption is completed. No agitation is to be employed. The fourth part of the diminution gives the oxygen. Suppose the diminution to amount to 84, the fourth of that number, or 21, represents the oxygen in 100 parts of air. On repeating this expe