the hypothesis of radiation, depending very much on the polish of their surfaces. Thus if those substances which supply the warmest coverings, such as furs, feathers, silk, &c. be viewed through a microscope, we shall find the surfaces of their fibres or minute laminæ not only smooth, but also very highly polished: and those substances will be warmest which excel in these respects, the fine white shining fur of a Russian hare being much warmer than coarse hair; and fine silk, as spun by the silk-worm, being preferable for warmth to the same silk twisted together into coarse threads.

A considerable part of the paper is now bestowed on the theory of heat, which the author attempts to deduce from the foregoing facts and observations. Heat and cold, he says, like fast and slow, are mere relative terms; and as there is no relation between motion and rest, so there can be none between any degree of heat and absolute cold, or a total privation of heat. It has long been thought, and it appears more and more probable, that motion is an essential quality inherent in all matter: this is illustrated by many examples; and by applying the analogy above given, and the observations since brought forward, there seems every reason to believe that, without having recourse to any specific element, all the phenomena of heat may be accounted for by the simple operations of motion; or that motion, in fact, constitutes the heat or temperature of sensible bodies.

It will no doubt occur that this theory will hardly account for the effects of frigorific rays; but this objection is answered by the observation, that as the rapid undulations occasioned in the surrounding ethereal fluid by the swift vibrations of a heated body will act as calorific rays on the neighbouring colder solid bodies; so the slower undulations occasioned by the vibrations of a cold body, will act as frigorific rays on neighbouring bodies of a higher temperature; and that these reciprocal actions will continue, but with decreasing intensity, till the two bodies have acquired the same degree of temperature, or until their vibrations have become isochronous.

According to this hypothesis, cold can with no more propriety be considered as the absence of heat, than a low or grave sound can be considered as the absence of a higher or more acute note; and the admission of rays which generate cold involves no absurdity, and creates no confusion of ideas.

As this theory, however, entirely supersedes the hypothesis of the calorific element, of late so much resorted to, it may be imagined that the author would not discuss the controversy in a slight or superficial manner; and accordingly many pages are here dedicated to this intricate and abstruse disquisition.

Among other important points, it was necessary to reconcile solidity, hardness, and elasticity, with the incessant motion he ascribes to the constituent particles of matter, and to obviate the objection founded on a supposition that there is not room sufficient for this motion. What increases the perplexity is, that, admitting the changes of temperature in bodies to be the effect of the calorific and frigorific radiations above described, a particular nicety will be required to

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distinguish between the effects of those simultaneous operations, and of ascertaining their relative intensities. A hot body, A for instance, heats a neighbouring colder body B, by its calorific radiations; but B emits at the same time frigorific radiations, which contribute to lower the temperature of A; nor is it clear that both these bodies, especially if they have polished surfaces, will not reciprocally, and perhaps repeatedly, reflect those incident rays, and that those rays will not be refracted by the media through which they pass, and be concentrated or expanded by the shapes of the reflecting surfaces, and thus create a combination of effects, which it will require much labour and ingenuity to unravel.

As it is impossible for us within our narrow limits to do justice to the connected series of observations and arguments here adduced, we shall refer those who wish for more ample information on the subject to the paper itself; and this the rather, that we may dwell more largely upon the practical uses that may be derived from a knowledge of the facts which the author now considers as fully established.

In all cases where it is intended to preserve the heat of any substance which is confined in a metallic vessel, it will greatly contribute to that end if the external surface of that vessel be kept very clean and bright; but if the object be to cool anything quickly in such a vessel, its external surface should be painted, or coloured with some of those substances which have been found to emit calorific radiations in abundance. Hence the sides of kitchen utensils should be kept bright, in order to confine the heat; while their bottoms should be blackened, in order that their contents may be made to boil sooner, and with a less expense of fuel.

Brewers, it seems, are mistaken when they employ broad shallow vessels, or flats, as they call them, of metal for cooling their wort. Wooden flats, it appears, ought in every respect to have the preference.

In all cases when metallic tubes, filled with steam, are used for warming rooms or hot-houses, the external surface of those tubes should be painted, or covered with some substance which facilitates the emission of calorific rays. Where, on the other hand, tubes are intended to convey hot steam from one place to another, they should be kept very clean and bright. This applies also to the cylinders of steam-engines, and the principal tubes used in that machine.

Gardeners should advert to the circumstance, that if walls painted black acquire heat faster when exposed to the sun's direct rays, they likewise cool much faster during the night, and in the shade when the weather is cold.

Black cloths are known to be very warm in the sun; but they are far from being so in the shade, especially in cold weather, when the temperature of the air is below that of the surface of the skin.

It having been shown that the warmth of clothing depends much on the polish of the surface of the substance of which it is made, we may conclude that in choosing the colour of our winter garments, those dyes should be avoided which tend most to destroy that polish.

Hence there is reason to think that, contrary to the general opinion, white garments are warmer than any other in cold weather; and indeed if they are well calculated to reflect calorific rays in summer, they ought to be equally well calculated to reflect those frigorific rays by which we are annoyed in winter. Fur garments have been found by experience to be much warmer in cold weather, when worn with the hair outwards, than when it is turned inwards.

This is alleged as a proof that we are kept warm by our clothing not so much by confining the heat of our bodies, as by repelling those frigorific rays which tend to cool us. The fur of several delicate animals we know becomes white in winter in cold countries; and bears which inhabit the polar regions are likewise known to be white in all seasons. Now if, in fact, as there is great reason to believe, white is the colour most favourable to the reflection of calorific and frigorific rays, it must be acknowledged that these animals have been greatly favoured in having a clothing assigned them so well adapted to their local circumstances.

The excessive cold which is known to prevail, in all seasons, on the tops of high mountains, and the frosts at night which frequently take place on the surface of the plains below, seem to indicate that frigorific rays arrive continually at the surface of the earth from every part of the heavens; and it is no doubt by the action of these rays that our planet is continually cooled, and enabled to preserve the same mean temperature for ages, notwithstanding the immense quantities of heat that are generated at its surface by the continual action of the solar rays. The action of these frigorific nocturnal rays will likewise justify the inhabitants of hot climates, who, in order to be more cool during their hours of rest, remove their beds in summer to the tops of their houses.

Experiments and Observations on the Motion of the Sap in Trees. In a Letter from Thomas Andrew Knight, Esq. to the Right Hon. Sir Joseph Banks, Bart. K.B. P.R.S. Read February 16, 1804. [Phil. Trans. 1804, p. 183.]

Some experiments are here described, the tendency of which is to prove, what the author had advanced as a conjecture in a former communication, that the vessels of the bark which pass from the leaves to the roots, are in their organization better calculated to carry the fluids they contain towards the roots than in the opposite direction.

In the first of these experiments several strong horizontal shoots of vines were depressed about their middle; and at that part, buried in the mould, contained in pots about ten inches in diameter: after some months of vegetation, when the shoots had nearly filled the pots with roots, they were separated from the parent stock, having at each side above the earth a certain length of the layer, with at least one bud upon each. The end towards the stock was called the inverted, and the other the proper end of the layer. If the author's

above-mentioned conjecture of the retrograde motion of the sap be founded, it would follow that in the subsequent vegetation the inverted would display a more vigorous growth than the proper end; and this accordingly was soon found to be the case, with this additional circumstance, that the parts beyond the buds on the inverted ends were observed to increase considerably, while the same parts on the proper ends not only withered, but even gradually died away.

In another experiment a number of cuttings of gooseberry and currant trees were planted, some in their natural erect, and others in an inverted position. Many of these, especially the gooseberry cuttings, failed altogether; but in those that survived, the same accumulation of wood was observed on the upper ends of the inverted cuttings as on the vine shoots: similar effects were likewise observed in inverted grafts of the apple-tree, and in some respects also in cuttings from the sallow-tree, where, however, they being of some length, the accumulation of wood did not take place at the summit, but about the base of the cuttings.

It will be needless to dwell minutely on these results, since they may all be deduced from the author's theory, which, in addition to what has been above stated, is, nearly in his own words,-that the vessels of plants are not equally well calculated to carry their contents in opposite directions; and that the vessels of the bark, like those which constitute the venous system of animals (to which they are in many respects analogous), are provided with valves, imperceptible indeed to our eye on account of their extreme minuteness, but whose effects in directing the course of the sap are sufficiently

obvious.

The paper concludes with some strictures on the experiments described by Hale and Du Hamel, and the reasons why these naturalists did not arrive at the same conclusions which are here brought forward, and an experiment which illustrates some parts of the paper the author gave last year on the descent of sap in trees.

Analytical Experiments and Observations on Lac. By Charles Hatchett, Esq. F.R.S. Read April 12, 1804. [Phil. Trans. 1804, p. 191.]

A brief historical account of the substance here treated of is prefixed to this paper. Though long in use, especially in India, yet, except what we have lately learnt from Mr. Kerr and Mr. Saunders, few inquiries have hitherto been made concerning its mode of production, first discovery, its nature and relative properties. We now know that it is the nidus or comb of the insect called Coccus, or Chermes Lacca, deposited on branches of certain species of Mimosa and other plants; and that the kingdom of Assam furnishes it in the greatest quantity. There are four sorts of it:-1. The stick lac, being the substance or comb in its natural state, incrusting small branches or twigs. 2. Seed lac, or the same substance granulated, but probably prepared in some manner, it being deprived of a great part of its colouring matter. 3. Lump lac, formed from seed lac,

liquefied by fire, and formed into cakes. And, 4. Shell lac, being the original comb, liquefied in water, strained through a cotton cloth, and spread upon a junk of a plantain-tree so as to form thin transparent laminæ this kind contains the least of the tinging substance, as may well be expected from the mode in which it is prepared.

Among the chemists who have hitherto analysed this substance, none deserve notice except M. Geoffroy; but our author's present labours render his investigations of little or no avail.

The first section of this paper treats of the effects of different menstrua on the varieties of lac, from which it appears that it is soluble in alkalies, and in some of the acids. And the second section contains an account of the analytical experiments made on stick, seed, and shell lac. From the ample series of facts herein contained, of which it is in vain to attempt a compendious abstract, we collect in general that the varieties of lac consist of four ingredients, namely, extractive colouring matter, resin, gluten, and a peculiar kind of wax; and that the resin is the predominant ingredient, insomuch that, strictly speaking, we ought to consider lac as consisting principally of resin mixed with certain proportions of a particular kind of wax, gluten, and colouring extract. The mean results of the experiments give the proportions as follows:-100 parts of stick lac are found to contain resin 68, colouring extract 10, wax 6, gluten 5, and extraneous matter 64;-seed lac, resin 884, colouring extract 24, wax 41, gluten 2;—and shell lac, resin 90·90, colouring extract wax 4, and gluten 2·80. Each of these ingredients, we must observe, has been separately and carefully analysed.

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The third and last section contains a number of general obserrations, chiefly relating to the uses of this substance. From the whole of the experiments here related, it appears that although lac be indisputably the production of insects, yet it possesses few of the characters of animal substances; and that the greater part of its aggregate properties, as well as those of its component ingredients, are such as more immediately appertain to vegetable bodies. Its uses are various, and some of them important. The Indians manufacture it into rings, beads, and other female ornaments. When formed into sealing-wax, it is employed as a japan, and is likewise manufactured into different coloured varnishes. The colouring part is formed into lakes for painters; and as a dyeing material it is in very general use. The resinous part is employed to make grindstones, by melting and mixing it with about three parts of sand, or with a like proportion of powder of corundum for those stones which are used by lapidaries. We owe to Mr. Wilkins the information, that a peculiar and excellent kind of ink is prepared by the Hindoos of shell lac, dissolved in water by the mere addition of a little borax, and by adding to the solution a certain quantity of ivory- or lamp-black. This process has the further advantage of teaching us to prepare an aqueous solution of lac, which probably will be found of very extensive utility, especially in the preparation of varnishes and pigments, which, when perfectly dry, will not be easily affected by damp or water.