Page:Supplement to the fourth, fifth, and sixth editions of the Encyclopaedia Britannica - with preliminary dissertations on the history of the sciences - illustrated by engravings (IA gri 33125011196181).pdf/889

 o’clock in the evening, for the measure of the day and of the night; the corresponding results are .092, .029,—.078, .012,—.050, .010,—.032, .013, .020. On the 21st of July 1815, the quantity of evaporation, during twenty-four hours, was 208; and, on the 27th of the same month, it was 200, the thermometer having sunk as low during the night as 46° and 49° of Fahrenheit’s scale.—If we reckon the mean daily evaporation through the year at .040, this would give 14 inches, for the whole amount, or about half the annual quantity of rain that falls in this climate.

The atmometer, in its most compendious form, is admirably fitted for delicate experiments on the evaporation which takes place in close vessels, when absorbent substances are introduced. Let the ball of the instrument, for example, be immersed in air variously rarefied or condensed, under a receiver covering a surface of sulphuric acid, which has different degrees of strength; and, things being thus disposed, on extracting the common air, and introducing hydrogen gas, the rate of evaporation will then be nearly tripled. But we purpose to take some future opportunity of stating the results of such curious and interesting researches.

We shall close this short notice, with mentioning a striking fact, which shows the necessity of extreme caution in all physical inquiries. Let the ball of an atmometer be cemented to a narrow glass tube of three or four feet in length, and the whole capacity filled with fresh distilled water. Now invert the instrument upright in a basin of quicksilver, and secure it in that situation; the quicksilver, following the column of water, will rise at first quickly, and then by degrees more slowly till it reaches, perhaps, an elevation of 28 inches, where it will remain stationary and afterwards sink down, when the evaporation is nearly spent. Ice-water is raised in this way about 26 inches only, and common water scarcely 24 inches; the air separated from such liquids forming near the top of the ball, a thin medium, which, by its elasticity, counteracts in part the pressure of the external atmosphere supporting the mercurial column. But a similar experiment, where the shoot of a vine was cemented to a tube holding quicksilver, has been thought, by Dr Hales and M. Du Hamel, quite conclusive, in proving the power of the living principle of vegetation. It is obvious, that the force of evaporation alone was sufficient to explain the facts advanced by those ingenious philosophers. See Leslie’s Short Account of Experiments and Instruments depending on the Relations of Air to Heat and Moisture.

ATMOSPHERE. We have to regret, that the experiments to which we alluded, in referring from to this article, are not yet completed; and must again postpone the consideration of the various points connected with this subject, till other opportunities shall occur, particularly under the articles and. There are already articles of some extent, both on and, in the Encyclopædia, to which we beg, in the meantime, to refer our readers. 

, a species of philosophy lately introduced into Chemistry, which deserves to be fully explained in this place.

It is well known to have been the doctrine of some of the most eminent of the Greek philosophers, that the ultimate elements of matter consisted of atoms or particles incapable of farther division or diminution. This doctrine was adopted by Sir Isaac Newton, and, indeed, has been almost universally embraced by modern philosophers. But it was in chemistry alone that it could be applied with any advantage. The object of chemists was to determine the component parts of bodies, and to ascertain the different elements out of which all substances are compounded. Considerable progress was gradually made in this difficult investigation. Thus it was soon ascertained that the salts, which constitute a very numerous class of bodies, contain always at least two constituents, namely, an acid and a base. Thus, saltpetre is a compound of the acid called nitric acid, and the base called potash. Now, how small a portion soever of saltpetre you examine, whether a grain or the millionth part of a grain, you will always find it to contain both the ingredients of which saltpetre is composed. The same thing holds with respect to all the other salts, and, indeed, with respect to all compound bodies whatever. Now, this could not be the case, unless these compounds were formed by the union of the minutest possible particles of the constituents with each other; that is to say, unless it were the ultimate atoms of the elements which united together and constituted the compound. Accordingly, it has been admitted as an axiom in chemistry, that chemical union consists in the combination of the atoms of bodies with each other.

Chemistry originated from the absurd pursuits of the alchymists; and many years elapsed before it was able to shake off its connection with the chimerical notions respecting the philosopher’s stone and the universal medicine. Dr Cullen was perhaps the first man that viewed the science as constituting a great and important branch of natural philosophy. His views were followed out by Black, Cavendish, Priestley, and a cloud of other eminent men, who have added so much lustre to the scientific pre-eminence of Great Britain. Margraaf, Bergman, and Scheele, were the first scientific chemists who appeared on the Continent. Bergman, in particular, was of the most essential service to the science. Educated in those branches of mechanical philosophy which had already made such progress, and accustomed to the rigid accuracy of mathematical reasoning, he introduced the same correct views, the same precise reasoning, the same generalization to which he had been already  1em