ESSAY XIII. ON WATER. IN the beginning of the last century it was supposed, that there were four elementary bodies in Nature, and that this terrestrial world was entirely composed of those elements. Of these first principles, WATER was one; and consequently this was always considered to be a simple substance that entered into the composition of most other bodies, but was itself incapable of decomposition. Boerhaave says, that it was in consequence of Moses having delivered a tradition that the Spirit of God, brooding upon the face of the waters, had communicated to them a prolific virtue, that the ancient Persians looked upon water as the principle of all bodies 1. Many other ancient nations considered water to be the first principle of all created things. Milton likewise favours the idea: 1 See Genesis i. ver. 2. "On the watery calm His brooding wings the spirit of God outspread, The same doctrine is also taught in the Koran. "Do not the unbelievers know that the heavens and the earth were solid, and I clave the same in sunder, and made every living thing of water 3?" Of late years, however, it has been discovered that this notion of the simplicity of water is erroneous, and that the four substances which the ancients had fixed upon as the simple elements of Nature are themselves all compound bodies. Water is composed of oxygen and hydrogen in the proportions of 85 parts by weight of the former to 15 of the latter; so that 85 ounces of oxygen gas when united with 15 ounces of hydrogen gas will form 100 ounces of water. These are what are deemed the usual proportions: but they cannot be considered as absolutely and undeniably correct, because the quantity of aqueous vapour which the gases usually contain renders it difficult, if not altogether impossible, to produce an accurate estimate 4. Some experiments by Ritter seem to prove that frozen water contains a less proportion of oxygen. These were communicated by Professor Jameson to Mr. J. Murray, who has given an interesting account of them in the 2nd volume of his 2 Paradise Lost, book vii. line 234. See Sale's Koran, vol. ii. page 155. * See a Memoir by Humboldt and Gay-Lussac on this subject in vol. liii. of the Annales de Chimie, pp. 239-259. System of Chemistry. The most beautiful, and apparently the most decisive of these experiments, is that of diffusing in various portions of water a quantity of newly prepared white prussiate of iron, (a substance which, by the blue colour it assumes when it receives oxygen, is a very delicate test of that principle,) and excluding the action of atmospheric air by a thin layer of oil on the surface of the liquid. These vessels were exposed to various degrees of cold, and as soon as the water in either of them froze, the white precipitate became blue. Since the compound nature of water has been understood, many ways have been discovered of decomposing and re-forming it from its original elements 5, so that there is now no longer the least doubt either of its composition, or of the proportion of each of the elements of which it consists. Water may be decomposed by the agency of some combustible substances; also by means of common and Voltaic electricity. Vegetables of all kinds, while alive and fresh, are likewise furnished with organs for the decomposition of water, by which they appropriate its hydrogen and part of the oxygen to the formation of oil, sugar, starch, and numberless others of the combinations to which we apply the term vegetable principles. Fish, especially those of the cetaceous tribe, constantly decompose water and live upon its hydrogen. A description of Mr. Cuthbertson's apparatus for producing water by the combustion of hydrogen in oxygen gas, will be found in the Philosophical Magazine, vol. ii. page 317. |