nexture. If the stop-cock be again opened a fresh portion of the mixed gases enters, and may be inflamed as before. Le p ic spa water by iron 237. Water may be decomposed or resolved into its elements by a Analysis of variety of processes, the most important of which are the following : i. a is a glass retort, into which is introduced a given weight of water; bb a small furnace through which passes the earthen tube cc, which terminates in the spiral pewter tube dd, immersed in water. A given weight of pure iron wire, coiled up, is introduced into the tube c, and the whole made red hot; the water in a is then made to boil, and the vapour, on coming into contact with the red-hot iron, is in part decomposed; the oxygen is retained by the iron, and the hydrogen escaping through the tube f, may be collected as usual. Any decomposed portion of water is condensed in the worm-pipe d, and drops into the vessel e. After this experiment the iron will be found to have increased in weight; and if attention be paid to the quantity of water which has collected in e, and to the weight of the hydrogen gas evolved, it will be found that the weight gained by the iron, added to that of the hydrogen, will be equal to the weight of the water which has disappeared. Lavoisier concluded, from an experiment thus conducted, that water consisted of 85 per cent. by weight of oxygen, and 15 by weight of hydrogen; that is, that for every 15 grains of hydrogen evolved, 85 grains of oxygen were condensed by the iron. ii. Decomposition by Voltaic electricity (137) best illustrates the By Voltaic composition of water, since it exhibits both the oxygen and hydrogen electricity. in the gaseous form. The woodcut overleaf represents a section of an apparatus for this purpose. It is a glass vessel containing water, having two wires of platinum, a a, passing through its bottom: over these are inverted the tubes bb, also filled with water. The wires are rendered positive and negative by connexion with a moderatelypowerful Voltaic apparatus. Oxygen is evolved at the positive wire, and hydrogen at the negative wire, which gases rise into the tubes, and it is seen that one volume of oxygen, o, and two volumes of hydrogen, h, are the constant results. If these gases be mixed and detonated, pure water is again formed. M 238. Water, in its ordinary state, such as spring and river water, is always so far contaminated with foreign substances as to be unfit for many chemical purposes, and frequently, as will be more fully shown hereafter, even for domestic use. Rain-water is much more pure, but it always contains a portion of carbonic acid and of the elements of at mospheric air, besides appreciable traces of vegetable or animal matHeterogene- ter; to the latter it owes its property of becoming putrid when kept. ous contents. The distinction of water into hard and soft has reference to its less or greater purity. The impurities of water are separated by distillation, which process is usually conducted upon the large scale in a copper boiler, a, placed either in a portable furnace, or set in brickwork, according to its dimensions, to which is annexed a head, b, of the same material, or of pewter connected with the pewter worm c, which is immersed in the worm-tub, or refrigerator, d, its lower end passing out at e. The water in this vessel must always be retained of a low temperature to effect the condensation of the vapour in the spiral tube. Distilled water, as commonly prepared, always affords minute traces of foreign matter, especially when subjected to Voltaic decomposition, and can only be considered as perfectly pure when re-distilled at a low temperature in silver vessels. 239. Pure water is transparent, and without either colour, taste, or Properties smell. In consequence of the facility of obtaining it pure, it is assum- Standard of ed as a standard to which the relative weight of all other bodies may specific gravi be compared, its specific gravity being called = 1.000, and hence the importance of estimating its weight with precision. At the temperature of 40° it is at its maximum of density, and at that temperature an English cubic foot weighs 437102.4946 grains (THOMSON'S System, Vol. ii. p. 12.) or 999.0914161 ounces avoirdupois, and a cubic inch 252.953 grains. 240. At the temperature of 32o water congeals into ice, which, if slowly formed, produces needles crossing each other at angles of 60° and 120. The specific gravity of ice is 0.94. Exposed to the air, ice loses considerably in weight by evaporation. 241. If water be exposed to heat in open vessels it boils, or is converted into steam, at 212o, the barometer being at 30 inches; but the boiling point of water varies considerably with the pressure (83.) Boiling point. The specific gravity of air being considered as = 1; that of steam is 0.6235. At mean pressure, and at the temperature of 212°, the bulk of steam is 1700 times greater than that of water. 242. Water is susceptible of compression, as was originally shown by Canton, and more lately by Mr. Perkins, who has estimated, in an Ingenious series of experiments, the rate of its compression (Phil. Trans. 1820.) If submitted to very sudden compression, water becomes luminous, as has been shown by M. Desaignes,-THENARD, Traité de Chimie, i. 432. 243. Water enters into combination with a variety of substances, and is retained with various degrees of force: where it contributes to the regular form and transparency of crystallized bodies, it is termed water of crystallization (12.) In other cases the compounds which water forms with substances are called hydrates, as with many of the metallic oxides (520); in both cases it may be considered as one of the constituents of the bodies, for it exists in them in a definite proportion. 244. Water, which has been exposed to the atmosphere, always contains a portion of air, as may be proved by boiling it, or by exposing it under the exhausted receiver of the air-pump. To separate the air, the water must be boiled for about two hours. It absorbs oxygen gas in preference to atmospheric air or nitrogen, and when the air is expelled by boiling, the last portions contain more oxygen than those first given off.-HUMBOLDT and GAY-LUSSAC, Journal de Physi que, 1805. Compressible. Unites to an 245. M. Thenard has shown that water may be united to a considerable excess of oxygen by means of peroxide of barium. (Quarterly excess of oxy Journal, Vol. viii. p. 114.) The specific gravity of oxygenated water gen. is about 1.45. It acts as a caustic upon the skin; detonates violently when dropped upon dry oxide of silver, or upon most of the metals finely divided. This singular compound may be termed peroxide of hydrogen: its properties, and the process for obtaining it, which is complex and circuitous, have been fully detailed by its discoverer, and will be adverted to under the article Peroxide of Barium, Vol. ii., Section 5. Par. 630. 246. The following table exhibits the quantity of different gases which water is capable of absorbing at mean temperature and pressure, the water being previously deprived of all aëriform matter by long boiling. 247. As hydrogen is the lightest known substance, it is assumed in this work as unity, in reference to the representative numbers of other bodies. The principle of numeric representation, or of equivalent or proportional numbers, has already been adverted to (50), and the following will be the representative numbers of the bodies described in the foregoing chapter, the number for oxygen being deduced from the composition of water (235), and of chlorine and iodine from the muriatic (248) and hydriodic acids (259). |