WATER 495 from the surface, and in a confined space ceases after the surrounding air has become saturated with the watery vapor. The process is there- fore accelerated by a free circulation, which removes the saturated atmosphere. "Water boiling at the ordinary pressure is converted into more than 1,600 times its volume of vapor, which at the temperature of ebullition (212 F.) has a specific gravity of 0'622 as compared with air at that temperature, or of 0'455 as compared with air at 32. The conversion of water into vapor is attended with absorption of heat. One pound of water at 212, in becom- ing vapor of the same temperature, consumes as much heat as would raise 5'37 Ibs. of water from 32 to 212 ; hence one pound of steam, at 212, will raise 5-37 Ibs. of water to 212, being itself condensed, so that the result is 6'37 Ibs. of water of 212. Aqueous vapor is color- less and transparent, and only becomes visible in the air when partially condensed, as in the case of escaping steam. "Watery vapor is pre- cipitated from air upon cold surfaces in the form of dew, but occasionally also as hoar frost, thus changing from gas to solid without passing through the intermediate condition of liquidity. At the heat of melting platinum it is separated into its constituent gases. "Water is widely distributed in nature : in the form of ice and snow in the polar regions, in the condi- tion of aqueous vapor, which forms a constant ingredient in the atmosphere, and in the liquid form not only in oceans, lakes, and rivers, but permeating the soil and most of the known rocks. It is the predominant element in the sap and juices of plants, and in the blood and flesh fluids of animals, and constitutes about five sixths of the weight of the human body. Water, or at least the elements of which it con- sists, hydrogen and oxygen, moreover exist in a great many bodies in such a state of combi- nation that these are generally described as com- pounds of water, and are known as hydrates. Thus gypsum when exposed to heat gives off 20 per cent, of water, which it will again absorb if brought in contact with it at ordinary temperatures. Serpentine rock contains 12 per cent., brown iron ore 15 per cent., and alum 45 per cent. From all of these bodies it is given off by heat. Some of these hydrates re- tain the water much more strongly than others, and it is a question with chemists whether the water is to be regarded as existing ready formed in these and similar compounds. Besides, there are numerous bodies, such as starch, su- gar, and woody fibre, which are often spoken of as hydrates of carbon, and may be repre- sented as compounds of carbon with water. But they are rather to be looked upon as triple compounds of carbon, hydrogen, and oxygen, in which water as such does not exist ; and in philosophical exactness, the same view should be extended to the mineral hydrates just men- tioned. Water is remarkable for its solvent power, by which we understand its capacity to unite with or take up into itself various solid, 828 VOL. xvi. 32 liquid, and gaseous matters, forming with them homogeneous liquid compounds called solu- tions. Familiar examples of this are seen in its action on salt and sugar. Different bodies have very different degrees of solubility, and many are soluble in water to so slight an ex- tent that they are generally classed as insol uble. Thus, while one part of common salt requires about three parts by weight of water to dissolve it, one part of gypsum requires about 400 parts, and one part of carbonate of lime under ordinary conditions 10,000 parts of water; while sulphate of baryta is very much less soluble, and for all practical purposes may be regarded as insoluble in pure water, though somewhat soluble in saline solutions. Cer- tain bodies, such as the metals, resins, carbon, sulphur, and oils, are regarded as wholly insol- uble; but all of these bodies form chemical compounds which are soluble in water. With regard to a great many substances it is known that they occur in two conditions, the one sol- uble and the other insoluble. Thus silica, which in the form of flint or quartz appears wholly insoluble in water, not only forms a soluble compound with the alkalies, the so- called soluble glass, but when separated from this compound is itself soluble in water to the extent of 14 per cent. In like manner the sesquioxides of iron, chrome, and aluminum, though completely insoluble in water in their ordinary artificial forms, and constituting more- over some of the most insoluble minerals in nature, may be by chemical means obtained in eminently soluble forms. In a great many other cases it can be shown that bodies when gen- erated in the presence of water by chemical reactions are soluble for a time, though when they have once passed into insoluble forms it is not easy to restore the condition of solu- bility. Solution is a process of condensation, in which the volume of the body dissolving is more or less completely lost in that of the sol- vent. Hence pressure, which favors condensa- tion, augments the solvent power of water; experiments have shown that the solubility of certain salts in water is notably increased under a pressure of many atmospheres. Heat exercises an important influence on the solvent power of water ; thus, while gypsum and lime are much more soluble in cold than in hot water, and sea salt has about the same solu- bility in both, the greater number of salts are much more soluble in hot than in cold wa- ter. Some bodies nearly insoluble in cold wa- ter possess a considerable degree of solubility at 212, while others apparently insoluble at this point enter into solution in water when heated under pressure to temperatures con- siderably higher. The presence of carbonic acid, which is found in most natural waters, greatly augments the solvent power of this liquid for many substances. Asa result of this wide range of solubilites, it follows that pure water, except as an artificial product, is un- known, and that all natural waters have their
