ments of Van Helmont and Mr. Boyle, who maintained that it could be changed into all vegetable substances, as well as into earth ; but it was substantially held until the middle of the last century, (1781,) when Mr. Cavendish proved that this liquid was a compound of oxygen and hydrogen. Natural History. In the inorganized kingdom. Water is very generally diffused over the surface of the globe, forming seas, lakes, and rivers ; it is mechanically disseminated among rocks, constitutes an essential part of some minerals, and always exists to a greater or less extent, in the atmos- phere. In the air, water is formed in two states ; as a vapor (which makes about one-seventieth by volume, or one one-hundreth by weight of the atmosphere) it is supposed to be the cause of the blue color to the sky ; and in a vesicular form, in which state it constitutes the clouds. Terrestrial water forms about three-fourths of the surface of the terraqueous globe. The average depth of the ocean is calculated at between two and three miles. Now as the height of dry land above the surface of the sea is less than two miles, it is evident, that if the present dry land were dis- tributed over the bottom of the ocean, the surface of the globe would present a mass of waters a mile in depth. On the supposition that the mean depth of the sea is not 32 126 greater than the fourth part of a mile, the solid contents of the ocean would be 32,058,939 cubic miles ( Thomson's Chemistry.) The quantity of water mechanic- ally disseminated through rocks, which serve merely as a natural reservoir for the time, must be, in the aggregate, very considerable, though it is impossible to form any very accurate estimate of it. Even in those rocks which merely supply springs, the amount of disseminated water must be enormous ; for they so far resemble fil- ters, that are necessarily charged with the fluid before they permit it to pass out. De La Beche has advanced the opinion that capillary attraction has great power, both in mechanically disseminating water among rocks, and in retaining it in them when so disseminated, and that it therefore keeps them, to a certain extent, satu- rated with moisture, and assists in promoting a more equal flow of water in springs. Capillary attraction and gravity probably carry water down far beyond those situa- tions where it can be returned in springs, at least cold springs, for there are certain circumstances connected with those which are thermal, which go to prove, that the water thrown up by them may have percolated to considerable depths. It is very evident that most rocks contain disseminated moisture, for there are few which, when exposed to heat, do not give water. Sulphate of lime, for example, or plaster of paris, contains about 20 per cent., and common serpentine, as much as 15 per cent, of it. Soap-stone has 4 per cent,, and even quartz 2 per cent, of water, in their composition. This fluid exists in minerals either as water of crystallization, or combined as a hydrate. But though water is thus generally diffused over the surface of the globe, yet it is not found perfectly pure in any place ; even the rain and the snow that de- scend from the clouds, the condensation, as it were, of a natural distillation, are slightly tainted by saline matters ; which circumstance can only arise from the great solvent power of water enabling it to take up a portion of most substances with which it comes into contact, in its natural condition. In many lakes, and in the ocean, the quantity of saline matter is so great as to render it unfit for diluent purposes ; but, when sea-water freezes, the saline impregnations are deposited ; and the ice affords fresh water. In the state in which water is generally employed as a diluent, its impregnations are in small quantity, and not usually sufficient either to dim its transparency, or to give it color, smell, or taste, and consequent- ly to render it unfit for the ordinary purposes of life. Water, therefore, which is transparent, colorless, inodorous, and tasteless, is called good and pure, and none other can be called such ; though some medical writers are of opinion, that it is not necessary it should be in this pure state for common use. Such opinion however is undoubtedly erroneous — II. In the organized kingdom. Water enters largely into the composition of 127 organic substances. It constitutes, at least, four fifths of the weight of the animal tissues, being the source of their physical properties, extensibility and flexibility • This water is not chemically combined in them :