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 descend 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 consequently 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 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: for it is gradually given off by evaporation, and can be extracted at once by strong pressure between blotting-paper. When deprived of its water, animal matter becomes wholly insusceptible of vitality; except in the case of some of the lower animals, which, as well as some plants, revive when again moistened. According to Chevreul, pure water alone can reduce organized substances to this state of softness; although salt water, alcohol, ether, and oil, are also imbibed by dry animal textures. Moist animal tissues, by virtue of their porosity, allow soluble matters, which come into contact with them, to be dissolved by the water which they contain, and which oils their pores: if the matters are already in solution, they are imparted by their solutions to the water of the tissues. Gaseous substances are taken up in the same way. Water exists in nearly as large a proportion in vegetable as in animal substances.