This water is always very soft; and is, consequently, well calculated for the dissolving of soap, in washing and other processes. It is also peculiarly adapted to the solution of alimentary or colouring matter, in the preparation of food and dyeing, and is accordingly used to great extent for these purposes. By the addition of a small quantity of a solution of barytes ([195]), it may be rendered sufficiently pure for all chemical uses.
If rain water be long kept, especially in hot climates, it acquires a disagreeable smell, and becomes putrid, and full of animalculæ.
275. ICE and SNOW WATER are equal to rain water in purity; and the air having been expelled from them during the process of freezing, they are consequently devoid of air when first melted.
Ice and snow, in their natural state, are highly important to mankind. It is a general law of nature that all bodies become more dense and heavy by exposure to cold; but the freezing of water is an exception to this law, and for a purpose of extreme benefit to mankind. By this ordination it is that ice always rises to the surface of the water, and thus preserves from the effects of the surrounding cold, a vast body of heat in the fluid beneath; and is itself ready to receive its own accustomed quantity upon the first change of the atmosphere. The expansion of water, in freezing, is owing to its assuming a crystallized form; and this expansion is often so great that glass bottles, filled with water, are burst by it.
During the intense cold of winter, snow, which is of a soft and spongy texture, is considered of great utility in preventing the immediate access of the atmospheric air to the ground; it has doubtless been designed by Providence as a garment to protect the incipient vegetation, at that inclement season, from injury.
The inhabitants of all the extreme northern parts of the world use thawed snow for their constant beverage during winter; and the vast masses of ice which float in the polar seas afford an abundant supply of fresh water to the navigators of those dreary regions. Snow water has, however, long lain under the imputation of occasioning those extraordinary swellings in the neck, which deform the inhabitants of some of the alpine valleys of Switzerland; but this opinion is not supported by any well-authenticated facts. Indeed it is rendered quite improbable by the frequency of this disease in the island of Sumatra, where ice and snow are never seen: and by its being quite unknown in Chili and Thibet, though the rivers of these countries are chiefly supplied by the melting of the snows.
276. SPRING WATER is nothing more than rain water, which having gradually filtered through the earth, collects at the bottom of declivities, and there makes its way to the surface.
It is obvious that spring water must be nearly as various in its contents as the substances through which it flows.
Ordinary springs pass insensibly into mineral springs, according as their foreign contents become more abundant; but it has not unfrequently happened that waters have acquired great medical reputation from their purity only. Although by far the greater number of springs are cold, some are hot, or at least are of a temperature which, at all times, exceeds that of summer heat: and this warmth is so little influenced by the state of the atmosphere, that it is nearly the same both in summer and winter.
The water of almost every spring is of such nature that it will not dissolve, but curdles, soap; and cannot be used for dressing several kinds of food. Water of this description is denominated hard, a property owing to the great proportion of earthy salts which it holds in solution, and which, at the same time, are not in such abundance as to impair its taste. The most common of these salts is selenite, or sulphat of lime ([192]), and chalk, or carbonat of lime ([140]); when it contains only the latter, the water is easily rendered soft by boiling, which expels the excess of carbonic acid ([26]), and thus causes the chalk to be precipitated. Hence originates the earthy crust or fur in such tea kettles as have had hard water several times boiled in them.