A chief part of the nutrition of vegetables is the water which they absorb from the earth through the pores of their roots. The great quantity so absorbed may readily be imagined, when it is stated that the driest and most compact kinds of wood, such as even heart of oak, when converted into charcoal, lose, during the process, full three-fourths of their weight; and that the fluid which escapes is nearly pure water. This fluid is found in the driest of solid bodies, whatever be their description. A piece of hartshorn kept for forty years, and thereby become as hard and dry as metal (so that if struck against a flint it would give sparks of fire), upon being distilled, was found to yield an eighth part of its weight of water.
Every being with life, in a great degree, lives by it; and whatever grows, through it receives its growth; and wherever it enters, it promotes and sustains life, preserving the whole of created nature in their proper classes of existence. And whether we consider it as productive of health to animals and vegetables, as requisite to the existence and beauty of the earth, or as one of the great powers by which the Almighty works in the support of the world, we cannot but admire and adore the wisdom by which it has been ordained.
In the various kinds of water, even of that which is commonly used in drinking, and for the preparation of food, there is great difference both of taste and appearance. This difference is chiefly occasioned by the foreign matters which they hold in solution or suspension. In some cases the quantity of these is so minute as to have but little influence on the taste; but in others they alter its properties altogether, and render the water noxious, or medicinal, or unfit for the preparation of food.
The chemical analysis of water, for the purpose of ascertaining the different substances which it holds in solution, is one of the most difficult and complicated operations that is known in this branch of science; and one that exercises, in a peculiar degree, both the skill and industry of the operator. The difficulty arises not only from the diversity of the bodies which occur, but from the very minute quantities of some of them.
These bodies are discovered by an addition to the water of certain substances, the consequence of which is some change in its appearance, and this change indicates the presence or absence of the bodies suspected.
The substances thus employed are very numerous, and have the name of tests. The methods of ascertaining the exact proportion of each of these ingredients are much too complicated to require a place in the present work.
Water cannot be obtained in any state of perfect purity except by the artificial process of distillation.
ORDER I.—COMMON WATER.
274. RAIN WATER is considered to be next in purity to distilled water, from its having undergone a natural distillation. Its foreign contents vary according to the state of the air through which it falls. Hence, for instance, when it passes through the atmosphere of a smoky town, it becomes impure: and when collected in towns, it frequently acquires a small quantity of sulphat of lime ([192]), and calcareous matter from the mortar and plaster of the houses.