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._ 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._