The meaning of the word " equivalent" has already been explained on p. 15, and we shall now consider how the equivalent of an element may be determined. As already stated, some compound of the element is analysed, preferably one with hydrogen, oxygen, or chlorine, and the weight of the element which is in combination with, or which replaces 8 parts by weight of oxygen, is termed the equivalent of the element. But it is seldom that a direct method of estimating the equivalent can be practised, for it is not always possible to obtain a compound of the element with hydrogen, or to deprive its oxide of oxygen, or its chloride of chlorine. In fact, each element has to be specially studied, and a method devised which will lead to the required information. It is, above all, necessary that the compounds dealt with shall be pure—that is, that they shall not contain any other elements than those which it is desired to estimate, and that their composition shall be definite. For instance, if it were desired to find the equivalent of barium by estimating the proportion of chlorine in its chloride, it would be essential to obtain barium chloride free from the very similar elements calcium and strontium, and it would also be of the first importance to make sure that in weighing the chloride, the specimen should be free from water adhering to the powdered substance.