PRACTICAL CHEMISTRY.

CHAPTER I.

INTRODUCTORY.

§ I.-CHEMICAL REACTIONS.

(1.) CHEMISTS are acquainted with about sixty different kinds of matter which have hitherto proved undecomposible, and are consequently termed simple bodies, or elements. These elements unite with one another in certain definite proportions to form an infinite variety of compounds; each particular chemical compound being always constituted of the same elements, combined together in the same proportion. Common salt, for instance, no matter how obtained, or when examined, is always found to consist of sodium and chlorine, united in the ratio of 23 parts by weight of the former element to 35.5 parts by weight of the latter.

The relative quantity of hydrogen which can enter into chemical combination being less than that of any other element, its combining proportion is taken as the standard of comparison or unity. It is found that I part by weight of hydrogen unites with 355 parts by weight of chlorine to form hydrochloric acid; and further that in very many chemical compounds I part of hydrogen may be displaced by 35 ̊5 parts of chlorine to produce chlorine-derivatives of the respective compounds. Again, when hydrochloric acid is treated with metallic sodium, every 23 parts of sodium is found to expel I part of hydrogen and form common salt, by uniting with the 35'5 parts of chlorine previously combined with the I part of hydrogen.

The proportion of an element which unites with 1 part by, weight of hydrogen, or which displaces I part by weight of hydrogen, to unite with 35'5 parts of chlorine, is called its equivalent. Thus 80 is the equivalent of bromine, 23 the equivalent

B

of sodium, and 39 the equivalent of potassium, because 80 parts of bromine, 23 parts of sodium, and 39 parts of potassium are respectively exchangeable for, or equivalent in combination to, I part of hydrogen.

(2.) But it is found in many cases that some multiple of the proportion of an element which unites with or displaces I part by weight of hydrogen, constitutes the smallest proportion of that element which actually enters into chemical combination. Thus although 25 parts of arsenic unite with 1 part of hydrogen to form arsenetted hydrogen, and with 35'5 parts of chlorine to form chloride of arsenic, yet it is allowed on all hands that the molecule* of arsenetted hydrogen contains three separable equivalents of hydrogen united with 75 parts of arsenic; and that the molecule of chloride of arsenic contains three separable equivalents of chlorine united with 75 parts of arsenic; and, in fact, that 75 parts of arsenic constitute the least indivisible proportion of arsenic which ever enters into a combination. The least indivisible proportion of an element which is found to enter into chemical combination is termed its atom, and the number expressing that proportion is called its atomic weight. Hence the atomic weight of an element sometimes coincides with its equivalent weight, as in the case of sodium, and is sometimes a multiple of its equivalent weight, as in the case of arsenic.

*The determination of the molecule of a compound body is based upon very many considerations, physical as well as chemical, which cannot be fully entered into here. The most important of these considerations relate to specific heat, atomic volume, direct combination, analogy, mode of derivation, and, above all, to metamorphoses by substitution. Thus the molecules of marsh gas, ammonia, water, and hydrochloric acid, might each be represented with one equivalent of hydrogen. But the molecule of marsh gas is represented with four equivalents of hydrogen, because in it or or or of the hydrogen can be displaced by substitution at four successive stages. The molecule of ammonia is represented with three equivalents of hydrogen, because in it or or of the hydrogen may be displaced at three successive stages. The molecule of water is represented with two equivalents of hydrogen, because in it or of the hydrogen can be displaced at two successive stages; while the molecule of hydrochloric acid is represented with one equivalent of hydrogen, because in it the hydrogen must be displaced at once or not at all.

The determination of equivalents is a purely experimental question, which, in the majority of instances, has been answered with almost absolute exactitude; but the determination of atomic weights is a question of judgment, to which in many cases very different answers were until lately accorded. Chemists are now agreed, however, as to the atomic weights of all the most important elements, and their agreement extends equally to atomic weights which are multiples of, as to those which are identical with, the equivalents of the respective elements. It is only with regard to a few of the less known elements that any great difference of opinion now exists as to the correlations of their respective equivalents and atomic weights.

(3.) The following tables exhibit lists of the most important elements, with their accepted atomic weights, their symbols or abbreviated names, and the names and symbols of their principal compounds with hydrogen or chlorine. All the hydrides are volatile, and when in the gaseous state occupy the same volume.