A term happily introduced into chemistry by Dr. Wollaston, to express the system of definite ratios in which the corpuscular objects of this science combine, referred to a common standard of unity. The two grand laws of chemical combination are, 1st, The general reciprocity of the saturating proportions; and 2d, The definite proportions in which bodies combine; and if any substances are capable of combining in more than one proportion, such combinations are always multiples, or submultiples of one of the proportions. The first of these laws was discovered by Richter, in 1792, who inferred it from the remarkable and well established fact that two neutral salts, in mutually decomposing each other, give birth to two new saline compounds, always perfectly neutral. Thus sulphate of soda being added to muriate of lime, will produce perfectly neutral sulphate of lime and muriate of soda. From this he concluded that the quantities of two alkaline bases adequate to neutralize equal weights of any one acid, are proportional to the quantities of the same bases requisite to neutralize every other acid.
Therefore to find the quantity of soda equivalent to the saturation of this quantity of nitric acid, we need not make any experiment, but merely compute it by the proportional rule of Richter. Thus - as 6: 4.4:: 2.93, the weight of soda required to saturate 5 parts of nitric acid; and this proportion of 6 to 4, or 3 to 2, will pervade all the possible saline combinatione of these bases, so that it will require only two parts of soda to saturate as great a quantity of any acid as could be saturated by three parts of potash. The doctrine that bodies combine chemically in certain definite proportions, was first promulgated by Dr. Higgins, in his Comparative View of the Phlogistic and Antiphlogistic Theory, published in 1789. This doctrine was subsequently maintained by Mr. Dalton, in his New System of Chemical Philosophy, first framed about the year 1803, and published in 1808. In this work Mr. Dalton fully establishes the proposition that bodies do not combine in all proportions, as Berthollet maintained, but that the different compounds of the same principles proceed in successive proportions, each a multiple of the first.
This proposition has been further illustrated and confirmed by the researches and reasonings of the most eminent chemical philosophers, and is now universally admitted to be true. In the first part of the Philosophical Transactions, appeared Dr. Wollaston's description of his scale of chemical equivalents; an invention which has contributed more to facilitate the general study and practice of chemistry, than any other invention of man. This singularly useful contrivance consists of a flat ruler, about 2 inches wide and 18 inches long, on which a list of substances are arranged on one or other side of a logarithmic line of numbers, in the order of their relative weights, and at such distances from each other that the series of numbers placed on a sliding scale can at pleasure be moved, so that any number expressing the weight of a compound may be brought to correspond with the place of that compound in the adjacent column. The arrangement is then such, that the weight of any ingredient in its composition, of any reagent to be employed, or precipitate that might be obtained in its analysis, will be found opposite the point at which its respective name is placed.
For example: - If the slider be drawn upwards till 100 corresponds with muriate of soda, the scale will then show how much of each substance contained in the table is equivalent to 100 of common salt. It shows with regard to the different views of this salt, that it contains 46.6 dry muriatic acid, and 53.4 of soda, or 39.8 sodium, and 13.6 oxygen; or if viewed as chloride of sodium, that it contains 60.2 of chlorine, and 39.8 sodium. With respect to reagents, it may be seen that 283 nitrate of lead, containing 191 of litharge, employed to separate the muriatic acid, would yield a precipitate of 237 muriate of lead; and that there would then remain in solution nearly 146 nitrate of soda. These, and many more such answers, appear at once by bare inspection, as soon as the weight of any substance intended for examination is made by motion of the slider to correspond with its place in the adjacent column. Dr. Wollaston took, as his standard or unity, oxygen, from its almost universal relation to chemical matter, and determined the equivalents of other substances by their relation to this standard. Sir H.
Davy proposed as unity, hydrogen, which is the lightest of all known substances, and which, therefore, seems preferable as the basis of the scale, since the equivalents of all other substances must necessarily be expressed in all numbers without fractions; and this scale has, accordingly, been extensively adopted.