It became evident, chiefly owing to the work of two German chemists, Wenzel and Richter, that when an acid, such as vitriol or vinegar, is mixed with a base, such as potash, and neutralised, as the expression runs—that is, rendered incapable of changing the colour of certain vegetable extracts and deprived of its sharp taste—the same weight of base was always required to neutralise the same weight of acid. And other examples of apparently constant proportions between the constituents of substances had also been observed. But the processes of analysis were very imperfect, and the results by no means always concordant; and there was some ground for the statement made by Count Berthollet, a contemporary of Lavoisier, in his Researches on the Laws of Affinitypublished in 1803, that the composition of chemical compounds was variable, and not constant; that, in fact, it depended on circumstances, such as the proportions of the substances present, on the temperature, on whether the substance produced was an insoluble solid, and so on. Berthollet's statement was disputed by his countryman Proust, who, by fairly accurate analyses, carried out during eight years of controversy, proved the truth of the doctrine of constant proportions. But in the course of his work, he found that in certain cases two elements form more than one compound with each other ; for example, tin combines with oxygen in two proportions, each of them fixed and constant ; and iron forms similarly two compounds with sulphur. Perhaps the most exact experiments which had at that time been made were those due to the Hon. Henry Cavendish, who having discovered that water was composed of oxygen in union with another gas, to which the name " hydrogen ⁹⁹ was subsequently given, determined the proportion of these constituents with very great accuracy. He found that two volumes of hydrogen invariably combine with one volume of oxygen to produce water, neither hydrogen nor oxygen being left over. Owing, however, to the method of expressing the composition of compounds, no relation was evident between the proportions of the constituents. Thus Proust expressed the results of his determh.ation of the composition of the two oxides of tin and of copper in parts per hundred :—

Had he Calculated by simple proportion how much oxygen is combined with the same weight of copper and tin in each case, he would have found that the ratio of the oxygen in the suboxide of copper to that in the protoxide is as 13.8 to 21.5 ; and in the two oxides of tin as 13 to 24. The correct figures are :—

The ratio should be as 1 to 2 in each case; and the fact that Proust did not remark this is to be ascribed partly to his method of stating his results, and partly to the inaccuracy of his analyses.

Attention was first drawn to the existence of simple proportionality between the amounts of one element forming more than one compound with another by John< Dalton, a Manchester schoolmaster, in 1802 and the next succeeding years. In the year named, he described experiments " On the proportion of the several gases in the atmosphere ; " and he then stated: "The elements of oxygen may combine with a certain portion of nitrous gas, or with twice that portion, but with no intermediate quantity." And he later illustrated the same fact by considering the composition of two compounds of carbon with hydrogen, marsh gas, and defiant gas, the former of which contains twice as much hydrogen as the latter, proportionally to the same weight of carbon.

The laws relating to the proportions in which various elements combine are therefore usually called Dalton's Laws ; they are : —