double binary or ternary compounds, and when these were made to act on each other the reaction was represented as a double decomposition. This was known as the dual theory of chemistry, and it organized and explained the facts of the science in the most beautiful manner. Electro-chemistry lent it powerful aid, as compounds were resolved into pairs by galvanic decomposition, and their elements were supposed to be in opposite electrical states and to be united by polar attractions. The atomic theory gave a basis of philosophy to the doctrine, and the admirable nomenclature which was adapted to it gave it wide currency and acceptance. Under this chemical system the science grew and flourished for more than half a century, spread out into branches, and became the guide in medicine, mining, agriculture, and numberless arts and manufactures. Yet this system, too, was true for its time; only true in relation to the facts known, and is now doomed to the fate of phlogiston. As it grew out of a preceding stage upon which it was a great improvement, so it has led to a subsequent and higher stage of knowledge, to which it must, in turn give way. Its facts live on; its partial truths survive and are expanded into new forms, and a system of doctrine has arisen so contrasted with the dual theory and so advanced beyond it that it is now characterized as the "new chemistry" in contradistinction to the old which it has superseded. We are now entering upon the new chemical epoch in which ideas that have long simmered in the brains of chemists, and were long contested, have emerged into distinctness and are passing into predominance. The simple splitting and pairing theory of chemical change has failed, and we are becoming familiar with the conception of unitary structure, molecular types, and transformations by substitution and replacement that leave the construction and character of chemical compounds unaltered. The dualist appealed to analysis, and asked only what are the constituents and what their proportions in chemical substances. The apostles of the new chemistry point to the failures of analysis, and aver that it is not so much what a compound is made of, as how its elements are arranged, that is the present concern of inquiry. And chemistry was probably never so active as now under guidance of the new theories, and never before answered so well to that highest test of science, the prevision and prediction of chemical results. There is no escape from the new chemistry. It absorbs the verities of the past and it is the highest truth arrived at by centuries of thought and labor. But it is not a finality. Its truth, though priceless, is imperfect, and is no doubt destined to still further and higher development. Historically regarded, the science of chemistry is a striking exemplification of the laws of mental evolution, as the doctrine of evolution is the grandest illustration of the relativity of truth.
An interesting illustration of the striking changes of view that have taken place in modern chemistry is furnished by the reversal of scientific rank assigned to those prime elements of Nature, oxygen and hydrogen gases. Oxygen was long enthroned both from its enormous distribution in earth, sea, and air, and its active participation in the great changes of matter, combustion, respiration, decay, all of which were generalized as different forms and grades of oxidation. It was supposed to be the acidifying principle in Nature, and was early taken as a standard in chemical scales. Hydrogen was also known as a widely-diffused and important element, but of far inferior import to oxygen, and received its name from the fact that it generates water by union with oxygen. But, as more was known about it, it was found to be deeply im-
