elements, combination proceeded, pair and pair, until all terrestrial products were educed. The members of each class of these products were designated by specific names, regularly formed and easily remembered.
Such a simple system could easily be comprehended and presented in such works as that of the late Dr. Turner, and, illustrated by the brilliant phenomena of combustion, had a great charm. I can remember most distinctly the impression it made on me as a boy, and I have heard many learned men, among others my late colleague, Dr. Asa Gray, speak in the most glowing terms of the impression it made on them.
Lavoisier himself regarded his system as perfectly true to nature, and often affirms that he accepts no conclusion not based on experimental evidence; but, with the progress of knowledge, the system soon became highly artificial. Indeed, it never would have been formulated had not its author's vision been restricted to the narrow field that had been cultivated in his time. As investigation extended, the class of hydrogen acids, and their products, which Lavoisier had hidden away under a mistaken interpretation of their constitution, assume an ever-increasing prominence; and the system was doomed when Berzelius felt obliged to withdraw this class of bodies from the general scheme, and place them by themselves in a special division, which he called the haloids. Then after a time it appeared that the simple oxides of the elements had neither acid nor basic properties in themselves, and only acquired active qualities of either kind when united with water; and that hydrogen and not oxygen was the acidifying principle. Moreover, multitudes of compounds were discovered in whose production oxygen took no part whatever, and, although attempts were made to classify these on the same general dualistic plan, assuming that sulphur, chlorine, or one of the allied elements might act in place of oxygen as a general binding agent in a chemical combination, yet the attempts were obvious failures.
Before I became a teacher of chemistry, in 1849, it had already become evident that Lavoisier's definition of a chemical element, as a substance that could not be decomposed, must be modified; or, at least, that even if our actual processes of analysis could not go beyond the substances regarded as elementary, the philosophy could not possibly be thus restricted. Many facts previously known but overlooked, and other facts then first discovered which exhibited the old facts in a stronger light, all combined to show clearly that the same chemical element might appear under the guise of different substances. By burning a gem in oxygen gas, Davy had proved that diamond was pure carbon; and when it was also shown that the iron in graphite was an accidental impurity, it appeared that carbon was known under three forms, dia-
