THE CHEMICAL ELEMENTS. 745 



elements, combination proceeded, pair and pair, until all terrestrial 

 products were educed. Tlie members of each class of these prod- 

 ucts were designated by specific names, regularly formed and 

 easily remembered. 



Such a simple system could easily be comprehended and pre- 

 sented in such works as that of the late Dr. Turner, and, illus- 

 trated 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 oth- 

 ers 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 na- 

 ture, 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 inves- 

 tigation 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 impu- 

 rity, it appeared that carbon was known under three forms, dia- 



