744 THE POPULAR SCIENCE MONTHLY. 



that they may result from the combined action of the oxygen and 

 nitrogen of the atmosphere on the organic materials in the process 

 of burning. Fourcroy goes still further. In his work on " Chemi- 

 cal Philosophy," to which I have referred, he writes (translation) : 



" We do not understand the composition of potash. It has 

 been suspected that it might result from a union of lime with 

 nitrogen, because it is often found in vegetables mixed with this 

 earth ; but this theory, which I brought forward some fifteen 

 years ago, has not been proved by any positive fact." It is inter- 

 esting to go back and watch this groping in the dark for what is 

 now positive knowledge, but the experience may teach us many a 

 valuable lesson, and will at least help us to realize the intense 

 enthusiasm with which, on October 6, 1807, Davy saw metallic 

 globules running from a lump of caustic potash under the influ- 

 ence of the current of his new voltaic battery. 



With this great achievement of Davy the formative period of 

 the Lavoisierian system of chemistry may be said to have closed ; 

 but in this connection it is amusing to notice that in a chemical 

 text-book studied in Harvard College by the class of 1815, and 

 given me by the late Hon. John G. Palfrey, of that class, the 

 alkalies and earths are included in the list of chemical elements, 

 and Davy's discovery is only briefly referred to in a note. 



Immediately after Davy's short but brilliant career, the sci- 

 ence of chemistry took the form which it retained for nearly fifty 

 years — a form in which it was first studied by all the older men of 

 the present generation. The form was essentially that given by 

 Lavoisier, and its chief merit was the simplicity of the classifica- 

 tion, and the admirable nomenclature in which this classification 

 was expressed. This nomenclature, which is to a great extent still 

 retained, although the terms have lost most of their original sig- 

 nificance, was devised by Lavoisier, with the co-operation of sev- 

 eral of his associates, and adopted with the sanction of the French 

 Academy of Sciences. It was a masterly production, and very 

 greatly strengthened the hold which the system acquired at all 

 the great centers of learning. The general features of the Lavoi- 

 sierian system can be stated in few words. 



Oxygen, which constitutes at least one half of the earth's crust, 

 is the common cement by which all the elementary parts are held 

 together. It is the universal acidifying principle, and the salifiable 

 bases owe their peculiar relations to the same element as well. 

 The elements may be divided into metals and non-metallic sub- 

 stances. The direct compounds of the non-metals with oxygen in 

 different proportions are acids, while the compounds of the metals 

 with oxygen are salifiable bases, and the compounds of the acids 

 and bases are salts ; and simple salts may still further combine 

 with each other to form double salts. Thus, beginning with the 



