LECTURE IX.] HISTORY OF CHEMISTRY. 151 



equivalent, and to follow up both of them consistently, almost 

 the opposite might be said of their opponents. The latter (/ 

 knew that one atom of alumina, A1 2 O 3 , requires three times as 

 much sulphuric acid to saturate it as one atom of potash, KO ; 

 they also considered that one atom of phosphoric acid, P 2 O , 

 required three times (in reality twice) as much base to form a 

 neutral salt as one atom of hydrochloric acid ; and yet they 

 did not hesitate to employ the name " equivalents " for these 

 quantities. 



Just because our chemistry of to-day is based essentially 

 upon the difference between the conceptions of atom and 

 equivalent, everything that could lead to a distinction between 

 these conceptions must be specially emphasised. I wished, 

 therefore, to point out that a new means of determining equiva- 

 lents was furnished, in the fourth decade of the century, by 

 the phenomena of substitution, and that this involved a real 

 advance in the question which is especially interesting us at 

 present. But it was further shown, from an entirely different 

 point of view, that the atoms of compound substances are not 

 necessarily equivalent. Decisive reasons were advanced to 

 establish the differences that exist, in this respect, in the case 

 of the acids, which formed one of the most fully investigated 

 classes of substances. The experiments relating to this matter 

 were carried out at an earlier period than the advancement by 

 Dumas of the theory of types, which was the next step in the 

 development of the substitution theory, and on this account 

 I think it should be treated of first. Even if both matters 

 seemed at that time to be extremely diverse, still the exercise 

 of some influence is not only conceivable, but it can actually 

 be observed ; and, for this reason, the chronological order must 

 not be altogether lost sight of. 



It is seldom that, in order to produce any .great result, so 

 few investigations have been necessary as was the case in the 

 founding of the theory of polybasic acids. The experiment and 

 the idea whereby this wide field of investigation was opened up 

 to experimental science, while new and secure footing was 

 afforded to theory, are alike elegant and precise. Only a few 



