LECTURE IV.] HISTORY OF CHEMISTRY. 49 



He may have arrived quite independently at those ideas which 

 exercised the greatest influence upon the subsequent develop- 

 ment of chemistry, but we must here take notice of all the facts 

 which are of importance in this connection, and we must not 

 overlook Dalton's predecessors. Almost simultaneously with 

 the conception of the atom, that of the equivalent was also de- 

 veloped. The recognition of the latter contributed to procure 

 admission and general favour for the atomic theory, and it is 

 therefore advisable, in my opinion, to deal with both ideas side 

 by side as they arose chronologically. In doing so, it will be 

 observed that Dalton's atom was introduced independently of 

 the equivalent, but that Wollaston, in particular, endeavoured 

 to replace the atom by the equivalent. This afterwards led to 

 the identification of the two ideas, a fact which had a detri- 

 mental effect upon the science. 



The first experiments which could have led to the establish- 

 ment of equivalent quantities, were carried out by Bergman in 

 the second half of last century. 2 Bergman observed that 

 neutral solutions of metals were precipitated by other metals, 

 without the production of an acid reaction and without the 

 evolution of gas. x\s an adherent of the phlogiston theory, he 

 explains the observations quite correctly in accordance with 

 the principles of the theory. He assumes that the precipitated 

 metal has taken up just as much phlogiston as the precipitating 

 metal has parted with; and in consequence of this view, he 

 perceives a means of determining the quantities of phlogiston 

 contained in different metals. The quantities of the dissolved 

 and of the precipitated metals respectively, must stand to each 

 other inversely as the supposed quantities of phlogiston con- 

 tained in equal weights of these metals. 



Lavoisier, who repeats and extends Bergman's experiments 

 a few years afterwards, 3 recognises that they must show, in 

 terms of his theory, the quantities of oxygen which combine 

 with equal weights of the metals. Where Bergman had spoken 



2 Bergman, Physical and Chemical Essays. Translated by E, Cullen, 

 M.D., 2 (1784), 349 et seq, 3 Lavoisier, Oeuvres. 2, 528. 



D 



