LECTURE IX. 



GRAHAM'S INVESTIGATION OF PHOSPHORIC ACID LIEBIG'S THEORY OF 

 POLYBASIC ACIDS, AND HIS VlEWS WITH RESPECT TO ACIDS IN 

 GENERAL ADOPTION OF THE DAVY-DULONG HYPOTHESIS DIS- 

 COVERY OF TRICHLORACETIC ACID ATTACK UPON THE ELECTRO- 

 CHEMICAL THEORY REPLIES OF BERZELIUS COPUL^E. 



I WISH to begin this lecture with some general remarks which 

 may serve as an appendix to what was stated in the preced- 

 ing lecture respecting the phenomena of substitution, and 

 especially to the conceptions of them which were entertained 

 by Dumas 1 and by Laurent. 2 



I desire to point out how, in consequence of the observed 

 phenomena of substitution, the conception of the equivalent 

 assumed a more definite form. Thus in assuming, with Dumas, 

 that the quantities replacing one another are equivalent (and 

 this was an assumption that had some justification, according 

 to Laurent's views which rendered possible a direct com- 

 parison between the original and the final products), a series of 

 experiments was all that was necessary in order to determine 

 the equivalents of the substances so replacing one another. 

 One section of chemists actually did work in this direction, 

 and, in consequence of this, it is necessary to observe par- 

 ticularly to which school the author of any paper published 

 at that time belongs ; for at this very period Gmelin's school, 

 which likewise wrote or desired to write in equivalent formulae, 

 began to acquire much influence. Whilst the adherents of the 

 substitution theory (who made use of the equivalents), in spite 

 of numerous shortcomings and mistakes, always endeavoured 

 to separate from each other the conceptions of atom and 



1 Dumas, Traite, Organic Part. I, 75. 2 Annalen. 12, 187. 



