The Evolution of the Sciences 



to do likewise, the first sound ideas concerning 

 reactions. Berthollet was the first to under- 

 stand the meaning of chemical equilibrium, and 

 in a particular case he saw how this equilibrium 

 could be destroyed and lead to a complete 

 reaction. 



These remarkably philosophical ideas of 

 Berthollet had long to wait for their logical 

 development at the hands of Henri Sainte- 

 Claire Deville. It was in the laboratory of the 

 Ecole Normale, quite near the one where Pasteur 

 discovered a new world, that Deville established, 

 by indisputable and repeated experiment, the 

 laws of dissociation. When chalk or carbonate 

 of lime is heated in a closed vessel it decomposes 

 into lime and carbonic-acid gas until this gas 

 reaches in the receptacle a pressure which 

 depends on the temperature and on it alone; 

 it is called the tension of dissociation. 



If part of the carbonic-acid gas is removed 

 more is formed by decomposition of the carbonate ; 

 if, instead, more gas is introduced into the re- 

 ceptacle it combines with the free lime until 

 the tension of dissociation iias been restored. 

 Thus, in compliance with Berthollet's views, 



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