The Evolution of the Sciences 



The second, developed from the theoretical 

 ideas of Berthollet, was opened in France by 

 Henri Sainte-Claire Deville, and continued by 

 the Dutch and German physico-chemical school, 

 of which Van t'Hoff is the acknowledged leader. 

 The conception of chemical affinity re- 

 mained to some extent incomplete and indefinite 

 until the conception of energy and the principle 

 of the conservation of energy made their ap- 

 pearance in science. Towards the middle of 

 the nineteenth century it began to be seen 

 clearly that natural phenomena invariably 

 imply the transformation of a single entity 

 appearing in the form of mechanical work, 

 electricity or heat. It was natural to include 

 in this catalogue of the various forms of energy 

 the force which appears in chemical reactions 

 and is betrayed by its mechanical effects when 

 gunpowder explodes, by an electric current in 

 the cell and by the production of heat in most 

 other cases. Henceforth the affinity of zinc 

 for sulphuric acid acquires a definite meaning; 

 it is measured by the heat released in the reaction 

 of the molecular weights of these two bodies. 

 Thus the course to be followed is already set; 



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