RELATIONS TO OTHER SCIENCES 251 



down the barriers that separated physical and chemical phenomena. 

 He explained many little-understood reactions, and showed how in- 

 verse reactions were produced and how the minerals were formed in 

 metallic veins. 



Henri Debray soon demonstrated the value of Deville's ideas by his 

 elegant experiments on the decomposition of carbonate of lime and 

 of hydrated salts. 



This matter of dissociation had also certain points of contact with 

 the phenomena of equilibrium of which mention was made in the im- 

 portant memoirs of Berthelot and Pean de Saint Gills, on speeds of 

 etherification. But I do not wish to enter upon the history of this 

 question, for my colleague Mr. van't Hoff will speak to you about it at 

 the Congress of Physical Chemistry, much better than I could. I will 

 only say that at all times the two sciences of physics and chemistry 

 have been of mutual aid and support. Victor Regnault began this 

 great movement of physical chemistry, illumined by the brilliant 

 discoveries of Deville, enlarged by the work of Joule, and continued 

 with such success by Gibbs, van der Waals, van 't Hoff, Arrhenius, 

 and Ostwald. 



Passing to a different order of ideas, we may recall the splendid 

 work of Pasteur on molecular dyssymmetry, from which started the 

 very original investigations of Le Bel and van't Hoff on the isomerism 

 of substances possessing rotatory power. 



At every turn, inorganic chemistry depends on the data of physics. 

 The determination of physical constants is an everyday performance 

 in our laboratories, and often is the only guarantee of the purity of 

 our preparations. In doubtful cases, when it becomes difficult to estab- 

 lish an atomic weight, the law of Dulong and Petit gives us valuable 

 information. The whole of thermo-chemistry, indeed, founded with 

 such success by Berthelot and by Thomson, makes use only of the 

 methods of calorimetry. 



There is another branch of physics which is called upon to render 

 service to inorganic chemistry, and which has had a great develop- 

 ment in the last few years; I refer to the easy production of high and 

 low temperatures. 



Metallurgy and ceramics have for thousands of years made use, 

 industrially, of high temperatures for obtaining metals, glass, and 

 terra-cotta. These high temperatures were secured by the combus- 

 tion of wood or coal. Later, savants concentrated the solar heat by 

 means of mirrors or burning-glasses for accomplishing some interest- 

 ing experiments. Two centuries ago, the importance of the action 

 of heat in the different reactions was so well appreciated that it served 

 as the basis for Stahl's theory of phlogiston. And when chemistry 

 established itself as a science, the ideas of Lavoisier on combustion 

 were the starting-point of this profound transformation. 



