328 HISTORY OF CHEMISTRY. [LECTURE XV. 



with absorption of heat), as in the case of the assimilation by 

 the green parts of plants, and those in which the chemical 

 process takes place with the evolution of heat, as in the case 

 of chlorine and hydrogen. The light appears, however, to do 

 work in both cases, although, in the latter case, it is merely 

 preparatory work, by which the obstacles to combination are 

 overcome. For this effect a certain time is required, and 

 Bunsen and Roscoe proposed to indicate this by the term 

 photo-chemical induction. 



Finally, there still remains a large department to be dealt 

 with, namely, that of molecular physics. This department has 

 to do with the determination of the physical constants of 

 chemical substances, and with seeking the relations between 

 these and the chemical composition and constitution. Her- 

 mann Kopp may be looked upon as the founder of this branch 

 of science. From the year 1842 onwards, he occupied himself 

 with the determination of the boiling points and of the specific 

 or molecular volumes of liquids. 179 In order that the numbers 

 obtained might be compared with one another, they had to be 

 determined under comparable conditions -the boiling-points 

 under the same pressure, and all the specific volumes at the 

 boiling-points, so that the corresponding vapours should be 

 under the same pressure. The guiding idea in the comparison 

 was that the same difference in composition corresponds to the 

 same variation in the property under investigation, or that the 

 particular property of a compound is the sum of the properties 

 of its elementary constituents. The values pertaining to the 

 atoms of the elements, with respect to this property, were cal- 

 culated empirically, and, by means of these numbers and of 

 the composition, the theoretical value of the property was 

 determined for the compound. This value was then compared 

 with that obtained by observation. Investigations of this kind 

 were carried out in the case of molecular volumes, in particular, 

 and harmonious results were frequently obtained. Deviations 



179 Annalen. 41, 86, 169; 50, 71 ; Pogg. Ann. 63, 283; Annalen. 

 94 2 57 J 95 "I. 307 5 9^, I, I53 33 J Supplementband 5, 323 ; etc. 



