124 Professor Julius Wilhelm Brilhl [May 26, 



fluid substances. The important fact was at once established that 

 Newton's expression for refraction, — - — , is not constant, but varies 



CL 



considerably with the temperature. On the other hand, it was found 

 that the more simple ratio 



n-1 

 cT 



remains practically constant. 



Now this ratio, just like the Newtonian constant confirmed by 

 Biot and Arago, and by Dulong, applies also to gases and vapours. 

 As, in the case of gases, the refractive index is very little different 



from unity, the numerical value of — - — is almost exactly twice that 



of !^. 



III. 



§ 6. Soon after 1860, Hans Landolt came forward with his 

 optical researches. He began by confirming the results of Gladstone 

 and Dale. He proceeded a step further, however, by following the 

 example of Berthelot, and comparing the refractivity not of equal, 

 but of molecular quantities of the substances. If P represents the 



molecular weight, the product ( — ~^ — ) P is the molecular refraction. 



§ 7. Landolt examined particularly the fundamental question 

 whether a different grouping of the same number of atoms of the 

 same elements — which is the cause of isomerism — has any influence 

 on the optical properties of bodies. 



He established the important fact that only the relative weight of 

 the elements is of influence on the molecular refraction of a compound, 

 wliile the different grouping of the atoms has no appreciable effect ; 

 and this made it possible to determine the atomic refractions of the 

 elements. The atomic refraction of carbon, for instance, was obtained 

 by comparing the molecular refractions of two compounds which dif- 

 fered only by one atom of carbon ; and in a similar manner the 

 atomic refractions of the remaining elements were determined. 



With the aid of these constants it was now possible to calculate a 

 priori the molecular refraction of many organic compounds from the 

 elements composing them, and Landolt showed that the calculated 

 molecular refractions agreed very well with those determined by ex- 

 periment. 



§ S. Gladstone, in the course of his researches, was able to 

 confirm Landolt's results in many cases. But he also found a con- 

 sideral)le number of substances in whicli the observed molecular 

 refraction was completely at variance witli that obtiiined by adding 

 the atomic refractions together. Tlie exceptions were so numerous, 

 that they really seemed to overthrow the whole law of summation. 



