354 M. R. Ruhlmann on the Alteration produced by Heat 



proof to gaseous mixtures. This error found further support 



ri*— 1 

 from the fact that the constant — j — = const, might be used, 



as Beer and Kremers* showed, to calculate the indices of refrac- 

 tion for saline solutions from their constituents. In the same 

 way, Hoekf? assuming this relation, repeated the calculations 

 for Deville's experiments on the refractive indices of liquid mix- 

 tures, and obtained perfectly concordant results. ForthommeJ 

 also adopted the refractive power (breckende Kraft) as basis for 

 similar investigations. Schrauf § published a very elaborate 

 criticism against the constancy of the quotient between the re-^ 

 fractive power and density, for which he proposed a new relation, 

 which he strove to support by numerical values. He proceeds 

 from Cauchy's formula of dispersion, and makes use of the first 

 approximation, 



A B 



A* = A+- 2 . 



In rather an arbitrary manner, the foundation of which I am 

 not alone in being unable to understand, Schrauf arrived for A 

 at the differential equation 



A.rfA=M.dD, (I.) 



where M represents a constant and D the density of the body. 



Schrauf gives the derivation of another equation in some such 

 terms as the following ||. 



The coefficient of dispersion depends, according to Cauchy, 

 upon a quadratic ; the dispersion can only be caused by disturb- 

 ances of a higher order ; and so, as is shown by comparison of 

 the differences, we may rightly assume that the alteration in B 

 is a function both of the density and its alterations, and that 

 accordingly 



dB = ND.dD, (II.) 



where N represents a new constant. 



Since the confirmation of these differential equations for other 

 reasons is not apparent, they are only valid provided the laws 

 derived from them are really confirmed by experience. For the 

 inferior limit of integration we make use of the relation that for 

 D = there is no refraction and no dispersion, and that accord- 



* Pogg. Ann. vol. ci. p. 133. \ Ibid. vol. cxii. p. 347. 



% Ann. de Chim. et de Phys. vol. lv. p. 307. 



§ " Ueber die Abh'angigkeit der Fortpflanzuiigsgesehwindigkeit des 

 Lichtes von der Korperdichte," Pogg. Ann. vol. cxvi. pp. 193-249. 

 || Pogg. Ann. vol. cxvi. p. 204. 



