33 HISTORY OF CHEMISTRY. [LECTURE XV. 



independent of the temperature, by employing, for the re- 

 fractive power, the expression discovered by Laplace 182 



2 [# = refractive index, d= density]. It soon appeared, 



however, that this does not satisfy the required condition of 

 being independent of temperature ; and besides, on the aban- 

 donment of the emission theory of light, it had lost all physical 

 importance. Gladstone and Dale 183 now showed empirically 



that the expression ^^- fulfilled this condition, in many cases 

 a 



at least. Landolt adopts the product of this value and the 

 molecular weight (i.e., the refraction equivalent) as the basis of 

 his extensive investigations, 184 and finds that it is dependent on 

 the constitution (the influence of chemical constitution is ascer- 

 tained, but is not followed up). He thus succeeds in calculat- 

 ing the refraction equivalents of the elementary atoms of carbon, 

 hydrogen, and oxygen, and in deducing from these, again, the 

 values pertaining to the individual compounds. These fre- 

 quently showed close agreement with the observed values. 

 Landolt, however, confined his observations to the fatty organic 

 compounds. These observations were further extended, first by 

 Haagen, 185 and then by Gladstone, 186 who determined the refrac- 

 tion of many inorganic compounds and the refraction equivalents 

 of almost all the elements. 



In the meantime another value, .--HL-, was theoretically 



(tf -f 2)d 



deduced as refraction constant by H. A. Lorentz 187 and by L. 

 Lorenz 188 in two ways that were independent of each other ; 

 and this value was employed especially by Landolt 189 and by 

 his pupil Briihl. They give the name molecular refraction to 

 the product obtained by multiplying this value by the molecular 

 weight ; and Briihl investigated this property in the cases of 

 strongly refracting substances, and of aromatic compounds in 



182 Mecanique celeste. 4, 232. 1S3 Phil. Trans. 1858, 887 ; 1863, 317. 

 184 Pogg. Ann. 117, 353; 122, 545; 123, 595. 185 Ibid. 131, 117. 



186 Proc. Roy. Soc. 16, 439 ; 18, 49 ; 31, 327. 187 Wiedem. Ann. 9, 641. 

 188 Ibid, n, 70. 189 Berichte. 15, 1031. 



