LECTURE XV.] HISTORY OF CHEMISTRY. 329 



were afterwards observed, however, and it proved necessary to 

 take the constitutions of the compounds into consideration 

 also ; so that the value for the atomic volume, pertaining to 

 an atom, was assumed to be different according to the way in 

 which the atom was combined. A means was thus furnished, 

 in certain cases, of checking the constitution which had been 

 deduced, in the first place, by chemical methods only. (Com- 

 pare p. 252.) 



This branch, which was investigated by Kopp with great 

 skill and success, was then followed up further by many inves- 

 tigators. Researches into molecular volumes continue up to 

 the present day, and the results obtained are discussed and 

 turned to account in the same way that they were by Kopp. 180 

 But other properties of substances were also examined, and 

 were considered in the same way in connection with com- 

 position and constitution. 



This was the case especially as regards the refraction of 

 light by liquids and gases. Since the refractive index of a sub- 

 stance is dependent upon the wave-length of the light as well as 

 upon the temperature, it is not itself employed for the purpose 

 of comparison. It is true that an endeavour was at first made 

 to render the refractive indices independent of dispersion, by 

 adopting as basis those for a particular wave-length. Thus 

 Landolt at first employed, in his investigations, the indices for 

 the C line of incandescent hydrogen. Briihl, on the other 

 hand, employing Cauchy's formula, and after determining the 

 refractive index for several wave-lengths, calculated a coefficient 

 which was independent of wave-length and held for waves of 

 infinite length. 181 



The next endeavour was directed towards obtaining results 



180 Compare, amongst others, Pierre, Annalen. $6, 139 ; 64, 158 ; 

 80, 125 ; 92, 6 ; Buff, ibid. Supplementband 4, 129; Ramsay, Berichte. 

 12, 1024; Thorpe, Journ. Chem. Soc. 37, 141, 327; Lossen, Annalen. 

 214, 138 ; Elsasser, ibid. 218, 302 ; R. Schiff, ibid. 220, 71 ; etc. 181 Anna- 

 len. 200, 1 66. In a subsequent paper (Annalen. 235, i) Briihl discards the 

 refraction coefficient derived from Cauchy's formula and readopts the 

 former one. 



