Refractive Power of Elastic Fluids. 215 



But if an analogous law really existed, it would show itself 

 in the numbers of the preceding table, for the gases having 

 been observed at the same temperature and pressure, the 

 inequalities in the refractive powers could arise only from 

 the inequality of the effects of each of the molecules consi- 

 dered individually. 



M. Dulong next proceeds to examine if there is any appre- 

 ciable relation between the refractive power of compound 

 gases and that of their elements. The following table con- 

 tains the refractive powers as calculated and observed in nine 

 different compounds. 



Observed Calculated 



Refract. Power. Hefract. Power. Difference. 



Ammonia, 1.309 1.216 +0.093 



Oxide of azote, 1.710 1.482 +0.228 



Nitrous gas, 1.030 0.972 +0.050 



Water, vapour of 1.000 0.933 +0.067 



Oxi-chloro-carb. gas, 3.936 3.784 +0.152 



Muriatic ether, 3.72 3.829 —0.099 



Hydrocyanic acid, 1-521 1.651 0.130 



Carbonic acid, 1.526 1.609 —0.093 



Hydrochloric acid, 1.527 1.547 0.020 



In Jive of the preceding gases, the refractive power of the 

 compound is greater than that of its elements, whilst in the 

 other/our it is the reverse. The particular kind of condensation 

 which accompanies the combination does not appear to have 

 any connection with this variation ; for, in hydrochloric acid, 

 for example, there is a diminution, and, in nitrous gas, an 

 augmentation, though the proportions of these two com- 

 pounds are the same, and the condensation is nothino- in 

 both. S 



The only remark which this kind of approximation leads 

 us to make is, that, in the binary alkaline or neutral combi- 

 nations, the observed refractive power is greater than that 

 deduced from the elements, while in acid compounds it is 

 weaker. 



Muriatic ether, which may be regarded as neutral, and 

 the oxi-chloro-carbonic gas, which is decidedly acid, are ex- 

 ceptions to the law. But we ought to remark, that those com- 

 binations are formed of three primitive elements, which are 



