Refractive Power of Elastic Fluids. 21 3 



stand of masonry before the prism, at such a height that we 

 can perceive across it a distant mark. If the telescope and 

 prism are invariable, or if the telescope is pointed to the mark 

 when the prism is filled with air, at m 76 of tension, for 

 example, then if another gas is made to replace the air, and 

 such a density given to it as to restore the coincidence of the 

 intersection of the wires at the mark, we are certain that the 

 deviation is the same for the same refracting angle, which can 

 only happen when the refractive powers are equal. 



In this process the limit of error depends on the magnify- 

 ing power of the telescope ; but as M. Dulong has remarked, 

 that we cannot determine to less than the ^\ z part the pu- 

 rity of the gas, it would be useless to estimate smaller frac- 

 tions in the measure of their refractive powers. 



This method is applicable, with some modifications, to gases 

 such as chlorine, which attack all metals, as well as va- 

 pours which cannot support atmospheric pressure. 



In order to verify the law that, in the same gas, the refrac- 

 tive powers are proportional to the density, M. Dulong de- 

 termines the refractive power of seven mixtures, formed by 

 gases which do not combine ; and as the results of observa- 

 tion always agree with those which are deduced from the re- 

 fractive powers of the ingredients of the mixture, we may con- 

 clude, that each gas preserves a refractive power exactly pro- 

 portional to its density. 



The refractive power of atmospheric air, for example, M. 

 Dulong has found to be equal to that of azote, oxygen, and 

 carbonic acid united, each of them being calculated for its 

 corresponding density in air. But as this equality is never 

 met with in any combination, we thus obtain direct proof, 

 that the elements of air are not combined together. 



The following table contains the refractive power of twenty 

 gases, as measured by M. Dulong, in relation to that of air 

 of equal density. 



Names of Gases. Refractive Powers. Densities. 



Atmospheric Air, 1 1 



Oxygen, - 0.92* 1.1026 



Hydrogen, - 0.470 0.068.5 



Azote, . . 1.020 0.970 



