212 



M. Dulong's Researches on the 



bination altered the force of refraction possessed by the ele- 

 mentary principles when considered separately. 



This last question had already been the subject of a Me- 

 moir, published in 1806, by MM. Biot and Arago ; but at 

 tbat time we possessed only very uncertain data respecting 

 the proportions of the greater number of compound bodies, 

 and it has since been demonstrated, that, in the passage of 

 elastic fluids to the liquid or solid state, considerable changes 

 are produced in the refractive powers. 



We cannot, therefore, expect to discover the relation in 

 question, unless by observing bodies in the gaseous stale. 

 The philosophers whom we have quoted, having, besides, exa- 

 mined only a very limited number of different species, it be- 

 came indispensable to have recourse to new determinations. 



The method of observation employed by M. Dulong, was 

 susceptible both of sufficient precision and of ready applica- 

 tion. It is founded on the law announced by MM. Biot 

 and Arago, and which M. Dulong has himself verified in other 

 gases, that the refractive power of the same gas is proportion- 

 al to its density. Hence it follows, that if we determine the 

 density of a gas when it refracts exactly as much as air, at the 

 same temperature, and at a convenient pressure, we can deter- 

 mine, by simple proportion, the ratio of the refractive powers 

 of the two gases under the same pressure. We thus obtain, 

 indeed, only the ratios of the refractive powers ; but that is 

 the only element which is necessary for the question which 

 M. Dulong proposes to resolve. 



The apparatus which he employed, consists of a hollow 

 prism of glass, with an angle of about 145*, into which differ- 

 ent gases may be successively introduced. A vertical tube, 

 filled with mercury, communicating with the interior of the 

 prism, permits us to dilate at pleasure the elastic fluid which 

 it contains. The tension of the gas is ascertained either by 

 the barometer of the air-pump, which forms part of the ap- 

 paratus, or, in some cases, where the communication with 

 the pump ought to be intercepted, by a small vertical tube, 

 opening at its lower end into the reservoir of mercury which 

 we have mentioned. An astronomical telescope, furnished 

 with cross wires in the focus of its object-glass, is placed on a 



