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RvVLKiGii ahvays compares flic j>avS, tlic refmcfivity of wliirli lie 

 wishes lo dcteniiine, with di'v air tVec from earbonic acid. He lakes 

 care that I lie gas and the air are both in such circiimslances liuit 

 tiieir refraclive indices are Ihe same in liiese circumstances, which 

 may be lesled by means of a Frannliofers' diirraction [)henomenon. 

 This equality of refraction indices is obtained by regulating the 

 pressure of the two gases. With the aid of a calhetomeler and (he 

 barometer the pressure is determined, and from the two pressures 

 the refractivity of the gas. 



This is applied to mixtures of vapours coexisting with mixtures. 

 The pressure of the air, the refractivily of which is compared with 

 that of the vapour, is regulated in such a way that the vapour and 

 the air are in such circumstances that their refractive indices ai-e the 

 same. 



From the relation between the pressures the molecular refractivity 

 of the vapour (or of the vapour mixture) may be determined, the 

 law of BiOT and Arago gives then the concentration of the vapour 

 mixture .i\. 



The vapour pressure is of course, also kno^^'n. The concentration 

 of the liquid was obtained by Aveighing the original quantities of 

 liquid brought into the apparatus, and diminishing each witli the 

 weight of the quantity of that substance in the vapour. 



The arrangement agreed on the whole witii that of Ounaeus. It was 

 a moditication of one given by Frauxhofer based on the interference 

 of light, which fell through two vertical parallel slits, after having 

 passed through two tubes of equal length, shut off by two plates of 

 plate-glass. Then it was converged by the object-glass of a telescope 

 and the diffraction phenomenon brought about in this way, was 

 observed by means of an eye-glass consisting of two cylindric lenses. 

 In one of the tubes mentioned the vapour is found, in llie other 

 ail' or dry carbonic acid, the refractivity of \vhich was known, and 

 tlie pressure of a\ hich was now regulated in such a way that the 

 diffraction phenomenon occupied the place which it would also have 

 taken, when the two tubes had been filled with the same gas under 

 the same circumstances. 



The two tubes were connected with open manometers and an 

 apparatus to increase or decrease the jiressure of the air. 



The improvements which were applied, were chiefly the following : 



Isf. The tw^o substances which were used for the determination 

 were sealed in two separate small pieces of glass lube, so thai the 

 air could be removed from the liquid as much as possible by boiling 

 the liquid under decreased pressure. 



