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LIII. 



ON A METHOD OF DETEKMINING THE ABSOLUTE DEN- 

 SITY OF A GAS. By J. JOLY, B.E., M.A. (Plate XI.) 



[Bead June 18, 1890.] 



The method still in general use for the accurate determination of 

 the density of a gas is that of Regnault, improved from the concep- 

 tion of Biot and Arago, by the addition principally of the counter- 

 poising vessel. This method is not free from objections. 



The gas is weighed in the vessel in which its volume is measured? 

 and as this is necessarily a large vessel, of considerable weight, the 

 weight of the gas bears but a very small proportion to the total 

 weight observed. Again, from the great bulk and surface of the 

 containing vessel, it is in itself a difficult object to weigh with a 

 high degree of accuracy, more especially when exposed, as it is, 

 between the first and the second weighings, to the manipulation 

 incident in filling it at a known temperature. 



Further, in the determination of the volume of this vessel there 

 arises, as Lord Rayleigh has lately shown 1 , an error due to the 

 variation of pressure within the vessel when it is weighed, vacuous 

 in the first instance, full in the second. This error, which escaped 

 Regnault, is very considerable in this method. 



If, again, there is an object in economising the gas, Regnault's 

 method is wasteful, as the " washing out " of so large a vessel with 

 the gas necessitates a far greater expenditure of gas than is subse- 

 quently required for the actual determination. Without referring 

 again to these objections to the method of Regnault, it will be seen 

 that they are either much reduced in importance or eliminated 

 in the method to be described. 



In this new method the measurements of the volume and of 

 the weight of the gas are effected in separate vessels. That in 



1 Froc. Hoy. Soc, vol. 43, p. 356. 



