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XLII. On a Method of Determinina the Absolute Density of a 

 Gas. By J. Joly, B.E., M.A* 



THE method still in general use for the accurate determina- 

 tion of the density of a gas is that of Regnault, improved 

 from the conception of Biot and Arago, by the addition 

 principally of the counterpoising 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 con- 

 siderable 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 

 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 f, 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 economizing the gas, Reg- 

 nault' 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 subsequently 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 which the gas is weighed is small, the gas being under 

 high pressure within it. That in which the volume is subse- 

 quently measured may, on the other hand, be as large as may 

 be desired. We are not concerned with its weight. 



The vessel which I have in use for holding the gas, when 

 being weighed, is of copper, spherical in form, about 6*7 centim. 

 in diameter, made up of two hemispheres, the walls being 

 about 1 millim. in thickness. The hemispheres are brazed 

 (not soft-soldered) together within a belt of copper about 



* Reprinted from the Scientific Proceedings of the Royal Dublin 

 Society, June 18, 1890. 



t Proc. Ptoy. Soc. xliii. p. 356. 



