134 Lord Rayleigh. [Mar. 23, 



his giving no means of saturating the gases with aqueous vapour. 

 He further measures his pressures to l/200th of a millimetre. His 

 ratio for the volumes is given as 



2-00023, 



or only l/10th of the difference from 2"0 exactly of that found by the 

 above-described experiments. 



II. " On the Densities of the Principal Gases." By Lord 

 Rayleigh, Sec. R.S. Received March 4, 1893. 



In former communications* I have described the arrangements by 

 which I determined the ratio of densities of oxygen and hydrogen 

 (15'8S2). For the purpose of that work it was not necessary to 

 know with precision the actual volume of gas weighed, nor even the 

 pressure at which the containing vessel was filled. But I was 

 desirous before leaving the subject of ascertaining not merely the 

 relative, but also the absolute, densities of the more important 

 gases, that is, of comparing their weights with that of an equal 

 volume of water. To effect this it was necessary to weigh the globe 

 used to contain the gases when charged with water, an operation 

 not quite so simple as at first sight it appears. And, further, in the 

 corresponding work upon the gases, a precise absolute specification 

 is required of the temperature and pressure at which a filling takes 

 place. To render the former weighings available for this purpose, it 

 would be necessary to determine the errors of the barometers then 

 employed. There would, perhaps, be no great difficulty in doing 

 this, but I was of opinion that it would be an improvement to use 

 a manometer in direct connexion with the globe, without the 

 intervention of the atmosphere. In the latter manner of working, 

 there is a doubt as to the time required for full establishment of 

 equilibrium of pressure, especially when the passages through the 

 taps are partially obstructed by grease. When the directly con- 

 nected manometer is employed, there is no temptation to hurry 

 from fear of the entrance of air by diffusion, and, moreover (Note A), 

 fthe time actually required for the establishment of equilibrium is 

 greatly diminished. With respect to temperature, also, it was 

 thought better to avoid all further questions by surrounding the 

 globe with ice, as in Regnault's original determinations. It is true 

 that this procedure involves a subsequent cleaning and wiping of 

 the globe, by which the errors of weighing are considerably 

 augmented; but, as it was not proposed to experiment further with 

 hydrogen, the objection was of less force. In the case of the heavier 

 * < Koy. Soc. Proc.,' February, 1888 j February, 1892. 



