Joly — A Method of Determining the Density of a Gas. 539 



after emptying, in distilled water] 1 ; and on the other hand the 

 use of the lighter vessel will probably confer still greater accuracy 

 on the weighings. 



Turning our attention to the volumenometer it appears possible 

 that a similar correction may arise in the assumption made as to 

 its capacity. For, in the experiments in which its volume is 

 determined, its walls are exposed to the pressure due to the weight of 

 the water contained in it ; subsequently, when it is occupied by a gas, 

 this pressure does not obtain. Hence there might be reason to fear 

 some small excess in the value ascribed to its capacity. To make 

 this a matter of experiment, I filled the jacket with water, and 

 provided a wide tube dipping into it, syphon- wise, enabling the 

 water to be all run off within a short space of time. The volu- 

 menometer within was filled with air, the upper tubulure was 

 closed, the lower one being arranged to dip into a vessel of water, 

 the water standing some little height in the tube, nearly to the 

 etched mark. Thick baize was now folded round and over the 

 jacket, and after the lapse of some time the cross wire of a 

 cathetometer was brought to read the level of the water in the 

 lower tubulure. A few minutes' observation sufficed to show when 

 this level was stationary. The syphon was now put into operation ; 

 and while the water was running out of the jacket the level of 

 the liquid on the tubulure was observed. No movement was 

 perceivable. It is thus, I think, legitimate to conclude that no 

 appreciable error arises from this source, the experiment being a 

 delicate one. 



Comparing the previous figures with those obtained in the 

 exact experiments of Lord Rayleigh, using Regnault's method, 

 which experiments may fairly be taken as representative of 

 modern refinement on the method, the following numbers are 

 obtained, assuming the same weight of hydrogen to be dealt with 

 in the present method as was used by Lord Rayleigh, and the 

 lighter sphere to be used. 



1 Two experiments on the heavier sphere afforded : (f) a shrinkage of 0-1013 c.cs. 

 for the release of 2*5728 grammes of air at 15°-4 ; (2) a shrinkage of 0*1732 c.es. for 

 4-3525 grammes of air at 12 C C. 



Two experiments on the lighter sphere gave: (1) 0-1720 c.cs. shrinkage for the 

 release of 1-6046 grammes of air at 16°; and (2) a shrinkage of 0-2636 c.cs. for 2-5284 

 grammes of air at 14° - 3. 



SCIEN. PE.0C. K.D.S. VOL. VI., PART IX. 2 S 



