384 Mr. J. Joly on a Method of 



a little more than the pressure due to two litres. The shrink- 

 age is, in fact, closely 0*257 cub. centim., and the cor- 

 rection on the weight of gas released is 0*000318. This 

 is higher than with the heavier sphere, but the correction 

 is perfectly definite [and easily ascertained once for all by 

 successive weighings, before and after emptying, in distilled 

 water] * ; 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 volumeno- 

 meter 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 jncket 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 be fairly 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. 



* Two experiments on the heavier sphere afforded : (1) a shrinkage of 

 0*1013 cub. centim. for the release of 2*5728 grammes of air at 15 0, 4; 

 (2) a shrinkage of 0*1732 cub. centim. for 4*3525 grammes of air at 12° C. 



Two experiments on the lighter sphere gave : (1) 0*1720 cub. centim. 

 shrinkage for the release of 1*6046 grammes of air at 16°; and (2) a 

 shrinkage of 0*2636 cub. centim. for 2*5284 grammes of air at 14°*3. 



