362 Relative Densities of Hydrogen and Oxygen. [Feb. 9, 



as it was by Regnault. The weight of the gas is not to be found by 

 merely taking the difference of the full and empty weighings, unless 

 indeed the weighings are conducted in vacuo. The external volume 

 of the globe is larger when it is full than when it is empty, and the 

 weight of the air corresponding to this difference of volume must be 

 added to the apparent weight of the gas. 



By filling the globe with carefully boiled water, it is not difficult 

 to determine experimentally the expansion per atmosphere. In the 

 case of globe (14) it appears that under normal atmospheric condi- 

 tions the quantity to be added to the apparent weights of the 

 hydrogen and oxygen is 0*00056 gram. 



The actually observed alteration of volume (regard being had to 

 the compressibility of water) agrees very nearly with an a priori 

 estimate, founded upon the theory of thin spherical elastic shells and 

 the known properties of glass. The proportional value of the 

 required correction, in my case about T oVo °^ the weight of the 

 hydrogen, will be for spherical globes proportional to a/t, where 

 a is the radius of the globe, and t the thickness of the shell, 

 or to V/W, if V be the contents, and W the weight of the glass. 

 This ratio is nearly the same for Professor Cooke's globe and for 

 mine ; but the much greater departure of his globe from the spherical 

 form may increase the amount of the correction which ought to be 

 introduced. 



In the estimates now to be given, which must be regarded as pro- 

 visional, the apparent weight of the hydrogen is taken at 0*15804, so 

 that the real weight is 0'15860. The weight of the same volume of 

 oxygen under the same conditions is 2*5186 + 0*0006 = 2*5192. The 

 ratio of these numbers is 15*884. 



The ratio of densities found by Regnault was 15*964, but the 

 greater part of the difference may well be accounted for by the omis- 

 sion of the correction just now considered. 



In order to interpret our result as a ratio of atomic weights, we 

 need to know accurately the ratio of atomic volumes. The number 

 given as most probable by Mr. Scott in May, 1887,* was 1*994, but 

 he informs me that more recent experiments under improved condi- 

 tions give 1*9965. Combining this with the ratio of densities, we 

 obtain as the ratio of atomic weights — 



2x15*884 



1*9965 



= 15-912, 



It is not improbable that experiments conducted on the same lines, 

 but with still greater precautions, may raise the final number by one 

 or even two thousandths of its value. 



The ratio obtained by Professor Cooke is 15*953 ; but the difference 



# Loc. cit. 



