148 



Lord Rayleigh. 



fMar. 23, 



The evaluation of the densities as compared with water is exposed 

 to many sources of error which do not affect the comparison of one 

 gas with another. It may therefore be instructive to exhibit the 

 results of various workers referred to air as unity. 





Oxygen. 



Nitrogen. 



Hydrogen. 



1 -10562 

 1 -10502 

 1 -1050 

 1 -10535 



0-97138 

 -97245 

 -9720 

 -97209 



-06949 



-06947 

 -06960 



1 -10525 



-97218 



-06952 



As usually happens in such cases, the concordance of the numbers 

 obtained by various experimenters is not so good as might be 

 expected from the work of each taken separately. The most serious 

 discrepancy is in the difficult case of hydrogen. M. Leduc suggests* 

 that my number is too high on account of penetration of air 

 through the blow-off tube (used to establish equilibrium of pressure 

 with the atmosphere), which he reckons at 1 m. long and 1 cm. in 

 diameter. In reality the length was about double, and the diameter 

 one-half of these estimates ; and the explanation is difficult to main- 

 tain, in view of the fact, recorded in my paper, that a prolongation 

 of the time of contact from 4 m to 30 m had no appreciable ill effect. 

 It must be admitted, however, that there is a certain presumption in 

 favour of a lower number, unless it can be explained as due to an 

 insufficient estimate for the correction for contraction. On account 

 of the doubt as to the appropriate value of this correction, no great 

 weight can be assigned to Regnault's number for hydrogen. If the 

 atomic weight of oxygen be indeed 15*88, and the ratio of densities 

 of oxygen and hydrogen be 15'90, as M. Leduc makes them, we 

 should have to accept a much higher number for the ratio of volumes 

 than that (2 "0002) resulting from the very elaborate measurements 

 of Morley. But while I write the information reaches me that 

 Mr. A. Scott's recent work upon the volume ratio leads him to just 

 such a higher ratio, viz., 2'00245, a number a priori more probable 

 than 2*0002. Under the circumstances both the volume ratio and 

 the density of bydrogen must be regarded as still uncertain to the 

 l/1000th part. 



* ' Comptes Rendus,' July, 1892. 



