198 Lord Kelvin on the Weights of Atoms. 



above, is calculated with this value of iV T ; but it is not 

 improbable that the true value o£ N may be considerably 

 greater than 10 20 *. 



§ 51. As compared with the value for argon, monatomic, 

 the smaller values of q for the diatomic gases, nitrogen and 

 oxygen, and the still smaller values for carbonic acid, triatomic, 

 are quite as might be expected without any special considera- 

 tion of law of force at different distances between atoms. It 

 seems that the diatomic molecules of nitrogen and oxygen 

 and still more so the triatomic molecule of carbonic acid, are 

 effectively larger when moving freely in the gaseous condition, 

 than when closely packed in liquid or solid assemblage. 

 But the largeness of q for the diatomic hydrogen is not so 

 easily explained.: and is a most interesting subject for 

 molecular speculation, though it or any other truth in nature 

 is to be explained by a proper law of force according to 

 the Boscovichian doctrine which we all now accept (many 

 of us without knowing that we do so) as the fundamental 

 hypothesis of physics and chemistry. I hope to return to 

 this question as to hydrogen in a crystallographic appendix. 



I am deeply indebted to Professor Dewar for information 

 regarding the density of liquid hydrogen, and the densities 

 of other gases, liquefied or frozen, which he has given me at 

 various times within the last three years. 



[To be continued.] 



* Maxwell, judging from ll molecular volumes " of chemical elements 

 estimated by Lorentz, Meyer and Kopp, unguided by what we now know 

 of the densities of liquid oxygen and liquid hydrogen and of the liquid of 

 the then undiscovered gas argon, estimated ^=-19 . 10 20 (Maxwell's 

 Collected Papers, vol. ii. p. 350) which is rather less than one-fifth of my 

 estimate 10 20 . On the same page of his paper is given a table of estimated 

 diameters of molecules which are about 3*2 or 3'3 times larger than my 

 estimates in col. 6 of the table in § 47. In a previous part of his paper 

 (p. 348) Maxwell gives estimates of free paths for the same gases, from 

 which by his formula (7), corrected as in col. 8 of my table in § 47, 1 find 

 values of .ZV ranging from 6-05 . 10 l8 to ^96 . 10 1S or about one-third of 

 •19 . 20 20 . His uncorrected formula k/Vtv (instead of */2 . it) gives values 

 of N which are V 73 " times, or 1*77 times as great, which are still far 

 short of his final estimate. The discrepance is therefore not accounted 

 for by the error in the formula as printed, and I see no explanation 

 of it. The free paths as given by Maxwell are about 1'3 or T4 times as 

 large as mine. 



