Dispersion and Size of Molecules of Hydrogen, tyc. 215 

 Table C. 



Before After lubricating 



lubricating. with vaseline. 



( Number of determinations = 9. 9. 

 _ _ _ .j Maximum total resistance = '00248 ohm "00182 ohm. 

 Box &o. V. J Minimum ?> = -00188 „ -00162 „ 



haying < Mean ?> ^ = . 002l7 ^ . 00170 >? 



!7 plugs- j Variation =±'00016 „ ±'00007 „ 



^ Percentage „ = +7'4 ±4*1 



In this case the plugs had been fitted with great care : 

 consequently we find that the variations in the resistance 

 both before and after applying the lubricant are much 

 smaller than in the cases previously cited. 



XIX. Dispersion and the Size of Molecules of Hydrogen^ 

 Oxygen, and Nitrogen. By L. Silberstein, Ph.D., Lec- 

 turer at the University of Rome*. 

 rpHE molecular refractivity of an isotropic substance 

 JL whose molecules consist o£ two equal atoms is given by 



where iV is the atomic refractivity, 



;« = 3m H /l-008 = 4-88 . 10~ 24 gr., 



and R the mutual distance o£ the " centres " of the two 

 atoms composing each molecule, i. e. of the positions o£ 

 equilibrium of the dispersive particles within their atoms f . 

 We shall henceforth use the abbreviation 



" ~ 2izR? 

 and write, therefore, 



(i) 



N: 



= P»[ 2+ I^] ( 2 > 



It will be remembered that the dimensions of 



N=£ 



2 -lM 



fi 2 + 2 d 

 are cm. 3 gr. -1 , so that crN is a pure number. When the 



* Communicated by the Author. 



t Cf. Phil. Mag. vol. xxxiii. (1917), pp. 92-128. Throughout that 

 paper, JR. 3 is to be replaced by 4wll 3 , our charges e, in formula (4) et seq., 

 being in rational units. Keeping this in mind, the above formula will 

 be found identical with (246) of the quoted paper. Another erratum 

 is on p. 113, line 13, repeated in formula (30), where iV in the denomi- 

 nator should read 2iV ; this, however, affects only the numbers of 

 pp. 113-114 concerning a purely fictitious example. The subject will 

 he taken up at a later opportunity in connexion with the real compounds 

 C 2 H 4 and C 2 H 2 . 



