532 On Molecular Refractivity and Atomic Interaction. 



which is converted by external agents into an electric 

 doublet, and the molecule is a system of such points. We 

 shall, now and then, need the above radii only in order to 

 estimate the kinetic dimensions of molecules composed of 

 these three and or' other atoms, or sometimes, to guide our 

 choice among all the possible configurations of the atomic 

 centres. 



4. Nitric Oxide, NO. 



The observed refractivity of this gas is, in normal 

 conditions, for \ = 0*589 (Landolt-Bornstein's tables, 1912, 

 p. 1019), 



/ u,-l = 2-94.1Q- 4 (Ciithbertson and Metcalfe). 

 Observations for other wave-lengths are not quoted. Taking 

 this value, we have for the molecular refractivity of nitric 



M 



oxide, N=% ^ G&-1), with jkf= 30-01, d = l'U2 . 10" 3 , 



N= 4-382, for X D . 



On the other hand, the atomic refractivities of nitrogen and 

 of oxygen for the same wave-length are, by Table I. and 

 by formula (12), 



N D = 2-114, D = 1'757, 



whence, their sum and their product, 



2 = 3-871, H = 3-714. 



Substituting these values and the observed JV= 4*382 into 

 our general formula (5) for the molecular refractivity of a 

 diatomic gas, we have the quartic equation for s — a/27rR 3 : 



a 4 s 4 — a 3 * 3 — a 2 s 2 + a = 0, .... (26) 

 where 



a ^NIP = 60-430, a 3 = 2n 2 = 27-581, 

 a 2 ^n(5i\ 7 -32) = 38-238, « = 4(iV r -2) = 2-044. 



The only positive root of (26), satisfying the condition of 

 stability, is, to four decimals*, 



s = 0-2221. 

 This gives, for the interatomic distance in NO, 



R= 1-518. lO" 8 (27) 



By (23) the atomic radii r Q and r^ are but slightly different 

 from one another. Thus we can, with sufficient accuracy, 

 take for both atomic spheres the mean 



* Of the remaining- three roots of the quartic equation two are negative, 

 viz. s— —0-2822 and —0-5201, and are thus out of question, and one is 

 positive, s = 1-037, but much too large to satisfy the condition of optical 

 stability of the NO molecule. 



