112 Dr. L. Silberstein on Molecular 



the sake of shortness of language, we shall speak of S t 

 simply as of the substance C - C. Similarly, let S 2 be the 

 substance = 0, with " double bond." Let, as above, iV' be 

 the molecular refractivity of the former, and JS" that of the 

 latter, say, for sodium light, that is, by Landolt-Bornstein's 

 Table (all JV's being in cm. 3 gr. -1 ), 



^' = 4*836; AN = 1-733 ; N" = 6*569, . . (A) 



always assuming, of course, that these cases are such as not 

 to call for other extra-terms besides that due to the double 

 bond. Next, let iV be the atomic refractivity of carbon. 

 At first one might be tempted to take for N the of that 

 table, i. e. 2*418 (the half of iV'), because this is the smallest 

 carbon- refractivity quoted by chemists. But, apart from 

 other reasons, there is a considerably smaller one which, 

 moreover, seems to have better claims to be the atomic 

 refractivity of carbon than 2*418, which is derived from 

 rather complicated organic compounds. In fact, it has 

 seemed interesting to see w T hat that refractivity is like when 

 it is calculated from purest carbon as found in nature, 

 viz. from diamond. Now, Martens (as quoted in Landolt- 

 Bornstein's Tafeln) finds for the refr. index of diamond, 

 for the D-line (\=589), at 14° 0., the value ^ = 2*4172, 

 and the density of that crystal can be taken equal to 3*51. 

 From these data I find 



4*8429.12 _ fl111 

 iVdiam ~ 7*8429. 3*51 ~ 2 



This is as much as 0*307 below the chemist's fundamental 

 C-value. I am fully aware that this circumstance is not, 

 by itself, a sufficient reason for taking the diamond-value as 

 the true atomic refractivity of carbon. For one could speak 

 here of "depression" due to the mutual linkage of the 

 carbon atoms in diamond. According to Bragg* each car- 

 bon atom in diamond is "linked up to the four carbon atoms 

 surrounding it " and placed at the corners of a regular 

 tetrahedron whose centre is occupied by the atom in 

 question, — this being the simplest way of considering the 

 originally double-lattice structure of the crystal as revealed 

 by the X-ray spectrometer. Thus, the difference 0*307 

 could well be considered as a negative " exaltation " due 



* See W. H. and W. L. JBragg's admirable book on 'X Bays and 



Crystal Structure,' London, Bell (1915), p. 106. 



