276 SCIENCE PROGRESS 



sidered, that refractivity is a periodic function of the atomic 

 weights of the elements. But the more they are studied the 

 more clearly it appears that Gladstone's figures are not capable 

 of elucidating much further the relation between the constitution 

 of the different atoms, or the nature of chemical combination. 

 The data from which his conclusions are drawn differ widely 

 among themselves, and the figure which he chooses as a constant 

 is often the mean of values which differ by 10 and 12 per cent. 

 Thus his value for potassium, which is the basis of his values 

 for the metals, is the mean of the numbers yg, 8'o, 8 - 2, 8*3, 8"i, 

 7'8> 77, 7'5- Subsequent experiment has shown that whatever 

 difference exists between the refractivity of a compound and the 

 sum of the refractivities of its constituents seldom exceeds 10 per 

 cent, of the whole, and hence values for the elements which may 

 contain an error as large as the whole difference which is being 

 investigated are comparatively worthless. 



The more carefully the figures are examined the more 

 numerous are the discrepancies which have come to light, and 

 the subsidiary hypotheses which have to be made in order to 

 reconcile them. Bri'ihl himself showed that when oxygen is 

 united to carbon its refractive effect is greater than when it is 

 combined with two different elements. Conrady confirmed the 

 existence of this distinction, and added a new one, that of oxygen 

 in the position which it occupies in the ethers. Gladstone 

 found that for weak acids the value for hydrogen must be taken 

 as i'4, while for those which are much ionised he gives 3*4 to 3*5. 

 Le Blanc found two distinct values for chlorine. Wiedemann 

 found two for sulphur. Nasini found a great variation in the 

 results he obtained. The refraction equivalents of nitrogen are 

 very anomalous. 



More examples might be given, but it would be superfluous. 

 It is evident that the conception of refraction equivalents and 

 the additive law is only a first approximation to the truth. So 

 long as we direct our attention to compounds which are made 

 up of two or three elements, combined in the same way, fairly 

 concordant results can be obtained : but even these are hardly to 

 be trusted beyond two significant figures. When we apply the 

 additive rule to the elements in general, it is only by courtesy it 

 can be called a success. 



The reason of this failure is not far to seek. The true 

 refractive constant of an element is, evidently, the retardation 



