148 ON THE REFRACTIVE INDICES OF GASEOUS POTASSIUM, ETC. 



and xenon (i.e., as 2 : 3 : 5) the table has been rearranged so as to bring these together 

 in such a way that a single factor is characteristic of each row.* 



From such a table we should expect the refractivities of selenium and tellurium to 

 be to that of sulphur as 3 and 5 respectively to 2. The values found are in both 

 cases rather low, but that for selenium is not notably so. In the other cases the 

 regularity breaks down. The value for arsenic is much nearer -f- that of phosphorus 

 than f. The values for zinc do not bear to those for cadmium the ratio 3 : 5, while 

 the refractivity of mercury is actually less than that of either, thus exhibiting the 

 only case yet observed of a gaseous refractivity which is lower than that of another 

 element of the same group and of lower atomic weight. 



In short, it may be said that the rule of simple integral ratios between their 

 refractivities is probably confined to the elements lying near the centre of the table 

 which are shown enclosed within the heavy rectangular boundary. To these we 

 may possibly add the group potassium, rubidium, caesium, in which the refraction 

 equivalents seem to indicate conformity to the rule. Within the heavily marked 

 enclosure the regularity is very striking, except for the two, or possibly three, 

 elements in the lower left-hand corner. 



A general survey of the table, with the new additions, confirms the view already 

 held that, in each group of elements, refractivity increases witli atomic weight. But 

 mercury, which stands in a lower horizontal row than the other elements examined, 

 forms a striking exception to this rule, and it remains to be seen whether this is 

 merely an isolated case, or whether it is characteristic of all the heaviest elements. 

 The refraction equivalents of GLADSTONE lend some colour to the conjecture that 

 there may be a falling off in refractive power when the atomic weight exceeds a 

 certain limit, but, in view of the discrepancies exhibited in Table VII., it would be 

 dangerous to place much reliance on their indications. It is also remarkable that in 

 each horizontal row of Table VIII. refractive power increases as we move to the left, 

 in spite of the decrease in atomic weight ; there can be little doubt, therefore, that it- 

 is intimately connected with the valency. 



We have to express our cordial thanks to Professor TROUTON and the staff of the 

 Physical Laboratory at University College, London, for assistance and advice, and to 

 the Royal Society for a grant in aid of the research. 



* This arrangement was first suggested by Professor A. W. POUTER. It will bo observed that it fits in 

 very well with the sequence of the atomic volumes. 



