142 Dr. J. H. Gladstone. The Relation between the 



The most notable change from previous tables is the increase in 

 the value of hydrogen and the decrease in that of carbon, but the 

 necessity of this has been gradually recognised by the principal 

 workers on the refraction of organic bodies. This in no way affects 

 the well-determined value CH 2 = 7'6. 



It should be borne in mind that the specific refraction cannot 

 claim a constancy equal to that of the atomic weight. The latter is 

 generally believed to be identical under all circumstances, though 

 the element may be capable of combining with another in two or more 

 multiple proportions. On the other hand, several of the elements, 

 as oxygen and iron, exhibit two or more specific refractions, 

 which are not in multiple proportion, but depend upon the manner 

 of combination. The best recognised of these are given in the third 

 column, and the existence of others is indicated by an " &c." Beside 

 these well-marked differences, there are many smaller variations, 

 scarcely, if at all, beyond the limits of experimental error, which 

 depend upon differences of physical condition or chemical structure. 

 The numbers given in column 3 are therefore subject to an uncer- 

 tainty, which may in some instances amount to 5 per cent. Where 

 there is a greater divergence among the values observed, or where 

 the deductions have been made from only one specimen, it is indi- 

 cated by a query. 



PART II. The Relation between the Specific Refraction and the 

 Combining Proportion of the Metals. 



In the paper " On the refraction equivalents of the elements " 

 previously referred to, it was shown that if the metallic elements be 

 arranged in the order of their specific refractions, they are roughly 

 in the inverse order of their combining proportions. 



In the lecture at the Royal Institution, I showed that this inverse 

 order followed an approximate law, namely, that the " specific 

 refractive energy of a metal is inversely as the square root of its 

 combining proportion." This generalisation was proved for uni- 

 valent metals, the figures showing (with the exception of sodium) a 

 practically constant value for the product of the specific refraction 

 and the square root of the combining proportion. 



By the aid of the table in the first part of this communication, the 

 generalisation can now be tested throughout the whole range of the 

 metallic elements. 



