286 SCIENCE PROGRESS 



much light would be thrown on the nature of the atom. 

 Unfortunately there is not much hope that many more indices 

 will be brought within our knowledge directly, as the vapour 

 pressures of the elements are too low at temperatures at which 

 even a quartz refractometer tube softens. The alkali elements 

 still remain to be the prize of whoever will find a material by 

 which they are not attacked at high temperature. Iron tubes 

 closed with plates of white sapphire would probably do ; but 

 the difficulties are considerable. The less volatile elements 

 zinc, cadmium, arsenic, selenium, and tellurium should certainly 

 be repeated with greater accuracy; and chlorine, bromine, and 

 iodine should be remeasured. It is possible that by a special 

 effort gallium, indium, germanium, tin, lead, and bismuth might 

 be secured. 



The investigation of the indices of gaseous compounds, with 

 the object of discovering the connection between the refractivity 

 of a compound and that of its constituents, is a branch of the 

 inquiry which promises to throw a great deal of light on the 

 nature of chemical combination. Refractivity, as has been 

 pointed out above, is probably in some manner intimately con- 

 nected with the number of electrons which help to constitute 

 the atom, and, as will be shown later, with the period of their 

 free motions with reference to the atom. In proportion as the 

 frequency of the light waves which impinge on the atom 

 approaches that frequency which is natural to the electrons, a 

 large part of the energy of light is absorbed in setting in motion 

 or in increasing the motions of the electrons. In so far as the 

 medium absorbs light, the loss of energy falls upon the ampli- 

 tude of the vibrations. In so far as it is transparent, it falls 

 upon the wave length, or in other words on the refractivity. 

 If it were found that the refractivity of a compound were the 

 sum of the refractivities of its constituents, the inference would 

 be that each atom preserved in the compound its whole 

 individuality and freedom of movement unchanged by the 

 proximity of that with which it was combined. But if it could 

 be shown that the refractivity of the compound is materially 

 different from this, it follows that to this extent combination has 

 altered the factors on which the refractivity of the atom depends, 

 by altering the position of the electrons, or the forces which 

 control their movements. It cannot be doubted that these 

 forces are complex ; and it is, therefore, to be expected that only 



