104 Scattering and Reflexion of Light by Gas Molecules. 



assume as little as possible regarding the processes of radia- 

 tion; the activity of a radiating molecule has been specified 

 merely by the disturbance produced at a distance, and it 

 may be remarked that the validity of the results is not 

 directly conditioned by the smallness of the molecules against 

 the wave-length involved; but it has been frequently as- 

 sumed that the molecules are distributed approximately as 

 in an ideal gas, and thus many of the formulae would fail if 

 the linear dimensions of the molecules were comparable with 

 their mean free path. The influence of the Doppler effect 

 and of collisions between molecules must be very small in 

 pure mercury vapour at room temperature ; this is indicated 

 by elementary gas-theory, and is confirmed by the experi- 

 mental evidence for the extreme narrowness of the resonance 

 line 2536. 



62. More doubt may be felt as to the assumed absence of 

 true absorption in the pure attenuated vapour. The assump- 

 tion is supported on general grounds by Wood, who insists, 

 however, on the urgent need for verification. And here I 

 venture to suggest that, provided sufficient sensitiveness can 

 be realized, a satisfactory way of testing for absorption of 

 radiation is to determine the value (whether zero or finite) 

 of some effect to which absorption would directly give rise. 

 Absorbed radiation will, in fact, heat the vapour and any 

 air that may be mixed with it, thus causing expansion ; and 

 there seems to be reason for supposing that the sensitiveness 

 obtainable on these lines would be abundant for the purpose 

 in view. The form of apparatus proposed is indicated in 

 plan in the diagram; it was suggested by F. W. Jordan's 



paper * " On a new type of thermo-galvanometer." A is a 

 little flask of fused silica, whose neck projects into, and is 

 fused to, the bulb B. C is a small thin circular disk, pre- 

 ferably of very thin fused silica, platinized. This is suspended 



* Phys. Soc. Proc. vol. xxvi. part iii. (April 1914). 



