282 Mr. W. Sutherland on 



whether conductivity is increased by radiation with Rontgen 

 rays. Experiments on liquid dielectrics should be easy enough. 

 One of Rontgen's observations is of special importance. He 

 found that air, through which Rontgen rays are passing, 

 emits Rontgen rays ; and this is exactly what our theory 

 would indicate, because, as we saw in discussing reflexion, 

 each atom scatters our small waves as a reflecting sphere dis- 

 tributes ordinary light. 



The remaining important positive facts concerning Rontgen 

 rays relate to their absorption in passing through different 

 substances. Our short waves in passing through a unit cube 

 of substance in a direction parallel to one of the edges, while 

 passing along the molecular interspaces, will be falling at 

 intervals directly on opposing surfaces of atoms; and if n be 

 the number of atoms per unit volume, and It the radius of 

 each, the quantity of surface encountered by unit area of 

 wave-front will be proportional to ?itR 2 , and the number 

 of encounters in passing unit distance will be proportional to 

 ni, so that as regards amount of encounter of wave-front with 

 atoms the energy absorbed by the atoms will be proportional 

 to 7?R 2 . But if the effectiveness of a collision in causing ab- 

 sorption from a given area of wave-front in a given time is 

 also proportional to the density of the matter in the atom, 

 that is to m/R 3 , as we had to suppose in discussing the col- 

 lisions of electrons and atoms, then the absorption of energy 

 from our short waves in passage through unit length of 

 different substances will be proportional to n?///R, that is to 

 the density and inversely to atomic radius as with Lenard 

 rays. The fact that Rontgen rays produce powerful fluores- 

 cence in certain substances shows that there are special re- 

 sonance phenomena that must be expected to produce decided 

 variations in absorption from the simple form just discussed ; 

 but the fact remains, as discovered by Rontgen, that by far 

 the most important factor in the absorption of Rontgen rays 

 is density. Benoist (Compt. Rend, cxxiv.) has found that 

 the absorption of Rontgen rays by certain gases is propor- 

 tional to the density, the factor of proportionality being 

 nearly the same as for solids such as mica, phosphorus, and 

 aluminium, though rising to a value six times as great in the 

 case of platinum and palladium ; density is the prevailing, 

 but not the only, property which determines the absorption 

 of the Rontgen rays. But under different circumstances 

 Rontgen-ray apparatus gives out rays of very different absorb- 

 ability, or, as it is usually expressed, of different penetrative 

 power. Thomson's theory of the Rontgen rays, as thin elec- 

 tromagnetic pulses, does not seem to offer any feasible ex- 



