404 Conduction in Solutions and Gases, 



furnished by the ionic theory is that in all three cases the gas 

 contains ions which act as centres of condensation for the 

 vapour. 



During the year which followed their discovery, the X-rays 

 were so thoroughly examined that at the end of that period 

 they were almost better understood than the cathode rays 

 from which they derived their origin. But the obscurity in 

 which this subject had been so long involved was now to be 

 dispelled. 



Lecturing at the Eoyal Institution on April 30th, 1897, 

 J. J. Thomson advanced a new suggestion to reconcile the 

 molecular- torrent hypothesis with Lenard's observations of the 

 passage of cathode rays through material bodies. " We see 

 from Lenard's table," he said, " that a cathode ray can travel 

 through air at atmospheric pressure a distance of about half a 

 centimetre before the brightness of the phosphorescence falls to 

 about half its original value. Now the mean free path of 

 the molecule of air at this pressure is about 10~ 5 cm., and if a 

 molecule of air were projected it would lose half its momentum 

 in a space comparable with the mean free path. Even if we 

 suppose that it is not the same molecule that is carried, the 

 effect of the obliquity of the collisions would reduce the 

 momentum to half in a short multiple of that path. 



" Thus, from Lenard's experiments on the absorption of the 

 rays outside the tube, it follows on the hypothesis that the 

 cathode rays are charged particles moving with high velocities 

 that the size of the carriers must be small compared with the 

 dimensions of ordinary atoms or molecules.* The assumption 

 of a state of matter more finely subdivided than the atom of 

 an element is a somewhat startling one ; but a hypothesis that 

 would involve somewhat similar consequences viz. that the 

 so-called elements are compounds of some primordial element 

 has been put forward from time to time by various 

 chemists." 



* A similar suggestion was made by E. Wiechert, Verhandl. d. physik.-ocon. 

 Gesellscb. in Konigsberg, Jan. 1897. 



