266 Professor Sir J. J. Thomson [April ?>, 



fleeted spot 0, is inversely proportional to the kinetic energy of the 

 particle hittins: the plate at A. This kinetic energy is due to the work 

 done by the electric field in the discbarge tube on the particle on its 

 way to the cathode, and this work will depend on the electric charge 

 and not on the ^^ eight of the particle ; thus different particles if they 

 had the same charge would have the same horizontal displacement, 

 and we find as a matter of fact that the heads of the parabolas 

 corresponding to the different elements, in the great majority of 

 cases, are in the same vertical line. Now let us suppose that the gas 

 we subject to analysis contains some Hg ; this in its course to the 

 cathode will get the'normal amount of kinetic energy. If after passing 

 through the cathode it broke up int'^ H, and Ho and the H., retained 

 the charge, the H., would appear on the Ho parabola ; but the kinetic 

 energy of this particle would not be the normal energy, because when 

 the Hg broke up one-third of its energy would go to H, leaving only 

 two-thirds for the Ho- As the horizontal displacement is inversely 

 proportional to the energy, the displacement of tnese particles will 

 be 1 • 5 times the normal displacement ; and I have here a photograph 

 which shows the Xg line accompanied by a bright spot on the 

 parabola correspondiiig to Ho at 1'5 times the normal horizontal 

 distance. 



I will now consider the question of the production of helium by 

 the bombardment of metals and salts to which I drew your attention 

 at a Friday Evening Discourse a year ago. I have made many experi- 

 ments on this subject since last year, and have found that when 

 helium is produced as the result of bombardment the supply eventually 

 fails when the bomV)ardment is prolonged for a long time. We must 

 remember that the positive ray analysis can detect the helium in one- 

 fifth of a cubic centimetre of air, although there are only about four 

 parts of helium to 100,000 of air. If, therefore, a trace of air were 

 left in the discharge tube, and the nitrogen and oxygen absorbed by 

 the charcoal cooled with liquid air, the helium would be left as a 

 residual gas, and would show itself on the photographs. Thus, when 

 metals were bombarded by cathode rays the helium might come from 

 air condensed on their surfaces and liberated by the bombardment. 

 The case of the bombardment of salts was more difficult, for salts 

 which gave off helium on bombardment would often continue to do 

 so after they had been dissolved in water, and reprecij^itated by 

 evaporating the solution and recrystallising or precipitating the 

 residue. The helium in this case can hardly be an impurity in the salt, 

 and comes, I think, from air imprisoned in the fissures in the crystals 

 or adhering to the surface. This view is supported by the fact that 

 the results of these experiments are very irregular ; sometimes more 

 helium was found after redissolving the salt than before, and some- 

 times bombarding one specimen of a salt several hours daily for a 

 fortnight did not yield more helium than another specimen of the 

 same salt gave after a bombardment lasting for a few hours. On the 



