210 Professor Silvanus P. Thompson [May 8, 



metals. I have found zinc, magnesium, aluminium, copper, iron and 

 platinum to answer — tlie last two best.* Mr. Porter, of University 

 College, and Mr. Jackson, of King's College, have independently found 

 out the merits of platinum foil, the former using an old Crookes tube 

 designed for showing the heating effect of the kathode discharge when 



Fig. 10. 



concentrated by a concave kathode. On the table are some of the 

 experimental forms | of tubes I have used. The best results are 

 found when the kathodic discharge is directed against an interior 



* [The author has since found metallic uranium to surpass all other metals.] 

 t See 'Philosophical Magazine,' August 1896, p. 1(32. 



