456 Messrs. Threlfall and Pollock on some 



internal diameter. A phosphorus-pentoxide tube was included 

 just above the fall-tube. The whole apparatus, including the 

 pentoxide tube, was repeatedly heated by a Bunsen burner 

 and exhausted until the discharge preferred to pass through 

 3 cm. of air-gap rather than through the tube. So sensitive, 

 however, was this means of testing the vacuum that for the 

 first few days, despite frequent heatings and puin pings, the 

 vacuum would not remain constant for more than some 

 minutes. After about a fortnight of heating and pumping, 

 however, the vacuum became so steady that the change in 

 twelve hours, which was sufficient to entirely stop the discharge 

 in the spark-gap, could be rectified by the fall of at most 

 50 drops of mercury in the fall-tube — i. e. by about 30 seconds' 

 pumping. The change of vacuum occuring during an hour 

 could still be easily detected by testing with a current in 

 alternate directions. 



Under these circumstances, experiments were made b 

 urging Pontgen tubes to their utmost, almost in contact wit 

 the bulb of the exhausted tube, and directing their radiation 

 on to the platinum-foil. Though many very active tubes were 

 fused or otherwise destroyed, yet during an hour's action on 

 several occasions no change of vacuum in the exhausted tube 

 could be detected when due allowance for the slight progressive 

 deterioration was made. 



This experment was repeated several times, and a tube 

 which had successfully exhibited the fluorescence of a screen 

 of tungstate of calcium through an aluminium plate *7 mm. 

 thick to an entire audience, was melted down in the operation ; 

 but no effect whatever was observed. 



Professor Wright, of Yale, has given some reasons for 

 thinking that Pontgen rays when passing through gold-leaf 

 carry particles (of gold ?) off with them ; so that if a positive 

 effect had been obtained in the experiment described it would 

 not have been quite conclusive ; neither is the negative effect 

 observed conclusive against any particles being carried for- 

 ward ; it only shows that the particles so carried (if any) 

 either refuse to behave as gaseous particles, or are exceedingly 

 few in number. 



The experiments of Professor Minchin which are now 

 available have, however, rendered the solid particle theory so 

 unlikely that it is hardly worth while pushing the investiga- 

 tion further — at all events until much more powerful appli- 

 ances are to hand. In case the experiment appears worthy 

 of repetition, we would call attention to the fact that it appears 

 desirable to use specially purified phosphorus pentoxide in 

 the drying-tube ; for the presence of the vapour of phosphorus 



