Bumstead — Heating Effects produced by Rontgen Rays. 17 



made with the full aperture of the circular window, so that 

 the ebonite was illuminated as well as the strips ; and also with 

 a cardboard screen in front of the window with a rectangular 

 hole which allowed the light to fall on the strips alone and 

 not on the ebonite. No difference was observed in the two 

 cases. 



5. The observed results cannot be due to electrostatic effects 

 produced by the negative corpuscles of the secondary rays. 

 The strips were earthed, and the two vanes were connected 

 by a conductor ; under these circumstances, no electrostatic 

 effect could be produced except an instability due to both vanes 

 having a different potential from the two strips. Such an 

 instability was frequently observed after the vanes had become 

 electrified by bumping against the strips, but no other electri- 

 cal effect. Moreover, the coincidence of the rates of motion of 

 the radiometer, when illuminated by .Rontgen rays and by 

 ordinary light, shows that the deflections produced by the rays 

 w r ere caused by changes of temperature and not by electrical 

 forces. 



6. The last consideration also excludes the (somewhat remote) 

 possibility that the effect is produced by direct impact of the 

 secondary rays. 



7. It is conceivable that a slight difference in the distance 

 of the lead and zinc strips from the vanes might cause a con- 

 siderable difference in sensitiveness of the two vanes and thus 

 account for the preponderance of the lead. If this were the 

 case, however, the effect should have appeared in the experi- 

 ments with ordinary light. I also tested the effect upon the 

 sensitiveness of varying the vane-distance by taking deflections 

 with the zero near the top and near the bottom of the scale. 

 For a variation in the vane-distance of 0'48 mm the change in 

 sensitiveness was less than 12 per cent. The positions of the 

 surfaces of the lead and zinc strips were afterward tested by 

 means of a point gauge and they certainly did not differ by 

 more than 0'l mm . 



8. It might also be imagined that the measurement of the 

 absorptions by means of the electroscope did not give the 

 relative amounts of energy absorbed by the lead and zinc ; 

 that the ionization in the electroscope was not a measure of 

 the energy of the rays. One may express this possibility in 

 slightly different form by saying that, accompanying the rays 

 which ionize the gas, there may be other rays which do not 

 cause ionization but which do carry energy ; and that these 

 latter may be more absorbed in the lead than in. the zinc. 

 In order to test this, measurements of the absorption were 

 made with the radiometer itself. One strip was shielded so 

 that as large a deflection as possible might be obtained with 



Am. Jour. Sci.— Fourth Series, Vol. XXI, No. 121.— January, 1906. 

 2 



