produced by Rontgen Rays in Lead and Zinc, 437 
a weak radium preparation which gave a few scattered 
scintillations ; this was placed near the Rontgen tube and 
observed while the tube was excited. A large Miiller water- 
cooled tube was used (20 cm. in diameter) and a heavy- 
discharge sent through it : the focus was only 2S cm. from 
the lead plate, so that the latter was exposed to very intense 
rays. The air between the lead and the screen would 
have formed at atmospheric pressure, a layer only 0'004 mm. 
thick. 
1 also tried to find out whether Rontgen rays had an 
accelerating effect upon the disintegration of a radioactive 
substance. The active deposit of thorium was used in 
preference to one of the more permanent radioactive sub- 
stances. An aluminium plate, one side of which had been 
exposed to thorium emanation for some hours, was placed over 
a hole in the wall of an electroscope with the exposed side 
inward. A number of measurements of the activity were 
made and then the plate (without being removed) was exposed 
for ten minutes to strong Rontgen rays from the large bulb 
with its focus 20 cm. from the plate. The ionization was 
then measured again several times. The following is an 
example of the results obtained : — 
Activity before exposure 7*56 + '056 
Activity after exposure 7*54 + *076 
Repetitions of the experiment gave similar results. 
Conclusions. 
1. The result previously obtained by the writer that the 
heat generated in lead by Rontgen rays was twice that in 
zinc is not confirmed by further experiments, which show 
that the quantities of heat are equal, with an uncertainty of 
from 5 to 10 per cent. The source of the error was imper- 
fect heat-insulation of the metals ; this escaped the control 
experiment on account of a difference in the coefficients of 
absorption of the surfaces for light, which, by an unfortunate 
accident, was just sufficient to compensate the other 
inequality. 
2 No rays capable of producing scintillations on a zinc- 
blende screen are present among the secondary radiations 
from lead when exposed to Rontgen rays, 
3. The disintegration of the active deposit from thorium 
emanation is not hastened by exposure to Rontgen rays. 
Sloane Laboratory, 
Yale University, New Haven, Conn., 
Dec. 6, 1907. 
