196 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. VI 
by the balance. With the electroscope it is possible to detect 
the radiation from 10~ gram of thorium, 7.e. the electroscope is 
capable of detecting the ionization which accompanies the disin- 
tegration of a single thorium atom per second. The electroscope 
is thus an extraordinarily delicate means for detection of minute 
changes in matter, which are accompanied, as in the casg of the 
radio-elements, by the expulsion of charged particles with great 
velocity. It is possible to detect by its radiation the amount of 
Th X produced in a second from 1 gram of thorium, although 
the process would probably néed to continue thousands of years 
before it could be detected by the balance or the spectroscope. It 
is thus evident that the changes occurrimg in thorium are of an 
order of magnitude quite different from that of ordinary chemical 
changes, and it is not surprising that they have never been ob- 
served by direct chemical methods. 
