at the end of the process, a mass comparable with that of the parent atom.
It will be shown later that a process of disintegration, very similar to that already described for thorium, must be supposed to take place also in uranium, actinium and radium. The full discussion of this subject cannot be given with advantage until two of the most important products of the three substances thorium, radium and actinium, viz. the radio-active emanations and the matter which causes excited activity, have been considered in detail.
137. Magnitude of the changes. It can be calculated by
several independent methods (see section 246) that, in order
to account for the radio-activity observed in thorium, about
3 × 10^4 atoms in each gram of thorium suffer disintegration
per second. It is well known (section 39) that 1 cubic centimetre
of hydrogen at atmospheric pressure and temperature
contains about 3·6 × 10^{19} molecules. From this it follows that
one gram of thorium contains 3·6 × 10^{21} atoms. The fraction
which breaks up per second is thus about 10^{-17}. This is an
extremely small ratio, and it is evident that the process could
continue for long intervals of time, before the amount of matter
changed would be capable of detection by the spectroscope or
by the balance. With the electroscope it is possible to detect
the radiation from 10^{-5} gram of thorium, i.e. the electroscope
is capable of detecting the ionization which accompanies the
disintegration 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 case 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 need to continue thousands of years
before it could be detected by the balance or the spectroscope. It
is thus evident that the changes occurring 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
observed by direct chemical methods.
