x] RADIO-ACTIVE PROCESSES B31 
that all elements are built up out of hydrogen, is an example of 
this point of view of regarding the subject. 
On the disintegration theory, the changes occurring in the 
radio-atoms involve an actual transformation of the atoms through 
successive changes. This change is so slow in uranium and thorium 
that at least a million years would be required before the amount 
of change is measurable by the balance. In radium it is a million 
times faster, but even in that case it is doubtful whether any 
appreciable change would be observed by ordinary chemical methods 
for many years, if the possibility of such a change had not been 
suggested from other lines of research. 
202. Amount of helium from radium. The appearance of 
helium in a tube containing the radium emanation may indicate 
either that the helium is one of the final products, which appear 
at the end of the series of radio-active changes, or that the helium 
is in reality the expelled « particle The evidence at present 
points to the latter as being the more probable explanation. In 
the first place, the emanation diffuses like a gas of heavy molecular 
weight, and it appears probable that, after the expulsion of a few 
a particles, the atomic weight of the final product is comparable 
with that of the emanation. On the other hand, the value of e/m 
determined for the projected @ particle points to the conclusion 
that, if it consists of any known kind of matter, it is either 
hydrogen or helium. 
If the « particles, when released, can exist in the gaseous state, 
an estimate can readily be made of the volume of the total number 
of particles liberated per year. It has been calculated that one 
gram of radium expels about 10" « particles per second. Since 
there are 3°6 x 10" molecules in one cubic centimetre of any gas 
at standard pressure and temperature, the volume of the « particles 
released per second from 1 gram of radium is 2°8 x 10~c.c. and 
per year 90 cubic millimetres. 
It has already been shown that the emanation released from 
1 gram of radium in a state of radio-active equilibrium is probably 
about 3 x 10-4c.c. Since the emanation passes through at least 
three stages, each of which gives rise to a rays, the volume of the 
a particles from the emanation, released from 1 gram of radium, is 
about one cubic millimetre. 
