[1] Being the Huxley Lecture, delivered at the University of
Birmingham on October 30th, 1912. Bedrock, Jan., 1913.
214
free electric charges becomes a conductor of electricity by
convection when an electromotive force is applied. The gas also
acquires other properties in virtue of its ionisation. Under
certain conditions it may acquire chemical activity and new
combinations may be formed or existing ones broken up. When its
initial velocity is expended the helium atom gives up its
properties as an alpha ray and thenceforth remains possessed of
the ordinary varying velocity of thermal agitation. Bragg and
Kleeman and others have investigated the career of the alpha ray
when its path or range lies in a gas at ordinary or obtainable
conditions of pressure and temperature. We will review some of
the facts ascertained.
The range or distance traversed in a gas at ordinary pressures is
a few centimetres. The following table, compiled by Geiger, gives
the range in air at the temperature of 15° C.:
cms. cms. cms.
Uranium 1 - 2.50 Thorium - 2.72 Radioactinium 4.60
Uranium 2 - 2.90 Radiothorium 3.87 Actinium X - 4.40
Ionium - 3.00 Thorium X - 4.30 Act Emanation 5.70
Radium - 3.30 Th Emanation 5.00 Actinium A - 6.50
Ra Emanation 4.16 Thorium A - 5.70 Actinium C - 5.40
Radium A - 4.75 Thorium C1 - 4.80
Radium C - 6.94 Thorium C2 - 8.60
Radium F - 3.77
It will be seen that the ray of greatest range is that proceeding
from thorium C2, which reaches a distance of 8.6 cms. In the
uranium family the fastest ray is
215
that of radium C. It attains 6.94 cms. There is thus an
appreciable difference between the ultimate distances traversed
by the most energetic rays of the two families. The shortest
ranges are those of uranium 1 and 2.
The ionisation effected by these rays is by no means uniform
along the path of the ray. By examining the conductivity of the
gas at different points along the path of the ray, the ionisation
at these points may be determined. At the limits of the range the
ionisation
{Fig. 13}