ays very great, attaining
as much as 2 x 109 cms. per second; a velocity which, if
unchecked, would carry the atom round the earth in less than two
seconds. The alpha ray carries a positive charge of double the
ionic amount.

When an alpha ray is discharged from the transforming element
into a gaseous medium its velocity is rapidly checked and its
energy absorbed. A certain amount of energy is thus transferred
from the transforming atom to the gas. We recognise this energy
in the gas by the altered properties of the latter; chiefly by
the fact that it becomes a conductor of electricity. The
mechanism by which this change is effected is in part known. The
atoms of the gas, which appear to be freely penetrated by the
alpha ray, are so far dismembered as to yield charged electrons
or ions; the atoms remaining charged with an equal and opposite
charge. Such a medium of

[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 deg. 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 proceedin