[eve] IONS PRODUCED BY THE BETA RAYS 77 
If that is so, 3-4 x 10” g particles from one curie of radium C are 
producing about 4 x 10" ions, and therefore one particle produces in an 
average path 1-2 x 10*ions. As the average path is 1/-004, or 250 cm., 
it follows that a B particle produces about 48 ions per centimetre.* If 
the radius of a molecule is taken as 10-8 em., and the number of mole- 
cules in a cm.’ of the atmosphere as N — 2-7 x 10”, it follows, from the 
formula of Clausius, that the mean path between collisions by a B 
particle with air molecules, at atmospheric pressure is 
I= 1/ N?ro = 1/8500 cm. 
so a B particle ionizes only about 48 ions out of every 8,500 molecules 
which it directly encounters. Unless the 8 particle is an adept at 
swerving, it must pass right through 180 molecules for every one which 
it succeeds in ionizing. Or, on the other hand, if every collision is 
supposed successful in ionizing, the radii of the molecules would have 
1 
to be as small as 8x10 em. 
If the supposition were made that a y ray is an entity with a linear 
path, and if it were further assumed that 3-410" such entities were 
ejected from a curie of radium C every second, it follows that one such 
entity, on an average path of 250 metres, makes 10-2X10“4/3-4X10", 
or 30,000 ions or 1-2ions per em. of path. In this case, only one collision 
in every 7,000 is effective in producing an ion. 
In Townsend’s work on “Ionization of Gases by Collision,” it 
appears to be proved that at low pressures practically every encounter 
of a negative ion is effective in ionizing, provided the velocity exceeds 
a certain limit. It seems certain, therefore, that the 8 particle pierces 
the molecule in its flight, and the ionization results only in about two 
per cent of the collisions. 
Heating Effect. 
It has been shown by Rutherford and Geiger f that the a particles 
from a gramme of radium and from its three succeeding 4 ray products, 
produce 244 x 10!# ions per second. The heating effect is about 100 
calories per hour, the greater part of which is due to the @ particles. It 
appears from recent work by Geiger ? that the & particles expend 
nearly the whole of their energy in ionization. It is thus possible to 
* Cf. Durack, Phil. Mag., May, 1903, who found a mean path of 6 em. for a B 
particle in air at 2 mm. pressure. 
{ Proc..R.S:, A..81,:141. 
{t Proc. KR. S., A. 83, .505. 
