i33 
its energy at a rate proportional, approximately, to the 
inverse square root of its speed. 
3. Determinations of the loss of range of alpha particles 
in consequence of their passage through various substances, 
from which it appeared that for all the materials examined 
the loss in traversing any atom was nearly proportional to 
the square root of the weight of the atom. The loss in the 
case of a complex molecule was proportional to the sum of 
the square roots of the weights of the constituent atoms. The 
results were presented in the following table : — 
Table, showing "'stopping power" of various atoms and 
molecules, as compared to air. The atomic weight 
of the imaginary standard atom of air is taken as 
14'4, and the atomic square root as 3"79 : — 
Ratio of 
Ratio of 
Substance. 
stopping- 
Power. 
Atomic or 
Molecular 
Atomic or 
Molecular 
Square Roots. 
Weights. 
Hydrogen 
•246 
•265 
•069 
Air 
1 
1 
1 
Aluminium 
. 1-53 
1-38 
1-88 
Copper 
. 2-42 
2-1 
4-53 
Silver 
. 3-12 
2-75 
7-5 
Tin 
. 3-42 
2^88 
8-2 
Platinum 
. 4-12 
3-7 
13^5 
Gold 
. 4-45 
3^7 
13^7 
Methyl bromide 
. 2-09 
2-09 
3^28 
Ethyl chloride 
. 2-30 
2-36 
2-23 
Methyl iodide 
. 2-49 
2-35 
4^9 
Ether 
. 3-30 
3-68 
2-56 
Carbon tetrachloride .. 
. 3-8 
3-61 
5^41 
When these results were plotted, the metals and gases seemea 
to lie on rather different lines. 
4. Discussion of these results. The authors suggested as 
a possible explanation that, if atoms had a disc-like form (see 
Meyer's Kinetic Theory of Gases, § 112), and if ions could only 
be produced on the edges of the discs, then the chances of 
ionisation by an alpha particle traversing any atom would be 
proportional to the square root of the atomic weight. This 
explanation involved the assumption that the energy required 
to produce a pair of ions was a constant under all conditions, 
as stated by Rutherford. The authors believed that this 
assumption was correct, in spite of the fact that in some of 
their experiments on gases with complex atoms the alpha 
particle did not produce as much total conductivity as in air, 
and they suggested, as an explanation of the apparent con- 
tradiction, that ions made in complex molecules sometimes 
re-combined before getting free of the molecules. 
