Ionization of Gases and the Ahsorjjtion of Bontgen Rays. 315 
'Theory of Matter, 1916, p. 504), Moore has extended the law of absorption 
of X-rays by matter. 
The law of absorption as it stands for solids is— 
Kco =z QN^At (i) 
Where K is mass absorption co-efficient of a radiation of wave length 
A in a substance whose atomic number is N, and lo is the actual mass of an 
atom of that substance and Q is a constant. It should be noticed that the 
assumption involved is that all the energy absorbed from the primary rays 
goes into the production of the corpuscular rays, and just as ionization is a 
secondary effect so is the production of secondary rays a secondary effect. 
We must suppose that the processes involved during the transmission of 
X-rays through gases are exactly analogous to those in solids ; that the 
emission of corpuscles by solids is not a surface effect — indeed it cannot be 
since thin plates of substances emit corpuscles both on their incidence and 
emergence sides — but that direct and secondary " ionization " takes place 
right through the solid just as it does through a gas. Some years ago 
an attempt was made to detect if there was any increase in the electrical 
conductivity of a solid during the passage of X-rays, but the results were 
vitiated by the direct heating effect due to the absorption of X-ray energy. 
In the experiments described above the potential difference between the 
walls of the ionization chamber and the electrode was sufficient to produce 
saturation current, consequently there was no recombination of the ions. 
The rapidly-moving (3 particle liberated within the solid loses its energy in 
the same manner as the (3 particle in the gas, but nothing is known of the 
" corpuscular factor " for the solid. It is known, however, that the higher 
the atomic weight of the solid the more copious is the emission of corpuscles 
from its surface, and that this emission is greatly increased when the 
primary radiation falls at grazing incidence. Moreover, the intensity of 
the secondary radiation around a radiator is uniform in all directions, 
pointing quite definitely to the conclusion that its production is a secondary 
effect, whilst the distribution of the scattered radiation — that radiation 
similar in type to the primary radiation — follows the law of scattering, and 
is polarized in the same plane as the primary radiation. These facts which 
I have brought together point definitely, I think, to the conclusion that the 
emission of characteristic secondary rays rests for by far the greatest part 
with the atoms which are ionized by collision within the substance, and that 
true X-ray absorption consists of an interchange of energy between the 
ether waves and the ionizing corpuscle. If more were known about the 
position of the orbital planes of the atoms in a crystal, probably the 
explanation of the point mentioned by Bragg (X-rays and Crystal Structure, 
1915, p. 198) would be forthcoming, viz. that the intensity of X-ray reflection 
of a given wave-length from a crystal, regardless of the crystal planes 
