316 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
A highly remarkable fact was found out by Bragg. The effect of 
the atom traversed by the ray to check the velocity of the ray is 
independent of the physical and chemical condition of the atom. 
He measured the “stopping power”’ of a medium by the distance 
the ray can penetrate into it compared with the distance to which 
it can penetrate in air. The less the ratio the greater the stopping 
power. The stoppmg power of a substance is proportional to the 
square root of its atomic weight. The stopping power of an atom 
is not altered if it is in chemical union with another atom. The 
atomic weight is the one quality of importance. The physical state, 
whether the element is in the solid, liquid, or gaseous state, is unim- 
portant. And when we deal with molecules the stopping power is 
simply proportional to the sum of the square roots of the atomic 
weights of the atoms entering into the molecule. This is the 
‘fadditive law,” and it obviously enables us to calculate what the 
range in any substance of known chemical composition and density | 
will be, compared with its range in air. 
This is of special importance in connection with phenomena we 
have presently to consider. It means that, knowing the chemical 
composition and density of any medium whatsoever, solid, liquid, 
or gaseous, we can calculate accurately the distance to which any 
particular alpha ray will penetrate. Nor have the temperature and 
pressure to which the medium is subjected any influence save in so 
far as they may affect the proximity of one atom to another. The 
retardation of the alpha ray in the atom is not affected. 
This valuable additive law can not, however, in strictness be 
applied to the amount of ionization attending the ray. The form 
of the molecule, or more generally its volume, may have an influence 
upon this. Bragg draws the conclusion, from this fact as well as 
from the notable increase of ionization with loss of speed, that the 
ionization is dependent upon the tume the ray spends in the molecule. 
The energy of the ray is, indeed, found to be less efficient in pro- 
ducing ionization in the smaller atoms. 
Before leaving our review of the general laws governing the 
passage of alpha rays through matter, a point of interest must be 
referred to. We have hitherto spoken in general terms of the fact 
that ionization attends the passage of the ray. We have said 
nothing as to the nature of the ionization so produced. But in 
point of fact the ionization due to an alpha ray is sui generis. A 
glance at one of Wilson’s photographs (fig. 2) illustrates this. The 
white streak of -water particles marks the path of the ray. The 
ions produced are evidently closely crowded along the track of the 
ray. They have been called into existence in a very minute instant 
of time. Now we know that ions of opposite sign if left to them- 
selves recombine. The rate of recombination depends upon the 
ry 
