ELECTRIC RADIATIONS—BRAGG. 201 
radiation to be scattered primary, is just what we should expect. In 
the case of the a rays no-secondary radiation other than 8 rays has 
been found; but a small reflection of canal rays has been observed, 
e. g., by Fuchtbauer. (Phys. Zeit., March 1, 1906.) Barkla has 
shown that the secondary radiation produced by X rays consists in 
part of scattered primary radiation, especially when the surface 
struck is of material whose atomic weight is low. The only cases in 
which a secondary radiation appears that is neither § radiation nor 
reflected primary rays are those in which £ rays are produced at the 
impact of X or y rays, and in which X rays are produced by cathode 
rays. It is remarkable that in the former of these cases there is very 
great difficulty in accounting for the high speed which is possessed 
by the secondary radiation, caused by X rays and y rays. (Wien, 
Ann. d. Phys., December 28, 1905.) It may well be that further re- 
search will bring these cases into better agreement with the rest. 
The next question which it is interesting to consider in relation to 
the various types of radiation is that of the law of absorption in pass- 
ing through matter. 
Absorption in the case of the material radiations appears to be due 
to two main causes: Loss of energy, which causes a gradual loss of 
speed, and scattering, which means a diminution in the number of 
particles in the primary beam. There is a possibility of a third, viz, 
absorption of the flying particle by an atom which it is traversing. 
In the case of the a particle, I have shown that the first of these 
causes operates alone, so that the particle pursues a rectilinear course 
throughout its career. (Australasian Association for the Advance- 
ment of Science, January, 1904; Phil. Mag., December, 1904.) It is 
the absence of any effective amount of scattering that makes the study 
of the motion of an individual a particle comparatively simple. The 
loss of energy in traversing an atom, or more exactly the probable 
loss in crossing a given space occupied by an atom, is nearly propor- 
tional to the square root of the atomic weight, and the effects appear 
to be exactly additive. 
On the other hand, if we consider a stream of £ particles projected 
into matter, and attempt to find the history of their motion, we are 
faced with a problem of great complexity. If we look for an answer 
expressed statistically, we must find the number of particles in each 
unit volume of the absorbing matter as a function of the time, the ve- 
locity, and the direction of motion. If, on the other hand, we try to 
follow the motion of any one particle, we must find the chance that 
the particle considered has any particular position, velocity, and di- 
rection of motion at any given time; which is really equivalent to 
finding the function just mentioned. Moreover, the data are very un- 
certain. We know so little of the interior of the atom that we are 
unable to say with what forces the electrons will be influenced when it 
