752 EVENING DISCOURSES. 
difficult to explain and illustrate to a large audience. Fortunately Mr. C. T. R. 
Wilson has recently developed a beautiful method of rendering the tracks 
of the rays visible. Through his kindness I am able to illustrate my state- 
ment by photographs which he has taken. His method depends upon the 
sudden expansion of a mass of damp air through which the rays are passing; 
the chill due to the expansion causes a fog deposit on the positive and negative 
particles in the tracks of the rays. When the gas is well illuminated the tracks 
stand out as bright lines on a dark background, and it is easy to obtain per- 
manent records of the effects. One picture shows a number of a-ray tracks 
radiating from a point at which a speck of radium has been placed. Another, 
on a larger scale, shows the end portions of one or two of the tracks; the 
occasional deflections, to which I have already referred, are plainly visible, and 
it is to be observed how straight the path lies between the sharp bends. At one 
of the severer deflections there is a slight spur from the point of deflection, 
which is believed to be due to the recoil of the atom which has been the cause of * 
the change in direction. Some tracks originate in a slight enlargement, and 
look like pins with small heads. No doubt this is also a phenomenon of recoil : 
the atom of radium emanation has exploded in mid-air, and while the shot has 
gone a relatively long distance one way, the gun has leaped back a short distance 
in the other. 
Other photographs show the tracks of B-rays, which are electrons moving 
with speeds approaching very nearly—in some cases—to that of light. The 
B-ray is much more liable to deflection than the a-ray, as is clearly shown. 
This is no doubt due to the smallness of its mass; and for much the same reason 
it is less effective in removing electrons from the atoms which it traverses. 
These new radiations make us familiar with the idea of material streams 
moving with velocities of the order of light; free, moreover, to traverse each 
other without mutual injury. Thus we find that both the reasons which Huygens 
gave for the abandonment of a corpuscular and the adoption of a pulse theory 
were quite mistaken. 
It is true that Huygens might have objected that neither a-rays nor f-rays 
can show the same power of penetrating great thicknesses of matter as the light 
which passes through miles of atmosphere or many yards of water. The utmost 
range in air of the swiftest a particle is less than 4 inches, and in the same 
medium the £-ray moves no more than a few feet. 
But there is a third form of ray, the y-ray, or x-ray, which possesses tlie 
most startling powers of penetration; it can pass even through inches of steel. 
Nor is there any reason to suppose that our discoveries have reached the limits 
of penetration of these rays. The y- or x-ray leaves no trail of positives and 
negatives behind it, and no fog settles to show where it has passed. We know 
of its existence only through its power of originating the more obvious 3-ray, 
and a fog-photograph of a gas through which an a-ray stream has passed shows 
the tracks of 8-rays beginning at irregularly spaced points in the path of the 
x-rays and completing their rather tortuous tracks within short distances there- 
from. The older methods of investigation have indicated that the 2-rays are 
separate entities, each of which has the power of originating one f-ray, and one 
only, handing over to it all the energy which it possesses. Indeed they have gone 
further, and shown that one f-ray originates each x-ray, and gives to it its 
own energy. Mr. Wilson’s photographs illustrate these deductions and go far to 
confirm them. Thus the 8-ray and the z-ray are interchangeable forms of carrier 
of the same quantum of energy: carried by the former form the energy is easily 
deflected; by the latter it is not. It is curiously reminiscent of the fits of easy 
reflection and transmission. ; 
Have we then in these rays the actual corpuscles which Newton imagined ? 
Much time and labour are even now being spent in the attempt to verify or to 
disprove their parallelism to the rays of light; and many interesting facts have 
already been discovered. For instance, light itself has the power of causing 
electrons to spring from matter upon which the light is incident, though to be 
sure the velocity is very much smaller than that of the B-ray. If verification is 
found, there will be strong reasons for supposing that Newton’s corpuscular 
theory of light was partially correct in form. 
But of course the main difficulty of a corpuscular theory of light lies in the 
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