HIGH FREQUENCY RAYS—MILLIKAN 199 
be transformed partially into soft rays of just about the hardness 
of the soft rays which we have actually observed on Pike’s Peak and 
Mount Whitney. The reason these soft rays were more plentiful 
on the mountain peaks than at Pasadena would then be found simply 
in the fact that there are about three times as many of the hard 
rays to be transformed at the altitudes of the peaks as at that of 
Pasadena. This seems to be the solution of the second of our 
summer’s problems. 
We can draw some fairly reliable conclusions of a general sort as 
to the origin of these very penetrating and very high-frequency 
rays. The most penetrating rays that we have known anything 
about thus far, the gamma rays of radium and thorium, are produced 
only by nuclear transformations within atoms. In other words, 
they are produced by the change of one atom over into another atom, 
or by the creation of a new type of atom. It is scarcely possible, 
then, to avoid the conclusion that these still more penetrating rays 
which we have here been studying are produced similarly by nuclear 
transformations of some sort. But these transformations must be 
enormously more energetic than are those taking place in any radio- 
active changes that we know anything about. For, according to our 
present knowledge, the frequency of any emitted ray is proportional 
to the energy of the subatomic change giving birth to it. We can 
scarcely avoid the conclusion, then, that nuclear changes having an 
energy value perhaps fifty times as great as the energy changes 
involved in observed radioactive processes are taking place all 
through space, and that signals of these changes are being sent to 
us in these high frequency rays. 
The energy of the nuclear change that corresponds to the forma- 
tion of helium out of hydrogen is known, and from it we have com- 
puted the corresponding frequency and found it to correspond closely 
to the highest frequency rays which we have observed this summer. 
The computed frequencies of these cosmic rays also correspond 
closely to the energy involved in the simple capture of an electron 
by a positive nucleus. Thus, the highest speed B ray emitted by 
thorium leaves its mother atom with a speed which is equivalent 
to the energy acquired by the fall of an electron through 7,540,000 
volts.11_ This electron in order to get out of the mother atom was 
obliged to move against the pull upon it of the positive nucleus, and 
in this act it gained a potential energy the equivalent of a fall 
through 4,400,000 volts.12 If this same electron had reversed its 
path and plunged into the nucleus it should have generated in so 
doing a 12,000,000-volt ray (7,540,000-+-4,400,000). The cosmic rays 
“4 Report of Committee on X Rays and Radioactivity of National Research Council, 1925, 
p. 92. 
“ Report of Committee on X Rays and Radioactivity of National Research Council, 1925, 
p. 68. 
