EVOLUTION OF THE UNIVERSE. 589 
an electroscope three different times in a balloon to heights which reached 
four and a half kilometres and found that its rate of discharge was there 
even higher than on the earth, he had discovered something new and 
important, namely, that although there are penetrating rays that do 
originate in the earth and are indeed abundantly given off from practically 
all the elements of the earth’s crust, as Kurz and the other workers prior 
to 1910 had rightly concluded, yet there must be other rays, abundant 
at high altitudes, that come in from above, originating either in the 
remoter regions of the atmosphere or else coming in from outer space ; 
in other words, that one or the other of the two alternatives which Kurz 
had explicitly considered and definitely discarded had been incorrectly 
set aside for at least some rays that actually exist. Which one of these 
two, namely, upper atmosphere or outer space, it took a great deal of 
work by Hess, by Kolhérster, by v. Schweidler, by Bowen, Otis, Cameron, 
myself and others, from 1910 to 1925 definitely to determine, and even in 
1927, at the Como conference, one of the most distinguished of living 
physicists declared himself still a believer in the theory of an upper 
atmosphetic origin. 
To-day, however, I think the cosmic origin has been generally conceded, 
and with that concession it follows from the meastrements themselves, not 
only that in the particular portion of our galaxy immediately around us, 
the energy carried by the cosmic rays is at least a tenth’ of that existing 
in the form of radiant heat and light, but also, since these latter radiations 
must be diminished greatly in intergalactic space, that the energy carried 
by the cosmic rays throughout the universe is of the same order of 
magnitude as, possibly greater than, all other radiant energies combined. 
In the light of this fact, when one reflects that the second law of thermo- 
dynamics, which has, strangely, been thought by some so determinative 
for theories of the origin and destiny of the universe, and which may be 
roughly said to be merely a generalisation of the fact always observed 
here on earth that all forms of energy tend to become converted into heat 
and then to be radiated away from the earth and hence lost to us, one sees 
how prone we are to make sweeping generalisations upon insufficient know- 
ledge. This is why the experimentalist has played and always will play so 
important a role in the progress of science. From the very inception of 
the experimental method he has continually been bringing to light facts 
which were not within the theorist’s ken even when that theorist had got 
observational phenomena pieced together, as he thought, into a beautifully 
consistent and ‘necessarily related’ scheme. With perhaps the largest 
source of radiant energy as yet unconsidered, may it not possibly be that 
the thermodynamic theorist has gone too far in his dicta about the origin 
and destiny of the universe? This is my excuse for forgetting, at least 
for the moment, all about theories, and asking first: What are the 
experimental facts in the field of cosmic rays ? 
There are three main facts that now seem to be quite well established, 
though discussion is still rife about some of them. I hope, however, that 
some of the new data that I am now able to present will help to bring about 
better agreement. 
> Millikan and Cameron, Phys, Rev., 31, 930; 1928. 
