THE SOURCE OF STELLAR ENERGY 
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material by the ordinary processes of collision. There is thus suitable 
mechanism for the conversion of the released energy into utilisable form. 
Penetrating Radiation from Interstellar Space. 
223. The wave-length 0-0004 A of the radiation released in the forma 
tion of helium from hydrogen is much shorter than that of known y rays, 
and.the radiation should be distinguished by greater penetrating power. 
A very penetrating radiation is known to exist in our atmosphere, ap 
parently entering it from outer space. It has been suggested that this 
may originate from the transmutation of elements in the stars and 
nebulae. 
The penetrating radiation can be detected in the following manner. 
An ionisation chamber is surrounded by a shield of heavy metal sufficiently 
thick to protect it from all ordinary radiation outside, and the rate of 
production of the ions is measured. Whilst there may be a constant pro 
duction of ions due to radio-activity of the inner part of the walls, any 
change in the rate must be attributed to radiation from outside which 
has succeeded in penetrating through the walls. Experiments are made at 
different altitudes, in ice caverns and below the surface of mountain lakes 
and the change in the quantity of radiation penetrating the walls is 
measured. It appears that the radiation travels downwards from the sky 
because the ionisation is reduced according to the amount of atmosphere, 
ice or water above the apparatus. 
The absorption properly so called of this radiation must be exceedingly 
small and practically it can only be stopped by scattering. Once scattered 
it is done with, since it loses its penetrating power by the Compton effect. 
Hence there is no backward scattered beam of penetrating radiation, and 
the intensity diminishes downwards according to the usual exponential 
law of absorption. 
Scattering depends only on the number of electrons in the material, 
which is practically proportional to the mass. It is convenient to keep in 
mind that the stopping power of the whole atmosphere is roughly equi 
valent to 10 metres of water or ice or to 1 metre of lead. Such a screen is 
sufficient to reduce the intensity considerably, but not to exclude the 
radiation entirely. 
In this provisional discussion we follow the results of W. Kohlhörster 
whose experiments were made on the Jungfrau. It should be stated, 
however, that R. A. Millikan, who has experimented at high altitudes in 
the United States, has obtained somewhat different results and I under 
stand that he is not in agreement with Kohlhörster’s conclusions. It is 
not for us to judge which is right; but since our only reason for treating 
of this radiation is its supposed extra-terrestrial origin we must tentatively