318 
THE SOURCE OF STELLAR ENERGY 
follow the authority whose experiments appear to demonstrate extra 
terrestrial origin*. 
Kohlhorster compared the ionisation when the apparatus was exposed 
in the open and in an ice cavern whose roof cut oft’ the radiation from above. 
He found that the intensity did not depend on the altitude of the sun. 
Hence the sun is not the source of the radiation. Maxima were found to 
occur when the Milky Way crosses the zenith; the greatest depth of the 
stellar system is then overhead. 
It must be realised that there is no possibility of the radiation coming 
from matter at a high temperature—high in the astronomical sense. All 
the matter of the universe at temperature much above 100,000° is securely 
tucked away behind screens of stopping power much greater than a metre 
of lead, and none of its penetrating radiation can escape into the open. 
Hence our choice is between the photospheric layers of the stars, the bright 
and dark gaseous nebulae, and the general unaggregated matter diffused 
through space. As regards temperature there is not much to choose be 
tween these; the temperature of diffuse matter in space is probably above 
10,000°. The chief theoretical objection to nebular origin is the low density 
which, one might suppose, would hin der the collection of material for 
atomic combination. 224 
224. To examine further the question of nebular or stellar origin, 
consider a column of 1 sq. cm. section extending to the limits of the stellar 
system. Let a be the mass contained in it with the proviso that only the 
first kilogram is to be counted since that would effectively screen any 
radiation emitted behind it. The limit will operate if the column intersects 
a star, but in general not otherwise. Divide the integral of a with respect 
to solid angle into four portions corresponding to (1) the sun, (2) the stars, 
(3) bright and dark nebulae, (4) apparently clear regions. If all matter 
liberated the radiation equally the amount due to each of these sources 
would be proportional to their respective shares of Jo-dan 
It is not very certain in what order of importance the four sources 
would be placed by this criterion but I should be inclined to adopt the 
order (3), (1), (4), (2). There can be little doubt that (2) comes last. The 
amount of matter diffused through space is likely to be comparable with 
the mass of the stars and it all counts in (4), whereas only a thin surface- 
film of each star counts in (2). The sun occupies 200 1 000 of the sky so that 
if the radius in a clear region meets more than 2 oo 1 ooo kg. per sq. cm. 
* [More recently Millikan has announced the results of a reinvestigation of the 
penetrating radiation by himself and H. Cameron in the summer of 1925. He is 
convinced of its extra-terrestrial origin. The absorption (scattering) coefficient is 
found to be 0-18-0-3 per metre of water ( Proc. Nat. Acad. Sci. 12, p. 48 (1926)).]