156 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
per square centimeter per mimute and corresponds to an effective 
temperature of a little less than 6,000 degrees absolute (Centigrade). 
Another method based upon the wave-length of the most intense 
radiation in the solar spectrum (Wien’s law), always assuming an 
emission from a black body, leads to a like result. 
This is very far, as you see, from the millons of degrees formerly 
supposed. It does not mean, however, that the heat sent out from 
the sun is not enormous. We may well ask by what means this 
immense loss of energy is compensated. It is indeed impossible that 
the sun should burn like an immense block of coal. The most intense 
combustion revealed to us by the chemist, for example, that of gun 
cotton, would not suffice to feed this radiation for more than a few 
thousands of years. We must look elsewhere: <A rain of asteroids 
has been suggested, which by its kinetic energy would restore the 
lost heat to the sun. Such an hypothesis is not possible. The mass 
of the sun would increase indefinitely and the planets should show 
an unobserved acceleration. The sun, however, as Helmholtz sup- 
posed, could contract little by little, changing mto heat the internal 
energy of the primitive nebula of Laplace. A contraction of 30 
meters per year would suffice to explain all. Unfortunately, a very 
suggestive calculation shows that all the heat that the sun could 
thus develop since its origin, by any mechanical process whatever, 
would not sustain the radiation for more than 15 million years. 
And the world is very much older than that, at least so the geologists 
affirm. Their arguments, taken separately, do not seem without 
value; but what is more remarkable, they all tend, by different 
paths, to lead us to admit a past very much longer than 100 million 
years, figuring perhaps into thousand millions. We must apparently 
search in the interior of the atoms themselves for the source of 
the solar heat. The infinitely small, as Pascal said, will explain the 
infinitely great. 
The intraatomic energy is indeed enormous and the sun, as well as 
many of the stars, shows in abundance one of the most characteristic 
elements of radioactive transtormations—helium. 'The mechanism of 
the radiation, it is true, remains unknown. But the necessity of such 
an explanation will perhaps not be so imperious in the near future. 
The problem tends to assume a new aspect of the highest philo- 
sophical import. The physicists of the new school are disposed to 
admit and appear to have proved a fundamental identity ' between 
the mass of a body and its internal energy. If matter is no more 
than energy, we may foresee what a beautiful simplicity physics may 
sometime assume. On this basis the sun would possess a total energy 
(easy to calculate since we know its mass) of 2 X 10° ergs, assuring 
e 1 Of course on the condition of the suitable choice of units. 
