1 877 .] Probable Origin and Age of the Sun . 
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II. ON THE PROBABLE ORIGIN AND AGE 
OF THE SUN. 
By James Croll, LL.D., F.R.S., 
Of H.M. Geological Survey of Scotland. 
HE total annual amount of radiation from the whole 
surface of the sun is 8340 x 10 30 foot-pounds. To 
maintain the present rate of radiation it would require 
the combustion of about 1500 lbs. of coal per hour on every 
square foot of the sun’s surface ; and were the sun composed 
of that material it would all be consumed in less than 
5000 years. The opinion that the sun’s heat is maintained 
by combustion cannot be entertained for a single moment. 
Mr. Lockyer has suggested that the elements of the sun are, 
owing to its excessive temperature, in a state of dissociation, 
and some have supposed that this fact might help to explain 
the duration of the sun’s heat. But it must be obvious that, 
even supposing we were to make the most extravagant 
estimate of the chemical affinities of these elements, the 
amount of heat derived from their combination could at 
most give us only a few thousand years additional heat. 
Under every conceivable supposition the combustion theory 
must be abandoned. 
It is now generally held by physicists that the enor- 
mous store of heat possessed by the sun could only have 
been derived from gravitation. For example, a pound of 
coal falling into the sun from an infinite distance would 
produce by its concussion more than 6000 times the amount 
of heat that would be generated by its combustion. It 
would, in fa fit, amount to upwards of 65,000,000,000 foot- 
pounds — an amount of energy sufficient to raise 1000 tons 
to a height of 5J- miles. 
There are two forms in which the gravitation theory has 
been presented : the first, the meteoric theory, propounded 
by Dr. Meyer ; and the second, the contraction theory, ad- 
vocated by Helmholtz. The meteoric theory of the sun’s 
heat has now been pretty generally abandoned for the con- 
traction theory advanced by Helmholtz. Suppose, with 
Helmholtz, that the sun originally existed as a nebulous 
mass, filling the entire space presently occupied by the solar 
system, and extending into space indefinitely beyond the 
