134 
[March, 
The Heat of the Sun. 
Moreover, the more rapid the shrinkage the gieatei the 
amount of heat radiated ; or, in other words, knowing the 
amount of radiation taking place, the weight and volume of 
the radiating body, and its specific heat, it is possible to 
calculate the rate of shrinkage necessary to keep up tn 
energy expended. Without going farther the superficial 
reader would at once discard the theory as untenable. 
“ Here,” he would say, “ you have the Sun emitting lacliant 
energy at this enormous rate, which has demonstrably no 
diminished during historic times, and has probably continue 
for 20,000,000 years ; and all produced by the shrinkage of 
his substance. Why long ere this he would have shrunk 
into a cold dead mass.” But let no one imagine the cele- 
brated German guilty of want of care in examining the 
subiedt. He has made the calculation indicated above, and 
he proves that a shrinking of the Sun’s diameter by 
i-io,oooth of its present length ( i.e ., 88 miles) would pro- 
duce an amount of energy capable of keeping up the solar 
emission at the present rate for two thousand years. But 
if the Sun be supposed to go on contracting, and its specific 
gravity to go on increasing until it equals that ot oui 
Earth, then Helmholtz shows that the heat evolved would 
supply the Sun with energy for no less than 17,000,000 
ye in brief outline this is the celebrated theory of Con- 
traftion. Simple in its assumptions, and quite competent 
to fulfil the demands made upon it, it is not a matter ot sui- 
prise that it is held by many of the first thinkers of our 
time Yet we cannot help thinking that two considerations 
weigh heavily against it. First, the researches of modern 
spedtroscopy seem to indicate that we have in the Sun an 
intensely hot nucleus, surrounded by an atmosphere or shell 
of less intensely heated matter. Now if the heat is pro- 
duced by condensation the whole body would be equal y 
heated, and consequently no such partial separation would 
take place. Also the formation of a nucleus is difficult ot 
explanation on such a theory, for although the . temper attire 
of the nucleus may be greater, the actual heat in the solar 
atmosphere must be greater. Secondly, as Siemens has 
pointed out, the heat being produced throughout the mass 
must reach the surface by conduftion and conveCtion. But 
we know of no substance capable of conducting the immense 
amount of heat which we find being ladiated. 
