1887 .] 
Mr John Aitken on Solar Radiation. 
119 
its loss of heat. Now there are certain physical facts which have 
induced me to think that this is not necessarily so. Nay, it seems 
even possible that the amount of heat radiated by the sun might 
go on increasing while its temperature was decreasing. 
The facts which seem to point to this conclusion are — - 
First , We know that different forms of matter vary greatly in 
their power of radiating heat. For instance, a non-luminous gas 
flame radiates far less heat than a luminous one, though actually at 
a higher temperature, and a red-hot surface of platinum radiates far 
less heat than an oxidised surface of iron of the same area and 
temperature. 
Second , We know that elementary bodies generally radiate far 
less heat than compound ones. It has been shown that the radiating 
powers of substances go on increasing with the increased com- 
plexity of their constitution. 
Third , We know that at high temperatures compound bodies are 
decomposed or broken up into simpler forms. Or, to put it in another 
way, and as a solar inhabitant would put it, bodies which have an 
affinity for each other do not combine unless their temperature is 
below a certain fixed point for the substances. 
Combining these statements we see that in the sun, owing to its 
high temperature, matter must he in much simpler forms than it is 
on the earth, and all recent investigation points markedly in this 
direction. It seems therefore in the highest degree probable that 
the average radiating power of the matter in the sun is much less 
than that of the matter of the earth. Again, the hotter the sun the 
simpler its constitution will he, and the weaker its radiating power. 
From this we see that there is no necessary proportion between the 
temperature of the sun and the amount of heat it radiates, as change 
of temperature is accompanied by change of constitution, and in- 
crease of radiating power. 
These considerations lead us on, and suggest that the store of 
energy from which the sun has drawn in long past ages may possibly 
he its own internal supply ; that the sun in its earlier ages was at 
a much higher temperature than it is at present, hut, owing to 
its simpler constitution, it radiated only about as much heat as it 
does now, and that as its temperature fell its matter became more 
compound, and its radiating power increased, which enables it now 
