1887.] Mr John Aitken on Solar 'Radiation. 121 
Pouillet supposed. Professor Langley, by more perfect methods, 
has shown that Forbes’s figure is also too small, and there are 
evident reasons for supposing that even Langley’s measurements are 
too low. The sun will therefore require to shrink a good deal 
more than 35 metres per year to develop from gravitational sources 
alone the energy radiated by it. 
Apart, however, from the energy developed by shrinkage, there will 
evidently he energy developed within the sun in another way while 
it is cooling. The falling temperature will be accompanied by com- 
bustion, though not in the manner supposed in the old combustion 
theory of solar energy. There will, however, evidently he a de- 
velopment of energy due to the combination or falling together of the 
molecules which will ensue on the decrease of temperature. Here 
we have a field for the chemist to come in and do for the chemical 
part of the subject what Sir W. Thomson has done for the gravita- 
tional. It must, however, be confessed that his task is a far more 
difficult one, at least at present, as we know very little about the 
condition of matter in the sun, and almost nothing about the 
amount of energy developed when the simpler forms of matter 
combine. 
Now, though the sun may receive an enormous amount of energy 
from those two sources, yet it is evident they can never keep the 
temperature of the sun constant, because, before energy can be 
developed in either of those ways, the temperature of the sun must 
fall, and the energy developed will he in proportion to the amount 
of the fall. 
4. On Laplace’s Nebular Theory, considered in relation to 
Thermodynamics. By Sir W. Thomson. 
5. On a Class of Alternating Functions. By Dr Muir. 
6. Note on Hoar-Frost. By Mr John Aitken. 
Hoar-frost is generally described as frozen dew, and is supposed 
to be deposited in the same manner and under the same conditions 
as dew ; the only difference being in the temperature at which it is 
