300 
SECOND REPORT — 1832 . 
nature to repeat with precision, owing to the necessity of wait¬ 
ing between each repetition for the thermometers to cool and 
become stationary. 
But it should be observed that there is nothing in my results 
which contradicts the idea that simple heat may have in a very 
slight degree a power of transmissibility through glass: all I 
have assumed is, that it is sufficiently distinguishable in this 
respect from the heating power which accompanies the light, 
and which undergoes no diminution. Connected with these 
points, again, is the question, whether if simple heat can 
radiate through solid transparent media, it cannot also com¬ 
mence radiating in them. It is commonly asserted that ra¬ 
diation can only take place, or commence, in elastic media. 
This, then, is an inquiry which will lead into a wide field of re¬ 
search, and may be found connected with the intimate nature 
of radiation. It will also be a question, whether, and how far, 
radiant heat passes through elastic media without heating them, 
and what support this gives to Leslie’s theory of pulsations. 
The whole subject should be viewed in connexion with the ad¬ 
mirable remarks of Sir J. Herschel in his Discourse on the 
Study of Natural Philosophy , p. 205. 
The radiation of heat in vacuo is another point on which 
further inquiry is much wanted. The greater capacity of air 
for heat, as it is more rarefied, would occasion a more rapid 
abstraction from the hot body ; and thus in an atmosphere of 
extreme rarity the cooling ought to be extremely rapid, and 
this must be accurately estimated in measuring the radiation. 
But it appears from the experiments of Gay-Lussac, (see Edinb. 
Phil. Journ. vi. 302,) that when air is reduced to the most ex¬ 
treme degree of rarefaction possible, a very considerable com¬ 
pression makes so little difference in its actual density, that the 
giving out of heat which ought to take place from diminishing 
its capacity is absolutely insensible. 
But even in this case it is very questionable whether so com¬ 
plete an approach to a real vacuum is obtained as to warrant in¬ 
ferences respecting the radiation of heat in an actual vacuum. 
In fact, we want a connected series of determinations to show 
the order and increase of conducting powers, as connected 
both with the radiation in and through different media, and the 
interception which they offer to its passage. 
In solids it is presumed no radiation can commence ; it is dis¬ 
puted whether it can continue even partially; but conduction 
goes on rapidly. 
In liquids it has been disputed whether there can be radia¬ 
tion; and they are worse conductors than solids. 
