270 
SECOND REPORT— 1832 . 
The substance of my observations is as follows : 
De la Roche found, that if radiant heat be intercepted by 
two transparent screens, the additional diminution of effect 
occasioned by the second, is proportionally much less than that 
produced by the first ; and the same conclusion is extended to 
any number of screens. This was explained by the supposition 
that the heat in its passage through the first glass undergoes a 
certain modification, in some respects analogous to polarization, 
by which it is enabled to pass, with very little diminution, through 
the second and subsequent glasses. 
In those cases where the source of heat is luminous, such 
phenomena would receive an obvious explanation on the prin¬ 
ciple investigated in my other paper. Vide infra . 
But if the same effect is still observable below the point of 
luminosity, we must have recourse to some other principle of 
explanation. That deduced by De la Roche appears at least 
plausible ; and though it should be considered proved, that, in 
general, heat is incapable of being radiated directly through glass, 
it perhaps would not necessarily follow, that it might not, under 
peculiar circumstances, have a power of doing so communicated 
to it. Though on the other hand it must be confessed, that 
in the present case some difficulty would attend such a suppo¬ 
sition. 
It certainly would not be easy to conceive such a property to 
be communicated to the heat, by the mere act of being conducted 
through the first glass. Again; a new property of heat is thus 
introduced, which, it must be conceded, is not absolutely and 
exclusively established. 
It appeared to me therefore a point of some interest to ex¬ 
amine, in the case of non-luminous heat,—in the first place, the 
accuracy of the fact; and secondly, if verified, whether there 
might not be circumstances observable in the conditions of the 
experiment by which it might be accounted for, without the 
necessity of supposing any peculiar property of heat, or a direct 
transmission even through the second glass. 
My apparatus in following up this inquiry was similar to that 
described by M. De la Roche, and consisted of two tin reflec¬ 
tors ;—in one focus the bulb of a thermometer coated with 
Indian ink, and in the other an iron ball two inches diameter, 
which was heated to redness, and then cooled till it ceased to 
be visibly red in the dark, at which point it was placed on its 
stand, and a thick screen withdrawn. The indications were 
observed, first, for the direct effect; secondly, with one glass 
screen interposed ; and thirdly, with two. The temperature of 
the screens was observed by means of a small thermometer 
