REPORT ON RADIANT HEAT. 
281 
anxious to devise as many variations of the experiment as pos¬ 
sible ;—these all tended to confirm the results just given. 
Thus I used a large differential thermometer having its bulbs 
differently coated, and exposed each of them in turn to the lu¬ 
minous source of heat, the other being completely screened, 
and invariably found the ratio of the effects on the black and 
white bulbs considerably greater when affected only by the 
transmissible part of the heat, than when exposed to the whole. 
As before, the part added on the removal of the screen was of 
a nature tending to add to the terms of the former ratio, quan¬ 
tities in a ratio much nearer equality ; viz. that which the effects 
of simple radiant heat would give when acting respectively on 
the two bulbs. 
Other variations of the fundamental experiment, were as 
follows: 
A differential thermometer having one bulb black, was ex¬ 
posed to the radiation from luminous hot bodies, first with and 
then without the interposition of a glass screen ; the same 
position being preserved. 
If the screen had no influence, it is evident that in whatever 
proportion the radiant matter affects the two bulbs, if it be of 
one simple kind, the only difference on removing the screen will 
be that its intensity will be increased, but will act on the two 
bulbs in the same proportion as before. Consequently an in¬ 
crease of effect, or motion of the liquid in the tube in the same 
direction as before, must take place. 
In various experiments of this kind, after using several pre¬ 
cautions against the influence of the screen, I never found an 
increase , and generally a decrease; that is, the action on the 
other bulb was now increased, or the portion of heat before 
intercepted and now admitted has a different relation to sur¬ 
faces from that transmitted. ( Quarterly Journal of Science , 
XIX. p. 45.) 
Similar experiments were tried with the two bulbs in a direct 
line from the hot body, each placed nearest alternately, with 
and without a screen. The difference of ratios in the two cases 
was very striking. (. Annals of Pliil . June 1825, p. 401 ; see 
also Edinb. Journ . of Science , No. IV. 323.) 
Upon the whole, the unavoidable conclusion is, that if the 
total direct effect were the result of one simple agent, the in¬ 
tervention of the glass would, by intercepting some portion of 
it, produce no other alteration than a diminution of intensity ; 
the ratio of the two effects would remain unchanged : but the 
reverse being the case, it follows that there are two distinct 
agents or species of heat acting together. 
