RADIANT HEAT. 317 



portion 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 increase 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 precautions 

 against the influence of the screen, I never found an increase, and gene- 

 rally a decrease; that is, the action on the other bulb was now in- 

 creased, or the portion of heat before intercepted and now admitted 

 has a different relation to surfaces from that transmitted. — (Quarterly 

 Journal of Science, xix, p. 45.) 



Similar experiments were tried with the fAvo 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 Phil., June, 1825, p. 401. See, also, Udinburg Journal 

 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 intervention 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. 

 Upon combining these results with those of previous experimenters, 

 we are led to the following general statement of the case: 



When a body is heated, at lower temperatures, it gives off radiant 

 heat stopped entirely by the most transparent glass, and affecting bodies 

 in proportion to the absorptive texture of their surfaces. 



At all higher temperatures it continues to give off such radiant heat, 

 distinguished by exactly the same properties. 



At a certain temperature it begins to give out light; precisely at 

 this point it begins also to exercise another heating power distinct 

 from the former; this is capable of direct transmission through glass, 

 and affects bodies in proportion to their darkness of color. 



This second species appears to agree with what the French philos- 

 ophers have called " calorique lumineux," or the "igneous fluid" of 

 Professor Leslie, but they seem to have considered it as constituting 

 the entire effect. 



The distinction thus established easily applies to the explanation 

 of De la Roche's results, before stated. On inspection, it appears that 

 the numbers in the column belonging to the blackened screen are 

 almost exactly in the same ratio to the first or direct effect through- 

 out the whole series. 



Upon the principle here laid down, the effects with the blackened 

 screen would be those arising from the absorption and subsequent 

 radiation of both species of heat; these in each- instance being ab- 

 sorbed in the proportions in which they existed in the original radia- 

 tion, produce a secondary effect proportional to the primary. 



The effect with the transparent screen does not follow any propor- 

 tion to the primary; and this is explicable as due to the glass inter- 



