368 Mr. Poioell on Terrestrial Light and Heat. [May, 



nor in any way altered in its properties so as to approach the 

 nature of light. It seems to be impossible to conceive that the 

 mere continued and increased action of one cause, or a fluid of 

 one simple nature can change a portion of it into a new sub- 

 stance, and yet leave a very considerable part in its original 

 state. Nor can the difticulty be diminished by supposing the 

 heat to be a compound of two different species, one convertible 

 into light, and the other not; because, as we have seen in these 

 experiments, in the same body the proportions of the two will 

 be constantly varying with the increasing intensity of ignition. 



(16.) After all it becomes a question, does any part of the 

 simple heat disappear so tliat we can sup|)oseit either converted 

 into light, or in any other way changed in its properties I It does 

 not appear to me that this" has been in any way established 

 either by the supporters of the theory just alluded to, or any 

 other experimenters ; yet its investigation is clearly a point of 

 importance. If it should be shown that it does not take place, 

 this theory (independently of the objections just urged) would 

 entirely fall to the ground. If it shuuld appear that some such 

 phenomenon does take place, the above objections would not bo 

 in the sbghfcest degree removed ; and we might then, perhajis, 

 have some ground for a more correct and inductive view of the 

 subject. 



(17.) This was one princi])al point I had in view in these 

 experiments, and I conceive to be by them sufficiently shown, 

 that a portion of the heat wl\i<jh we know upon independent 

 grounds is generated, is actually lost, or does not appear either 

 as heat of temperature, or in a radiant form. 



The general result of my second set of experiments is, that at 

 first the heating of a body causes it to continue radiating heat in 

 a proportion which is nearly that of the increase of temperature. 

 At a certain point which we call the temperature of luminosity, 

 light begins to be given out., possessing a heating power when 

 absorbed again; the hght estmiated both by this power, and 

 generally also by its illuminating effect, continues to increase ; 

 whilst the simple radiant heat, distinct from it, continues to 

 increase also, but in a less ratio than the light. 



The radiant heat probably tends to increase in a certain ratio 

 to the elevation of temperature ; at the same time (from the 

 peculiar constitution of bodies) an increasing quantity of it is 

 continually abstracted, or ceases to appear as radiant heat, and 

 this loss corresponds to the increase of heating power in the 

 light. 



This law applies to the case both of the same body at different 

 stages of ignition, and to the comparison of different luminous 

 bodies, as different flames, which have been shown to have 

 different temperatures of luminosity, and which on arriving at a 



