446 Mr. B. Powell on terrestrial radiant Heat. 



theory, a conversion of light into heat. These views of the 

 subject are certainly gratuitous assumptions. We have no 

 right whatever to identify those two agents, or to suppose that, 

 because a heating effect very closely accompanies the course of 

 the rays of light, the light is therefore converted into heat : but 

 the theories above alluded to seem to regard the voltole heating 

 effect of a luminous body as of this latter character. In this 

 particular, the present inquiry has led us to an essential dis- 

 tinction ; and if the experiments are to be relied upon, this 

 peculiar sort of heat constitutes only a part of the total effect. 

 These results do not indeed present so simple a theory as that 

 alluded to, but they apply very obviously to the explanation 

 of many phaenomena recorded by various experimenters. 



(28.) The peculiar heat above spoken of, and which for the 

 sake of distinction and brevity we may call "transmissible 

 heat," is similar to that which acts in the solar rays, and which 

 there constitutes the total effect. It is this kind of heat which 

 has been employed as a principle of photometry, on the as- 

 sumption that it is precisely proportional to the intensity of 

 liorht. Within certain limits this may be the case ; but there 

 are unquestionably circumstances under which the relation is 

 very different; such, for example, as difference of colour in 

 the light : and in general it cannot be assumed to hold good 

 in light from different sources. To show this, there is a re- 

 markable instance in incandescent metal, which produces but 

 very faintly illuminating raj'S, j-et its " transmissible heat" is 

 very considerable. I have repeatedly tried the experiment with 

 a small " photometer," having one bulb painted with Indian ink 

 and the other plain : the bulbs being in a vertical line, this in- 

 strument, whether employed with or without its case or a glass 

 screen, always gave an effect of about 10° in 30" at eight 

 inches distance from a ball of iron heated to the brightest point 

 in a common fire. 



(29.) In making these last experiments, the effect was al- 

 ways greater when the instrument was used without its case or 

 a glass screen. This was no doubt in part owing to the 

 greater action of the simple heat now admitted to the instru- 

 ment on the coated than on the plain bulb ; but it was also in 

 part occasioned by the circumstance, that the stem going to 

 the upper bulb passes in contact with the lower, and being a 

 solid mass compared with the thin bulb, is slower in acquiring 

 heat, and therefore cools it, — thus increasing the apparent effect 

 on the other. 



(30.) In a variety of other experiments which I have tried, 

 using either this " photometer," or another having the bulbs 

 at equal heights, various apparent anomalies presented them- 

 selves ; 



