RADIANT HEAT. oT3 



as it is emitted by different bodies, or is excited in them in a different 

 way. ; J 



This inquiry was pursued by a series of experiments, in which (1) 

 a vessel of water at 212° and (2) the flame of a lamp had in contact 

 with them various ad lather manous substances, su<*h as metal, porce- 

 lain, leather, wood, &c, in the heat given out, by which the series 

 of screens gave exactly similar series of effects. As also with the 

 heat from a hollow cylinder of copper or iron surrounding a flame. 

 But when the direct radiation of the flame was employed, the series 

 of results were in proportions considerably different. — (Tables XIII 

 to XVI.) 



The author, indeed, remarks, at the conclusion, that these last 

 differences are due to the heat transmitted by the screens, i. e., the 

 heat conveyed by the luminous rays; as, indeed, would be manifest, 

 according to the views at first noticed. 



In another series (1) an adiathermanous body at several tempera- 

 tures, from 88° to 212°, (2) the flame of a lamp and (3) a metal cylin- 

 der round a flame below 234°, were severally tried with the same 

 series of screens; the results in cases (1) and (3) being found exactly 

 similar, in (2) in a very different proportion. 



The same sources were next tried with two screens interposed; the 

 first being, successively, metal with holes, silk, ivory, &c, and each 

 of these combined with the several screens of the former series. In 

 all cases the results gave proportional series. — (Tables XVII to XIX.) 



Another series was conducted with, 1st, a flame, and, 2d, water at 

 212°; each in succession with a screen used first plain and then 

 blackened, the screens being black glass, lac, ivory, paper, &c. ; the 

 results being always less with the blackened surface, very similar in 

 each case, and all less with the hot water. — (Table XX.) 



Another set, with a heated mass, and with the hand at its natural 

 temperature for sources, gave similar results with various screens. — 

 (Table XXI.) 



This, the author says, "disproves the opinion of Forbes, that the 

 heat emitted by boiling water and the hand must be considered as 

 different." I am unable to find in what respect Professor Forbes 

 supposes them different. 



At the conclusion of this section the author adverts to two practical 

 inferences from what has preceded. 



(i.) The fact that the amount. of heat absorbed by a given body is 

 the same, from whatever source it was derived, is important in regard 

 to the determination of specific heats by the colorimeter ; for if the heat 

 absorbed by the ice were different as it might be derived from differ- 

 ent sources, no correct measure of specific heat would be obtained. 

 That it is not so insures the accuracy of the results, so far as this 

 source of error is concerned. 



(ii.) The second application is, that "these results lead to anew 

 method of ascertaining whether any substance transmits rays of heat 

 or not," (235;) that is, of determining whether any given instance of 

 transmission of heat is really due to diathermancy, or is merely sec- 

 ondary radiation. 



