1824.] Mr. Poivell on Terrestrial Light and Heat. 183 



de Vet at de lumiire semble indiquer leprogres d'un meme pheno- 

 mene qui dans ses modifications diverses, agit sur nous inegale- 

 rnent, comnie si les emanations calorijiques n'etaient que de la 

 lumiere obscure, et la lumiere du caloriqne lumineux." (P. 612.) 



(4.) Some observations bearing upon this subject occur in 

 Dr. Brewster's elaborate paper on " New Properties of Heat, 

 &e." in the Phil. Trans. 181b', Part I. His 40th proposition is 

 directed to prove that radiant heat is not susceptible of refrac- 

 tion, and is incapable of permeating glass like the luminous 

 rays. The truth of this is demonstratively shown from the 

 curious properties examined in the previous parts of the paper, 

 and shown to be communicated by heat to glass ; and by the 

 progress of which, the passage of the heat through the glass 

 may be as clearly traced as if the heat itself were visible. 



He applies this conclusion to the experiment of Sir W. Her- 

 schel, in which the concentration of simple heat by a lens 

 appears to be proved. The thermometer must have received 

 the heat radiated by the lens itself, and from the circumstance 

 that the edges will cool first, the most copious radiation of heat 

 will be in the direction of the axis. By a reference to Sir W. 

 Herschel's paper (Phil. Trans. 1800, Partll. No. 15, Exp. 19, 20), 

 it will obviously admit of question, whether in addition to the 

 causes thus shown to be operating, the light from the red-hot 

 metal may not have affected the thermometer. I have tried 

 several experiments (an account of which will hereafter be 

 given), from which it appears to me that the light from incan- 

 descent metal possesses a much greater heating power (using 

 that term in the sense before defined) than might be anticipated 

 from its small illuminating effect. Dr. Brewster's proposition, 

 however, is a most important and conclusive one as to the inca- 

 pacity of simple heat to undergo refraction, or to be transmitted 

 in the way of direct radiation through glass. In connexion with 

 the same point he also examines the conclusions of MM. De la 

 Roche and Prevost. These observations I give in his own 

 words : — 



" The ingenious experiments of M. Prevost, of Geneva, and 

 the more recent ones of M. De la Roche, have been considered 

 as establishing the permeability of glass to radiant heat. 

 M. Prevost employed moveable screens of glass, and renewed 

 them continually, in order that the result he obtained might not 

 be ascribed to the heating of the screen ; but such is the rapidity 

 with which heat is propagated through a thin plate of glass, that 

 it is extremely difficult, if not impossible, to observe the state of 

 the thermometer before it has been affected by the secondary 

 radiation from the screen. 



" The method employed by M. De la Roche of observing the 

 difference of effect, when a blackened glass screen, and a trans- 

 parent one, were made successively to intercept the radian) 



