306 RADIANT HEAT. 



Rise of focal thermometer (centig.) in l m . 



No screen. Transparent Blackened 



screen. screen. 



Mercury at 180° 3°.94 0°.22 0°.07 



Mercury at 346 16 .33 1 .36 .17 



He hence infers a partial transmission of heat at these high tem- 

 peratures; and the more so, viewing these results in connexion with 

 the rest of the subsequent series (considered in another place.) 



These are the only ones of his experiments referring really to 

 simple radiant heat; and the inference of an actual transmission in the 

 way of direct radiation is open to several objections. 



6.) The blackened screen causes a greater diminution of heat than 

 the transparent, and it was therefore inferred that a portion of heat 

 radiates freely through the transparent screen, and is stopped by the 

 opaque one; but there are several circumstances which show that this 

 is not a necessary conclusion. 



The coating was towards the source of heat, and rendered this 

 screen more absorptive of heat where exposed to it, that is, at its 

 central part, and a better radiator towards the edges without the 

 area of the incident rays; so that it radiated its heat most copiously 

 on the side away from the thermometer. With the plain screen there 

 was no such tendency to radiate more on one side than on the other, 

 and hence the greater effect on the thermometer. 



This explanation I suggested in the Annals of Philosojiliy, xlv, 181. 



Some observations bearing upon this subject occur in Sir David 

 Brewster's elaborate paper on "New Properties of Heat," &c, in 

 the Phil. Trans. 1816, Part I. His 40th proposition is directed to 

 prove that radiant heat is not susceptible of refraction, and is incapa- 

 ble 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 Wm. Herschel, 

 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. 



In connexion with the same point, he also examines the conclusions 

 of MM. De la Roche and Prevost, and observes: "The ingenious ex- 

 periments 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 ob- 

 tained 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 



