236 Professor Powell 07i the Influence of Colour on Heat. 



lateral and less direct conduction of heat : or, again, if some 

 portion of the absorbing surface be not exposed to the direct 

 rays of heat, from that portion, a radiation of acquired heat will 

 take place on the same side. Now, on this principle I formerly 

 offered an explanation of the results. 



De la Roche's apparatus consisted of conjugate reflectors: 

 in the foci were a heated ball and a thermometer, between them 

 the screen, having its coated side towards the ball. The por- 

 tions of the screen which fall without the area of the rays will 

 not radiate their heat so as to produce much sensible effect on 

 the thermometer : and they will at the same time give out their 

 heat more rapidly on the coated side, by virtue of its better ra- 

 diating power, and consequently abstract more from the other 

 parts on the same side, than when the glass was plain ; and the 

 same to a certain extent may take place even within the section 

 of the rays, since the central point of the screen will be that 

 most heated by the additional direct action of the hot ball. 

 With the plain glass there was neither so great an excess of 

 heat (from its less absorptive texture), nor such a tendency to 

 radiate it on one side rather than the other. 



It would, however, be satisfactory to see whether experiment 

 will shew any instance of a screen having a more absorptive sur- 

 face exposed to the radiating body, and yet not acquiring a cor- 

 responding increase of temperature on its other surface ; or whe- 

 ther that surface would, even in any case, shew a less acquisition 

 of heat than when the exposed surface was plain, so that the re- 

 lative temperatures of the outer side in the two cases should at 

 all correspond with De la Roche's result of less transmitted ef- 

 fect with the more absorptive surface ; and thus the dependence 

 of the latter effect on the former as its cause, be in any degree 

 rendered more probable. 



With this view, I may be permitted to mention a few experi- 

 ments which I tried long ago, comparing the temperature of the 

 outer surface of a plate of glass when the inner, or side exposed 

 to the source of heat, was respectively plain, or coated with In- 

 dian ink. The hot body was an iron ball heated and then cool-, 

 ed to just below visible redness in the dark. A small thermo- 

 meter was attached to the outer side of the glass, and its bulb 

 kept in contact by a wire spring ; before experiment, the ther- 



