46 Mr. Powell on Light and Heat 



(at least of ordinary thickness,) Iiowever transparent. Whatever 

 4oubts ijiay have been entertained on this point, I think are com- 

 pletely removed by the investigations of Dr. Brewster, in his 

 paper on " New Properties of Heat," ^-c — Phil. Trans. 1S16, 

 Part I., Prop. 40, ^-c. 



3. That from luminous hot bodies a very considerable heating 

 effect is produced, essentially different from that just described. 

 It passes through glass by direct radiation, without heating it, ajid 

 affects bodies in proportion to the darkness of their colour, without 

 relation to the texture of th^ir surfaces. This is evident from 

 the results of M. De la Roche, (^Biot. Traitt de Physique^ Vol. iv., 

 p. 640, 8^c.) Mr. Brande, {Phil. Trans. 1820, Part I.) ; and other 

 experiments. 



4. That of the total heating effect from this class of bodies a 

 considerable portion is stopped by glass. This appears from De 

 la Roche's experiments ; and further it is shewn that the degree 

 in which this interception takes place, decreases in proportion as 

 the body becomes more intensely luminous. 



The theory adopted by Professor Leslie, as also apparently by 

 M. Biot, in his account of the Relations of Light and Heat, (Traiie 

 de Physique, Vol. iv., p. 640, <§:c.), is, that this interception takes 

 effect upon one simple agent, which is heat, more or less con- 

 verted into light, according to the stage of combustion, ^-c, and 

 is greater, as the agent approaches more nearly to the form of 

 simple heat, in which case it is entirely stopped, or nearly so. In 

 order to establish this theory, it would be necessary to shew, that 

 whatever may be the particular law of relation to surfaces, by 

 which the action of the " igneous fluid" is determined at any stage 

 jpf its evolution, the portion intercepted should bear the same 

 relation in this respect as the portion transmitted. At the higher 

 degrees of incandescence, for example, this relation is to the 

 darkness of colour. The same relation ought consequently to 

 hold good with the portion intercepted, as with that transmitted 

 by the glass. At lower stages, an approach to the preference for 

 absorptive texture would be displayed, and this again should be 

 equally found in both cases. 



