158 Experimental Investigations 



SECTION IV. The Heat produced in a Body by a given 

 Quantity of solar Light is the same whether the Rays be 

 denser or rarer, convergent, parallel, or divergent. 



In all cases where the rays of the sun strike on the sur- 

 face of an opaque body without being reflected, heat is 

 generated and the temperature of the body is increased; 

 but is the quantity of heat thus excited always in pro- 

 portion to the quantity of light that has disappeared ? 

 This is a very interesting question and has not hitherto 

 found a decisive solution. 



When we consider the prodigious intensity of the 

 heat excited in the focus of a burning mirror or a lens, 

 we are tempted to believe that the concentration and 

 condensation of the solar rays increase their power of 

 exciting heat; but, if we examine the matter more 

 closely, we are obliged to confess that such an augmen- 

 tation would be inexplicable. It would be equally so 

 on both the hypotheses which natural philosophers 

 have formed of the nature of light ; for if light be analo- 

 gous to sound, since it has been proved, both by calcu- 

 lation and experiment, that two undulations in an 

 elastic fluid may approach and even cross each other 

 without deranging either their respective directions or 

 velocities, we do not see how the concentration or con- 

 densation of these undulations can increase their force of 

 impulse; and if light be a real emanation, as its velocity 

 is not altered either by the change of direction it under- 

 goes in passing through a lens or by its reflection from 

 the surface of a polished body, it seems to me that the 

 power of each of these particles to excite or impart 

 heat must necessarily be the same after refraction or 

 reflection as before, and consequently, that the heat 



