Light. 137 



prebend this fact more clearly, I must observe, 

 that whatever light passes through is called a me- 

 diiim, and those substances which do not reflect 

 the rays, but which may be seen through, are 

 called transparent ; those, on the contrary, which 

 intercept or reflect the rays, are called opaque. 

 Now a phial in which spirit of wine is contained 

 is a transparent medium, and in that state the 

 spirit will not be set on fire : if, on the other hand, 

 the spirit is poured forth into a spoon, or any 

 opaque vessel, which, in fact, intercepts the rays 

 of light, stops them in their progress, and thus 

 collects them in a mass, it will immediately be 

 inflamed. This, I think, proves, that the par- 

 ticles of light must follow each other at a great 

 distance, and that they must be in the first place 

 compressed together by the force of the burning- 

 glass, and then stopped and condensed by an 

 opaque body, to enable them to produce a consi- 

 derable degree of heat. 



That light may be exceedingly diluted, as well 

 as condensed, we may easily perceive ; for the 

 light of the glow-worm, of rotten wood, and of 

 what are called the solar pJwsphori, can never be 

 condensed by any burning-glass, so as to pro- 

 duce the slightest degree of heat. The expe- 

 riment has also been made with the light of the 

 moon, and that has been found too faint and 

 rare to be condensed into a burning focus. 



The principle upon which the rays of light are 

 collected in the focus of a burning-glass NY ill be 



