136 Experimental Philosophy. [Lecture 10. 



dred and fifty particles in that space of time, each 

 particle must be more than one thousand miles 

 distant from the other*. Indeed it is reasonable 

 to suppose that they must be at great distances 

 asunder, or they could not pass so continually as 

 they do in all directions, without interfering with 

 each other. 



If, in fact, light were not thus thinly diffused 

 it must be extremely injurious to our organs, 

 since we find that when it is condensed or com- 

 pressed, as in the focus of a burning-glass, there 

 is no substance that can withstand its force. 

 Gold, when exposed to its influence, is instantly 

 melted, and even the diamond itself, which re- 

 sists a very intense chemical heat, is suddenly 

 dissolved. To show, however, still more de- 

 cisively, that the particles of light are naturally in 

 this extremely rare or diffused state, or, in other 

 words, follow each other at an immense dis- 

 tance, it is a well-known fact, that the rays of 

 light, even when collected in the focus of the 

 strongest burning-glass, will not inflame spirit of 

 wine, or any other combustible matter, while they 

 merely pass through it. To make you com- 



* This is, in truth, quite an extreme estimate. It ap- 

 pears from the accurate experiments of M. D'Arcy (Mem. 

 Acad. Par. 1?65), that the impression of light upon the 

 retina continues two minutes and forty seconds : and as a 

 particle of light would move thirty-two millions of miles 

 during that interval, constant vision would be maintained 

 by a succession of luminous panicles, thirty-two millions 

 of miles distant from each other. 



