on the terrestrial Rays that occasion Heat. 509 



incidence and refraction of the mean rays is not the same in 

 both. Heat is evidently less refrangible than light; whether 

 we take a mean refrangible ray of each, or, which I believe to 

 be the better way of proceeding, whether we take the maximum 

 of heat and light separately. This appears, not only from the 

 view we have taken of the two spectra already mentioned, but 

 more evidently from the 23d experiment, by which we find, that 

 heat cannot be collected by a lens, to the same focus where light 

 is gathered together. 



Our fifth article, in which an account has been given of the 

 proportions of heat and light stopped by glasses and other sub- 

 stances, will afford us now an ample field for pointing out a 

 striking difference between these two principles. From the 

 24th to the 30th experiment, we have the quantities intercepted 

 by colourless substances as follows. 



Table I. 



Bluish-white glass stops 



250 rays of heat, and 86 of light 



White flint glass - 



91 - _ _ 34 _ _ 



Greenish crown glass - 



259 - - - 203 - - 



Coach glass - 



214 - 168 - 



Iceland crystal - 



244 - - - 150 - - 



Talc - 



139 - 90 - - 



Calcinable talc - 



184 - - 288 - - 



Now, by casting an eye on the above table, it will be seen 

 immediately, that no kind of regularity takes place among the 

 proportions of rays of one sort and of another, which are 

 stopped in their passage. Heat and light seem to be entirely 

 unconnected. The bluish-white and flint glasses, for instance, 

 stop nearly three times as much heat as light ; whereas, the 



mdccc. 3 U 



