on the terrestrial Rays that occasion Heat. 519 



occasioned by visible rays ; then will 1000 — x be the remainder, 

 which must be ascribed to rays that cannot be seen. Now, by 

 our table, we find that crown glass, of which one side has been 

 rubbed on emery, stops 854 rays of light. These alone, if not a 

 particle of invisible heat were stopped, would be equal to ,854 x 

 visible rays of heat, that must be intercepted by the glass. When 

 the other side of this glass has also been rubbed on emery, it will 

 stop 932 rays of light, which will give ,932 x, for the quantity 

 of heat to be intercepted, on the same supposition, that all invi- 

 sible rays of heat are transmitted. But, by our fourth table, we 

 have the actual stoppage of heat of these glasses ; which will 

 therefore give us the two following equations ; ,854, x == 464, 

 and ,932 x = 66y. Then, taking the first from the last, and 



reducing the remaining equation, we obtain x = ~x, for the 



visible part of the total heat. But ~1 or 2602, being only a 



part, comes out greater than 1000, which is the whole; and 

 this being absurd, it follows that visible rays of heat cannot be 

 admitted, in any proportion whatsoever. This will equally hold 

 good with any additional stoppage of invisible heat, provided it 

 be equal in both glasses ; and, of this equality, the 165th and 

 167th experiments can leave us no room to doubt. 



But it is high time that we should now take into consideration 

 a more direct proof, which may be drawn from our prismatic 

 experiments. The results of them are here brought into a table, 

 as follows. 



3X2 



