VOL. XC.] PHILOSOPHICAL TRANSACTIONS. ^J 



the rays of light be also those of heat, there can hardly come any heat through it 

 but what must be occasioned by rays that are invisible. It will therefore become 

 a question to be examined, how many of this sort we can admit, if we proceed 

 on a supposition that heat consists of light, as far as that will go. Now this we 

 find has already been ascertained, in a great measure, by our 1 3th, 17th, and 

 18th experiments. In the 13th, 120° of heat were given to a thermometer, in l 

 minute, by the rays which accompany the coloured part of the spectrum. In the 

 17th experiment, on the contrary, we find only 45° of heat communicated to the 

 same thermometer, in the same time, by the invisible rays of the same spectrum. 

 If we would be more scrupulous, the 18th experiment limits the heat from rays 

 totally invisible even to 21°; but in order to make every possible allowance, let 

 the proportion be the most favourable one of 120 to 45, which, reduced to mean 

 rays of heat, will give 727 of them visible, and 273 invisible, to make up our 

 thousand. 



To return to the experiment : if the total number of rays of heat ascribed to 

 light should accordingly be rated at 727, it is evident, from the stoppage of light 

 of this glass, that 726 rays of heat at least must also be intercepted ; and, in 

 consequence of the 153d experiment, which shows that our glass opposes no 

 obstruction to any of the invisible rays, we shall require no more. But, by our 

 present experiment, this glass stops only 606 rays of heat; so that 120 of them 

 will remain unaccounted for. Now the moment we give up the hypothesis that 

 heat is occasioned by the rays of light, the difficulty becomes fully resolved by our 

 100th experiment, which shows that full -^ of the rays that have the refrangi- 

 bility of the red are actually transmitted. In order however to make a 2d attempt 

 to overcome this difficulty, without giving up the hypothesis, it may be supposed, 

 " that perhaps the lens, which has been used in the 13th, 17th, and 18th experi- 

 ments, might stop a greater number of invisible than visible rays, and that its 

 report therefore ought not to be depended on." Now, though it does not appear 

 from the 148th experiment that such a supposition can have much foundation, yet 

 since those experiments were not made with a view to ascertain the proportion of 

 heat contained in each part of the prismatic spectrum, we cannot lay so much 

 stress on them as the accuracy which is required in this case renders necessary. 

 Let it therefore, contrary to our 100th experiment, be admitted, in order to 

 explain the phenomenon of the red glass, which stops so much light and so little 

 heat, that all the heat which it intercepts consists entirely of the rays which are 

 visible, and that every one of the invisible rays of heat is transmitted. Then 

 will 999.8 intercepted rays of light be equal to 606* rays of heat ; and the remain- 

 ing 394 will be the number of rays we are now to place to the account of the 

 invisible heat which is transmitted. 



Having thus also got rid of this difficulty, we are next to examine how other 

 facts, collected in the same table, will agree with our new concession. A violet- 

 coloured glass, for instance, stops 955 rays of light ; these, at the rate of 999. 8, 



vol. xviii. 5 G 



