on the terrestrial Rays that occasion Heat. 515 



ascertained, in a great measure, by our 13th, 17th, and 18th 

 experiments. In the 13th, one hundred and twenty degrees of 

 heat were given to a thermometer, in one minute, by the rays 

 which accompany the coloured part of the spectrum. In the 

 17th experiment, on the contrary, we find only 45 degrees 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 degrees; but, in order to make 

 every possible allowance, let the proportion be the most favour- 

 able 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 shews 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 occa- 

 sioned by the rays of light, the difficulty becomes fully resolved 

 by our 100th experiment, which shews that full three-tenths of 

 the rays that have the refrangibility of the red are actually 

 transmitted. 



In order, however, to make a second 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 experiments might stop a greater number of invisible 



