VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 6QI 



line of the red colour as near the outside of the ball as could be, without touching 

 it. Here the thermometer N° 1 rose, in 10 minutes, another degree higher than 

 in its former situation it could be brought up to ; and was now 9° above the standard. 

 The ball of this thermometer is exactly half an inch in diameter; and its centre 

 therefore was -£- inch beyond the visible illumination, to which no part of it was 

 exposed. 



It would not have been proper to compare these last observations with those 

 taken at an earlier period this morning, in order to obtain a true maximum, as the 

 sun was now more powerful than it had been at that 

 time: for which reason, I caused the line of termina- 

 tion of visible light, now to fall again just 4- inch from 

 the centre of the ball; and had the annexed result. 

 And here the thermometer N° 1 rose, in 10* minutes, 

 84-°, when its centre was 4- inch out of the visible rays of the sun. 

 we had a rising of 9°, and here 8^, the difference is almost too trifling to suppose, 

 that this latter situation of the thermometer was much beyond the maximum of 

 the heating power; while at the same time the experiment sufficiently indicates, 

 that the place inquired after need not be looked for at a greater distance. 



It will now be easy to draw the result of these observations into a very narrow 

 compass. The first 4 experiments prove, that there are rays coming from the sun, 

 which are less refrangible than any of those that affect the sight. They are invested 

 with a high power of heating bodies, but with none of illuminating objects; and 

 this explains the reason why they have hitherto escaped unnoticed. My present 

 intention is, not to assign the angle of the least refrangibility belonging to these 

 rays, for which purpose more accurate, repeated, and extended experiments are 

 required. But, at the distance of 52 inches from the prism, there was still a con- 

 siderable heating power exerted by our invisible rays, 1± inch beyond the red ones, 

 measured on their projection on a horizontal plane. I have no doubt but that their 

 efficacy may be traced still somewhat farther. The 5th and 6th experiments show, 

 that the power of heating is extended to the utmost limits of the visible violet rays, 

 but not beyond them; and that it is gradually impaired, as the rays get more refran- 

 gible. The last 4 experiments prove, that the maximum of the heating power is 

 vested among the invisible rays; and is probably npt less than half an inch beyond 

 the last visible ones, when projected in the manner before mentioned. The same 

 experiments also show, that the sun's invisible rays, in their less refrangible state, 

 and considerably beyond the maximum, still exert a heating power fully equal to 

 that of red-coloured light ; and that consequently, if we may infer the quantity of 

 the efficient from the effect produced, the invisible rays of the sun probably far 

 exceed the visible ones in number. 



To conclude, if we call light, those rays which illuminate objects, and radiant 

 heat, those which heat bodies, it may be inquired, whether light be essentially dif- 

 ferent from radiant heat? In answer to which I would suggest, that we are not 



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