780 PHILOSOPHICAL TRANSACTIONS. [ANNO1800. 



invisible heat from the coloured spectrum, by throwing that which is less refran- 

 gible than light to one side. But it has also been proved, that heat of the same va- 

 riety in refrangibility as the different colours, is also contained in every part of 

 the coloured spectrum. The question which we are discussing at present, may 

 therefore at once be reduced to this single point. Is the heat which has the re- 

 frangibility of the red rays occasioned by the light of these rays? For, should that 

 be the case, as there will then be only one set of rays, one fate only can attend 

 them, in being either transmitted or stopped, according to the power of the glass 

 applied to them. We are now to appeal to our prismatic experiment on the sub- 

 ject, which is to decide the question. First, with regard to light, I must anticipate 

 a series of highly interesting observations I have made, but which, though they 

 certainly claim, cannot find room in this paper. These have given me the means 

 of acting separately on either of the extremes, or on the middle of the prismatic 

 spectrum ; and by them I am assured that red glass does not stop red rays. Indeed 

 the appearance of objects seen through such coloured glasses, till I can give those 

 observations, will be a sufficient proof to every one that they transmit red light in 

 abundance. Next, with regard to the rays of heat, the case is just the reverse ; 

 for, by our preceding table, the red glass stops no less than 692, out of 1000, of 

 such rays as are of the refrangibility of red light. The incipient stoppage also, or 

 that in 2 minutes, of which something will be said hereafter, amounts even to 7 50 rays. 

 Now, if it should be suspected, " that on account of the great breadth of prism, 

 some invisible heat may be thrown on the spot where the red colour falls," I do 

 not only agree to it, but am certain it cannot be otherwise : but this again will 

 give additional weight to our present argument; for by the 153d experiment, as 

 our last table shows, it has already been ascertained, that all such heat will be trans- 

 mitted through a red glass ; so that, were it not for some of this admixture, the 

 stoppage might be still greater. Here then we have a direct and simple proof, 

 in the case of the red glass, that the rays of light are transmitted, while those of 

 heat are stopped, and that thus they have nothing in common but a certain equal 

 degree of refrangibility, which, by the power of the glass, must occasion them to 

 be thrown together into the place which is pointed out to us by the visibility of the 

 rays of light. The manifest use of the union of these rays, arising from their 

 equal refrangibility, will be explained at a future opportunity, when I may perhaps 

 throw out several hints that have already occurred to me, where the contents of 

 this paper may be applied to the useful purposes of life. 



There still remains a general argument, that heat and light are occasioned by 

 different rays, which ought not to be omitted. This, on account of the contracted 

 state in which the experiments have been given, cannot appear from my paper ; 

 but, by an inspection of them at full length, it is proved, that the stoppage of 

 solar heat, setting aside little irregularities, to which all observations are liable, has 

 constantly been greater in the 1st, 2d, or 3d minute, than in the 4th or 5th ; or, 

 more accurately, nearer the beginning of the 5 minutes, than about the end of 



