682 PHILOSOPHICAL TRANSACTIONS. [ANNO ] 800. 



on 2 shining, red points; they are pretty bright, o, I see many more points : the 

 object is better illuminated than in the red: the points are surrounded by black; 

 but are orange-coloured, y. The points now are yellow, and white surrounded by 

 black : the object is better illuminated than in orange. The maximum of illumi- 

 nation is in the brightest yellow, or palest green, g. The points are green and 

 white, as before surrounded by black : better illuminated than in orange, b. The 

 illumination is nearly equal to red. i. Very indifferently illuminated, v. Very 

 badly illuminated. 



The phenomena of the differently-coloured points being now completely resolved, 

 since they were plainly owing to the former admixture of colours, and the illumi- 

 nating power remaining ascertained as before, I attempted also to repeat the experi- 

 ments on the thermometer, with the prism covered in the same manner ; but I 

 found the effect of the coloured rays too much enfeebled to give a decisive result. 



I might now proceed to my next subject; but it may be pardonable if I digress 

 for a moment, and remark, that the foregoing researches ought to lead us on to 

 others. May not the chemical properties of the prismatic colours be as different 

 as those which relate to light and heat ? Adequate methods for an investigation 

 of them may easily be found; and we cannot too minutely enter into an analysis of 

 light, which is the most subtle of all the active principles that are concerned in 

 the mechanism of the operations of nature. A better acquaintance with it may 

 enable us to account for various facts that fall under our daily observation, but 

 which have hitherto remained unexplained. If the power of heating, as we now 

 see, be chiefly lodged in the red- making rays, it accounts for the comfortable 

 warmth that is thrown out from a fire, when it is in the state of a red glow ; and 

 for the heat which is given by charcoal, coke, and balls of small-coal mixed up 

 with clay, used in hot-houses; all which, it is well known, throw out red light. 

 It also explains the reason why the yellow, green, blue, and purple flames of burn- 

 ing spirits mixed with salt, occasion so little heat that a hand is not materially 

 injured, when passed through their coruscations. If the chemical properties of 

 colours also, when ascertained, should be such that an acid principle, for instance, 

 which has been ascribed to light in general, on account of its changing the com- 

 plexion of various substances exposed to it, may reside only in one of the colours, 

 while others may prove to be differently invested, it will follow, that bodies may be 

 variously affected by light, according as they imbibe and retain, or transmit and 

 reflect, the different colours of which it is composed. 



Radiant Heat is of Different Refrangibility. — I must now remark, that my 

 foregoing experiments ascertain beyond a doubt, that radiant heat, as well as light, 

 whether they be the same or different agents, is not only refrangible, but is also 

 subject to the laws of the dispersion arising from its different refrangibility ; and, 

 as this subject is new, I may be permitted to dwell a few moments on it. The 

 prism refracts radiant heat, so as to separate that which is less efficacious, from 

 that which is more so. The whole quantity of radiant heat, contained in a sun- 



