3«» Refrangihility of radiant Heat, &e. 



The obfervations which agreed uncommonly well together, afforded the conclufion that 

 the red making rays were far from poffefling an eminent degree of illumination : that the 

 orange poflefs more of it than the red ; the yellow ftill more ; the maximum of illumination 

 lies in the brighteft yellow, or the paled green ; the green itfelf is nearly as bright as the 

 yellow; but from the full deep green the illuminating power decreafes very fenfibly. That 

 of the blue is nearly upon a par with the red ; the indigo much lefs than the blue ; and 

 the violet is very deficient. With regard to the principle of diflinftnefs, none of the co- 

 lours appeared to be deficient ; that is to fay, that though for want of illumination in the 

 lefs powerful colours in this refpeft, fewer bright fpots could be difcerned, yet thofe which 

 were vifible were perfedtly diflinft. 



Before the Dr. proceeds to the next part of his fubje£l:, he digrefles for a moment to 

 remark, that the foregoing refearches ought to lead us on to others. " May not" fays he, 

 *' the chemical properties of the- prifmatic colours be as different as thofe which relate to 

 light and heat ? Adequate methods for an inveftigation of them may eafily be found; and 

 we cannot too minutely enter into an analyfis of light, which is the moft fubtle of all the 

 adtive principles that are concerned in the mechanifm of the operations of nature. A 

 better acquaintance with it may enable us to account for various fafts that fall under our 

 obfervation, but which have hitherto remained unexplained. If the power of heating, as 

 we now fee, 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 ftate of a red glow; and for the 

 heat which is given out by charcoal, coke, and balls of fmall coal mixed up with clay ufed 

 in hot houfes ; all which it is well known throw out red light. It alfo explains why the 

 yellow, green, blue, and purple flames of burning fpirits mixed with fait, occafion fo little 

 heat, that a hand is not materially injured, when paflTed through their corufcations. If the 

 chemical principles of colours alfo when afcertained, (hould be fuch, that an acid princi- 

 ple, for inftance, which has been afcribed to light in general, on account of its changing 

 the complexion of various fubftances expofed to it, may refide only in one of the colours, 

 while others may prove to be differently inverted, it will follow, that bodies may be vari- 

 oufly affe6led by light, according as they imbibe and retain, or tranfmit and reflect, the 

 different colours of which it is compofed." 



Tiat radiant heat is of different refrangibilityi is alfo dcducible from thefe experiments, 

 whether it be one and the fame thing, or different from light; becaufe, as the author ob- 

 ferves, if this were not the caf«, the whole of the heat would be confined in a fpace equal 

 in breadth to the prifm itfelf ; the contrary to which is proved by the fads. He alfo ftates, 

 as is more fully developed in the fubfequent paper, that as the maximum of light is found 

 at a much higher degree of refrangihility than that of heat in the vifible fpeftrum, the ab- 

 folute maximum lies even fliort of the limits of vifible radiation in the fpeftrum. And 

 admitting, as is highly probable, that the organs of fight are only adapted to receive im- 

 preffions from particles of a certain momentum, an explanation will be had, why the maxi- 

 mum of illumination Ihould be in the middle of the refrangible rays, as thofe which have 

 4 greater 



