Obfervations and Expirimenis on Light and Heat. 34$ 



afcetiding cold Jlream is certainly original; at Icaft, it ftands in dire£l oppofition to all the 

 eftabliftied principles of philofophy, though it is not calculated to infpire the moft favour- 

 able opinion of the' author's acumen in phyfical inquiries. The tardy progreffion frorii 

 violet to red compared with the numbers which I have afligned, feems to betray the ac- 

 cefTion of extraneous influence. Befides, the dark ball of a thermometer will, for the moft 

 part, receive its full imprefllon in the fpace of one or two minutes ; but the Do£lor's 

 thermometers required, or at leaft were allowed, an expofure of ten or thirteen minutes. 

 This fa£l affords a ftrong prefumption that other matters were brought into aftion, capable 

 of a more prolonged abforption, and of a greater accumulation of heat, than a mercurial 

 bulb. Nor can I hefitate to impute the principal derangement to the platform on which 

 the therrnxsmeters were laid, which, receiving light, though partially, would, as before 

 obferved, acquire Jieat in a qujldruple ratio, and communicate this to the contiguous 

 ftratum of air. The fame confideration will explain the origin of the miftake on which the 

 paradoxical aflertion is grounded, of " invifible refrangible light." Indeed, a more ob- 

 jeftionable plan of conducing experiments could fcarcely have been dcvifed. The prifmi 

 was fixed in an inverted pofition, to direft the rays downwards upon a table, which was 

 covered with white paper -, and the thermorrieter was placed a little beyond and above the 

 edge of the red light. In fuch a fituation no wonder that the bulb was notably afFe£ted,, 

 being immerfed in a warm atmofphere which extends to a certain diftance over the illu'- 

 minated fpace, and receiving likewife a large portion of the feveral coloured rays refl'e£led 

 from the paper. That joint effeft would be ftill further increafed by the thermometer 

 having its bulb dire£tly oppofed to the afcent of the heated air. The a£l:ion might even be 

 greater than if the bulb were placed in the colirfe of the red rays, fince it would then in- 

 tercept that light which would have produced an accumulated effedt on the wood below.. 

 But why is the maximum of invifible light ftated at half an inch beyond the red rays .''• 

 Does it move in ftraight lines? Does it diverge at a certain angle, or has it various 

 degrees of refrangibility ? Thefe were queftions to be refolved. And after all, what has 

 invifible matter to do with that peculiar ftru£lure of the furface of bodies which conftitutes 

 black or white ? 



Should any doubt remain after this long examination, there is a fingle fa£t which at 

 once demoliflies the whole fabric. If thofe invifible calorific rays had any real exiftence, 

 the aflion of a burning glafs would be moft powerful, not at the proper focus, but a con- 

 fiderable way beyond it. For the fame reafon, the hole burnt in a black piece of doth, 

 would not be confined to the lucid focus, but would include a ring fwelling all round to- 

 more than double that diameter.. 



Mm to deny that every current of air which afcends, in confequence of the proximity of an heated body,, 

 muft be cold when compared with that body itfeif ; or (to keep more ftriftly to' the expreflion alluded to) 

 that it cannot receive the heat which makes it afcend, without cooling, or exerting a cooling agency on, the. 

 body which heats it,. N. 



3 I havft 



