HISTORT of the SOClETi: 9 



the different fpecies of light, when of equal intenfiCy, as eftima- 

 ted by the eye, are of unequal intenfity when their effecft is 

 meafured by the thermometer. In thefe experiments he ren- 

 dered light of different colours equally intenfe to the eye, 

 by increafing or diminifliing the diflance from the luminous 

 body, till he could juft read by the light of it. In this way he 

 compared the red light from a fire of coals, with the white light 

 of flame, and found, that while they were equally powerful iA 

 affording vilion, the red was incomparably the moil powerful 

 in producing heat. 



When a body, therefore, is heated to incandefcence, like the 

 iron ball in M. de Saussure's experiment, it emits at firfl the 

 white or compound light, but as it cools, the light which it 

 emits becomes of the red fpecies, and this is the laft that dlfap- 

 pears. As the body cools, therefore, the power of its light, to 

 produce heat, increafes in proportion to its power to afford vi- 

 lion, and, therefore, when this laft vaniflies, or ceafes entirely, 

 the other may ftill remain in a certain degree. Thus, in the 

 experiment juft defcribed, the iron ball, after it had loft all light 

 to the €ye, continued to emit rays of light, which, though they 

 made no impreflion on the organ of vifion, had power to pro- 

 duce heat, and expand the mercury in the thermometer. To the 

 principle, therefore, of the irradiation of obfcure heat, by which 

 M. deSaussure explains the above phenomenon, Dr Hutton 

 fubftitutes that of obfcure^ or invifihle light, which, though it be 

 in appearance more paradoxical, is in reality free from the very 

 ftrong objedlions which prefs againft the other hypothefis. 



We muft not omit to obferve, that M. Pictet varied the ex- 

 periment, by placing a matrafs full of boiling water, inftead of 

 the iron ball, in the focus of one of the fpecula. The thermome- 

 ter in the other was ftill affe^ed, and raifed a httle more than a 

 degree. The irradiation of invifible light explains this alfo ; for it 

 is natural to fuppofe, that fuch an irradiation takes place from 



Vol, IV. b all 



