HIS TORT of the SOCIEtr. n 



To account for this phenomenon, M. Pictet conluleis the 

 thermometer as the body irradiating heat, and the matrafs with 

 the ice as the body which receives it, fo that the experiment is 

 the fame with the former, only that the obfcure heat moves in 

 the contrary dlredlion. 



This explanation, however, is not only liable to the objec- 

 tions that have been made, in general, to the fuppofition of ra- 

 diant and obfcure heat, but it involves in it new difficulties. It 

 implies, for inftanee, that the irradiation from the heated bodv 

 is affedled by the ftate of the body which receives that irradia- 

 tion, a principle furely contrary to all analogy. In the inadia- 

 tion of light from a luminous body, nothing fimilar to this is 

 obferved : Whether the light of a candle fall on a white wall, 

 by which it is refledled, or on a black wall, by which it is ab- 

 forbed, no difference is produced in the quantity of light emit- 

 ed, but it remains in both cafes the fame. In no cafe, it fhould 

 feem, can the quantity of the radiating matter depend on the 

 condition of the recipient bodies ; yet, according to the prece- 

 ding explanation, a body mud be fuppofed to irradiate heat 

 more copioufly when the body on which the irradiation falls 

 is cold than when it is hot ; a fuppofition which, being, as has 

 been faid, contrary to analogy, cannot be admitted. 



The Dodlor next proceeds to offer his own explanation, but 

 witli the diffidence that ought to accompany every attempt to 

 account for a phenomenon fo fingular as this, and having fo little 

 analogy with any other fadl that relates to the communication of 

 heat. He fuppofes that all bodies irradiate invilible light, when 

 they are of an ordinary temperature, and that this irradiation di- 

 miniflies as their heat diminifhes. The temperature of the 

 thermometer, therefore, in the above experiment, is to be confi- 

 dered, like that of all other bodies, as being maintained by the 

 adion of two caufes, viz. the irradiation of invifible Ught from 

 the furrrounding bodies, and the communication of heat from 



b 2 them 



