10 HlSTORr of the sociEir, 



all bodies, when above a certain temperature, whether they be 

 in the a6l of cooling down from incandefcence or not. 



The fame ingenious and accurate obferver, made another 

 phange in the circnmftances of the experiment, by fmoking the 

 bulb of the thermometer ; in confequence of which it was heat- 

 ed fooner, and rofe higher than before. This appearance is per- 

 fedlly conformable to Dr Hutton's theory, and feems quite 

 inconliftent with the other. The black coating of the bulb, by 

 its well known property of abfbrbing light, tended to accelerate 

 and increafe the effedl of the light in heating the thermometer ; 

 but the fame coating being of fmoke, and a very bad con- 

 ductor of heat, muft have oppofed the tranfmiffion of heat 

 through the glafs, and have both retarded and diminiflied its 

 effea. 



Nothing, indeed, can be more unlike than the laws which 

 ufually regulate the propagation of light and heat. To move 

 with extreme velocity through the tranfparent fubftance of 

 fome bodies, without heating them in any fenfible degree ; to 

 be reflected from the furfaces of others, without entering them 

 at all ; and, laftly, to be abforbed by certain bodies, neither 

 pafling through them, nor being reflected from them, thefe are 

 the properties of light. Heat, on the other hand, is lowly pro- 

 pagated through all bodies, combines with them intimately in 

 its paflage, and often remains at reft without any motion what- 

 ever. 



I The converfion of thefe experiments, which was very inge- 

 nioufly imagined by M. Pictet, led to a fadt ftill more fmgu- 

 lar and unexpe(fted. Inftead of the heated body, he placed a 

 matrafs, with ice in the focus of one of the fpecula \ the confe- 

 quence was, that the thermometer in the focus of the other was 

 fenfibly depreffed. When the cold was increafed, by pouring 

 nitrous acid on the ice, the depreffion of the thermometer was 

 alfo increafed. 



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