lo HlSrORT of the SOCIEir. 



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

 in the adl of cooling down from incandefcence or not. 



The fame ingenious and accurate obferver, made another 

 change in the circimiftances 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- 

 fe<5\ly conformable to Dr Hotton's theory, and feems quite 

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

 its well known property of abfbrblng light, tended to accelerate 

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

 l)ut the fame coating being of fmoke, and a very bad con- 

 duftor of heat, muft have oppofed the tranfmifhon of heat 

 through the glafs, and have both retarded and diminilhed its 

 effea. 



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

 ufually regulate the propagation of light and heat. To move 

 with extreme velocity through the tranfparent fubflance of 

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

 be reflefted from the furfaces of others, without entering them 

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

 pafllng through them, nor being refledled 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 palTage, and often I'emains at reft without any motion what- 

 ever. 



The converfion of thefe experiments, which was very inge- 

 nioufly imagined by M. Pictet, led to a faft ftill more fingu- 

 lar and nnexpecled. Inftead of the heated body, he placed a 

 matrafs, with ice in the focus of one of the fpecnla ; 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. 



To 



