14-3 



station till the effect was compensated by proportionably altering 

 the distances of the heated cylinders from the balls. In some further 

 experiments, instead of blackening one of the flat surfaces of one of 

 the cylinders, the other coverings used in the foregoing trials were 

 applied, and the results were such as might have been expected. 

 They all tended to prove that different bodies, or rather different 

 surfaces, emit heat not by any conducting power in themselves, or 

 in the surrounding bodies, but by a power which is here called ra- 

 diation, the nature of which had hitherto escaped our notice. 



Several experiments were next made with heated cylinders of dif- 

 ferent metals, but the results proved that all metals give off heat with 

 the same facility, or rather with the same celerity. May not this, it 

 is asked, be owing to their being all equally wanting in transparency ? 

 And does not this afford us a strong presumption that heat is in all 

 cases excited and communicated by means of radiations, or as they 

 may more properly be called undulations ? 



Before these questions can be solved, another and a very important 

 point in this inquiry must be decided, viz. whether bodies are cooled 

 in consequence of the rays they emit, or by those they receive ? Our 

 author was manifestly led to this problem by the celebrated experi- 

 ment of Prof. Pictet, from which it appears that rays or emanations 

 which (like light) may be concentrated by concave mirrors, proceed 

 from cold bodies ; and that these rays when so concentrated, are ca- 

 pable of affecting an air thermometer in a manner perfectly percep- 

 tible. The first experiment on this subject was to ascertain the ex- 

 istence of these cold emanations universally ; and this being success- 

 fully effected, it is proved by other processes, the detail of which 

 would far exceed our bounds, that the radiation of cold as well as of 

 hot bodies being established, the rays which proceed from cold bodies 

 have likewise the power of generating cold in warmer bodies which 

 are exposed to their influence. 



The object of another set of experiments was to ascertain whether 

 all cold bodies at the same temperature emit the same quantity of 

 rays ; or whether (as is the case with respect to the calorific rays 

 emitted by hot bodies,) some substances emit more of them than 

 others. Here it was a great gratification to the author to find in the 

 first experiment that the frigorific rays, from a blackened metallic 

 surface, were much more powerful in generating cold than those 

 which proceeded from a similar metallic surface of exactly the same 

 temperature, but without any coating. 



Observing that the approach of the hand to one of the balls of the 

 thermoscope affected the indications very sensibly and rapidly, it oc- 

 curred that perhaps animal substances emit both calorific and frigo- 

 rific rays more copiously than other substances, and that probably 

 living animal bodies emit them in still greater abundance than dead 

 animal matter. This was confirmed by a very conclusive experiment, 

 in -which one of the metallic surfaces was covered with goldbeater's 

 skin, and which surface emitted at least twenty-five times more ca- 

 lorific rays than a naked surface. The frigorific rays from the animal 



