substance were likewise found to be much more efficacious in pro- 

 ducing cold than those from the polished surface ; though in what 

 proportion could not be ascertained with any degree of accuracy. In 

 general, however, there is every reason to conclude that at equal in- 

 tervals of temperature, the rays which generate cold are just as real 

 and just as intense as those which generate heat, or that their actions 

 are equally powerful in changing the temperature of neighbouring 

 bodies. 



Our author, ever doubtful of the existence of the caloric of the 

 modern chemists, thinks himself authorized here to throw out the 

 following observation respecting that favourite hypothesis. On a 

 supposition that caloric has a real existence, and that heat or an in- 

 crease of temperature in any body is caused by an accumulation of 

 that substance in such body, the reflection of cold would indeed be 

 impossible ; and to maintain its reality must to all unprejudiced minds 

 appear an absurdity. 



By further experiments it is proved that all those circumstances 

 which are favourable to the copious emission of calorific rays from 

 the surfaces of hot bodies, are equally favourable to the copious 

 emission of frigorific rays from such bodies when they are cold. That, 

 on the other hand, those substances which part with heat with the 

 greatest facility or celerity, are those which acquire it also most 

 readily. Also that an animal substance, for instance goldbeater's 

 skin, will throw off more heat, and be more sensibly affected by the 

 frigorific rays of colder bodies when blackened, than when they are 

 of their natural colour. This latter fact is applied as a proof of the 

 great utility of the inhabitants of hot climates being of a black co- 

 lour ; and it is suggested that Europeans might find some relief by 

 availing themselves of this circumstance when they visit the torrid 

 zone. It is also surmised that the custom of savages inhabiting cold 

 countries, of besmearing their bodies with oil or other unctuous 

 matter, may have its utility by enabling their skins to reflect the 

 parching frigorific rays that reach them from the atmosphere. 



Another subject, which is here minutely investigated, is to ascer- 

 tain what proportion of the heat emitted by a hot body is acquired 

 or retained by the circumambient air; and the result yielded by 

 several experiments and calculations turns out, rather unexpectedly, 

 that this proportion is so little as -j l r th of the whole. And it is also 

 proved that a heated body, of a globular form, being suspended in 

 the centre of another larger thin hollow sphere, at the same tempe- 

 rature as the air and the walls of the room, the vicinity of the two 

 surfaces will sensibly retard the cooling of the hot body ; and that 

 if instead of one there be a number of thin concentric spheres of dif- 

 ferent diameters, the retardation of the cooling will be still greater. 

 Combining with this the results of some former experiments, from 

 which it appears that the cooling will be slower when the opposite 

 surfaces are bright, than when they are unpolished or blackened, 

 some inferences are derived concerning the warmth of different sub- 

 stances used as clothing, their effect in this respect, consistently with 



