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 qf 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 011'" more heat, and be more sensibly alfected by the frigorific rays of colder bodies when blackened, than when they are of their natural colour. Tlu's 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 ,‘fih 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
