the hypothesis of radiation, depending very much on the polish of
their surfaces. Thus if those substances which supply the warmest
coverings, such as furs, feathers, silk, &c. be viewed through a mi-
croscope, we shall find the surfaces of their fibres or minute laminae
not only smooth, but also very highly polished : and those substances
will be warmest which excel in these respects, the fine white shining
fur of a Russian hare being much warmer than coarse hair; and fine
silk, as spun by the silk-worm, being preferable for warmth to the
same silk twisted together into coarse threads.
A considerable part of the paper is now bestowed on the theory of heat, which the. author attempts to deduce from the foregoing facts and observations. Heat and cold, he says, like fast and slow, are mere relative terms; and as there is no relation between motion and rest, so there can be none between any degree of heat and absolute cold, or a total privation of heat. It has long been thought, and it appears more and more probable, that motion is an essential quality inherent in all matter: this is illustrated by many examples; and by applying the analogy above given, and the observations since brought forward, there seems every reason to believe that, without having recourse to any specific element, all the phenomena of heat may be accounted for by the simple operations of motion ,- or that motion, in fact, constitutes the heat or temperature of sensible bodies.
It will no doubt occur that this theory will hardly account for the effects of frigorific rays; but this objection is answered by the ob- servation, that as the rapid undulations occasioned in the surrounding ethereal fluid by the swift vibrations of a heated body will act as ca- lorific rays on the neighbouring colder solid bodies; so the slower undulations occasioned by the vibrations of a cold body, will act as frigorific rays on neighbouring bodies of a higher temperature; and that these reciprocal actions will continue, but with decreasing in- tensity, till the two bodies have acquired the same degree of tem- perature, or until their vibrations have become isochronous.
According to this hypothesis, cold can with no more propriety be considered as the absence of heat, than a low or grave sound can be considered as the absence of a hith or more acute note; and the admission of rays which generate cold involves no absurdity, and creates no confusion of ideas. _
As this theory, however, entirely supersedes the hypothesis of the calorific element, of late so much resorted to, it may be imagined that the author would not discuss the controversy in a slight or superficial manner; and accordingly many pages are here dedicated to this in- tricate and abstruse disquisition.
Among other important points. it was necessary to reconcile so- lidity, hardness, and elasticity, with the incessant motion he ascribes to the constituent particles of matter, and to ohviate the objection founded on a supposition that there is not room sufiicient for this motion. What increases the perplexity is, that, admitting the changes of temperature in bodies to be the effect of the calorific and frigorific radiations above described, a particular nicety will be required to
