14-5 



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 higher 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 obviate the objection 

 founded on a supposition that there is not room sufficient 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 



