Page:Proceedings of the Royal Society of London Vol 1.djvu/166

Rh distinguish between the effects of those simultaneous operations, and of ascertaining their relative intensities. A hot body, A for instance, heats a neighbouring colder body B, by its caloriﬁc radiations; but B emits at the same time frigoriﬁc radiations, which contribute to lower the temperature of A; nor is it clear that both these bodies, especially if they have polished surfaces, will not reciprocally, and perhaps repeatedly, reﬂect those incident rays, and that those rays will not be refracted by the media through which they pass, and be concentrated or expanded by the shapes of the reﬂecting surfaces, and thus create a combination of effects, which it will require much labour and ingenuity to unravel.

As it is impossible for us within our narrow limits to do justice to the connected series of observations and arguments here adduced, we shall refer those who Wish for more ample information on the subject to the paper itself; and this the rather, that we may dwell more largely upon the practical uses that may be derived from a knowledge of the facts which the author now considers as fully established.

In all cases where it is intended to preserve the heat of any sub- stance which is conﬁned in a metallic vessel, it will greatly contribute to that end if the external surface of that vessel be kept very clean and bright; but if the object be to cool anything quickly in such a vessel, its external surface should be painted, or coloured with some of those substances which have been found to emit caloriﬁc radiations in abundance. Hence the sides of kitchen utensils should be kept bright, in order to conﬁne the heat; while their bottoms should be blackened, in order that their contents may be made to boil sooner, and with a less expense of fuel.

Brewers, it seems, are mistaken when they employ hroad shallow vessels, or ﬂats, as they call them, of metal for cooling their wort. Wooden ﬂats, it appears, ought in every respect to have the pre- ference.

In all cases when metallic tubes, ﬁlled with steam, are used for warming rooms or hot-houses, the external surface of those tubes should be painted, or covered with some substance which facilitates the emission of caloriﬁc rays. Where, on the other hand, tubes are intended to convey hot steam from one place to another, they should be kept very clean and bright. This applies also to the cylinders of steam-engines, and the principal tubes used in that machine.

Gardeners should advert to the circumstance, that if walls painted black acquire heat faster when exposed to the sun’s direct rays, they likewise cool much faster during the night, and in the shade when the weather is cold.

Black cloths are known to be very warm in the sun; but they are far from heing so in the shade, especially in cold weather, when the temperature of the air is below that of the surface of the skin.

It having been shown that the warmth of clothing depends much on the polish of the surface of the substance of which it is made, we may conclude that in choosing the colour of our winter garments, those dyes should be avoided which tend most to destroy that polish.