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.
