Fig. 380.
a b. The cone of paper, gilt inside. c. The red-hot ball. d. Stand with wood supporting a slice of phosphorus, which is brought into the focus of the rays of heat reflected through the cone.
Dr. Bache has determined by experiments that the radiation of heat from a body is not affected by colour, so that in winter all coloured clothes are alike in that respect, and radiate heat without any appreciable difference. The power of absorbing heat, however, is greatly dependent on colour; and as a general rule, good radiators of heat (such as a black cloth, or indeed any surface covered with lamp-black), are also excellent absorbents of heat. Dr. Hooke and Dr. Franklin placed pieces of cloth of similar texture and size on snow, allowing the sun's rays to fall equally upon them. The dark specimen always absorbed more heat than the light ones, and the snow beneath them melted to a greater extent than under the others; and they both remarked that the effect was nearly in proportion to the depth of the shade, as in the following order:—After black, the maximum absorbent quality was possessed by, first, blue; second, green; third, purple; fourth, red; fifth, yellow. The minimum absorbent power was observed to belong to white.
When radiant heat is allowed to pass through glass, the latter substance is not found to be transparent to heat rays as it is to those of light, but a considerable proportion of heat is arrested and stopped; consequently glass fire-screens are to be found in the mansions of the wealthy, because they obstruct the heat but do not exclude the cheerful light and blaze of the fireside.
Melloni's researches on the nature of the rays of heat, and also on the media which affect them, would demand and merit a chapter to themselves; want of space, however, obliges us to omit the consideration of thermo-electricity, and the refined and beautiful experiments of Melloni, whose labours are a model for the imitation of all original seekers after truth.