RADIATION OF HEAT. There are cer tain experiments on the radiation of heat which possess great practical value, in so far as they show what substances Should be em ployed, and what avoided, when heat is re quired to be retained in any vessel or other body. In the jackets or envelopes for boilers and steam pipes this matter is always attended to.
Sir John Leslie placed a tin canister filled with hot water in the focus of a parabolic mirror of the same metal, and a differential thermometer in the focus of another such mirror, which was placed opposite to it. The four sides of the canister were covered with the substances whose radiating powers were required. When three of the sides were covered respectively with lamp black, paper, and ground glass, and then turned so as to radiate directly on one speculum, the heat reflected by the other raised the thermometer to 100°, 98° and 90° ; but when the fourth side, which was uncovered, was similarly directed, the thermometer fell to Thus it appears that polished metallic substances are bad radiators, which may be attributed to the internal reflection of the heat from their surfaces. A similar apparatus served to mea sure the absorptive power of different sub stances, the bulb of the thermometer being covered with an envelope of the substance to be examined ; and this power is thus found to be nearly in proportion to that of radiation. In another experiment Leslie filled a canister with ice or snow, and found that the cold apparently emitted from the varnished side was the greatest, and that from the polished side the least; he observed also that the cold like the radiant heat, varied with changes in the absorbent power of the thermometer and of the surface of the mirror.
Count Rumford laid down, as a rule justified by numerous experiments, that if we would confine heated substances, solid or fluid, in a vessel, the surface of the latter should be highly polished ; on the other hand, if the object be to cool the substances, the surface should be painted or varnished, or be covered with a soft coating which is not metallic. Also, in warming apartments by steam, the intention being to promote radiation as much as pos sible, the tubes conveying the steam should be unpolished or painted.
The colours of bodies have some effect on the velocity of radiation and on the absorption of heat. Dr. Stark of Edinburgh surrounded the bulb of a thermometer successively with equal weights of black, red, and white wool, and placed it in a glass tube, which was heated to the temperature of by immer sion in hot water; the tube was then cooled down to by immersion in cold water, and the several times of cooling were respectively 21, 20, and 27 minutes.
Those phenomena of radiation which are still the subject of scientific enquiry, and which are not yet applied in the arts, do not come under our notice here.