Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/611

Rh HEAT 577 required to raise one gramme of water from 273 to 274 (compare 6 above). 69. There is scarcely any subject upon which more skilled labour in scientific laboratories, chemical and phy sical, has been spent than the measurement of specific heats, whether of solids, liquids, or gases. An ample and well-arranged table of results is to be found in Clarke s Constants of Nature, a compilation of numerical results of scientific experiments made in all parts of the world by various observers and experimenters, a most valuable aid to scientific knowledge given to the world as No. 255 of the Smithsonian Miscellaneous Collections. It is most interesting as showing how very differently dif ferent substances behave in respect to constancy or variation of specific heat with temperature. Thus it shows that, according to the results of all the experimenters, the specific heats of all the substances experimented on, whether simple or compound, are very nearly constant at all events for ranges between - 10 and 200 or 300 C., except the three elementary substances, boron, carbon, silicon. The specific heats of these three have been found by F. Weber to vary greatly with temperature. Thus for diamond he finds the specific heat to be ! at C. and *27 at 206 C., or nearly threefold of the amount at 0; at - 50 C. the specific heat is 063 ; and at + 985 it is 459 or about seven and a half times the specific heat at - 50 (a curious practical commentary, we may remark in passing, on the doc trine of the calorists on specific heat referred to in 8 above). The specific heats of carbon in its other forms of graphite and charcoal through wide ranges of temperature according to the same observer, F. Weber, are particularly interesting and significant. The approximate equality of the product of specific heat into the atomic weight for the simple metals is interesting and important ; no less so is the utter want of constancy and uniformity in the corresponding product for other substances, whether simple or compound. If we were to define a metal as a substance for which through the range of temperature from to 250 C. the product of the specific heat into the atomic weight is not less than 5 86 and not greater than 6 93, we should include every substance commonly called a metal, and no substance not commonly called a metal, except phosphorus, and solid sulphur lately fused. Some important results of Regnault s regarding the specific heats of gases under constant pressure have been already quoted in 56, 58, and 62 above. Further infor mation from experiments aided by thermodynamic theory, regarding specific heats of gases and vapours under constant pressure and of gases in constant volume, will be found under the heading THERMODYNAMICS. To this also, and to the articles MATTER (PROPERTIES or), LIQUID, and STEAM, the reader is referred for information respecting latent heats of liquefaction of solids and of evaporation of solids and liquids, also respecting the thermal capacity of a portion of homogeneous substance in two different statss, such as the water-liquid and water-steam of an ordinary cryophorus or philosopher s hammer, or of the sulphurous acid liquid and steam of a sulphurous acid steam thermo meter ( 43 above). TRANSFERENCE OF HEAT. Trans- 70. When two contiguous portions of matter are at dif- ofTl ferenfc tem P eratur es, heat is transferred from the warmer to by con- tlie colder - Tnis process is called conduction of heat. When two bodies at different temperatures are separated by a transparent medium, such as air, or water, or glass, or ice, heat passes from the warmer to the colder irrespectively By va- of the temperature of the intervening medium, except in so diation. f ar as i ts transparency may in some slight degree be affected by the temperature. Thus the colder of the two bodies be- by con duction. comes actually heated above the temperature of theinterveu- Light ing medium if the warmer be kept above this temperature, and and if heat is not otherwise drawn off from the colder body ? a(ll t al in greater quantity than the heat entering it from the warmer. ident This process of transference from one body to another body C al. at a distance through an intervening medium is called radia tion of heat. The condition of the intervening matter in virtue of which heat is thus transferred is called light ; and radiant heat is light if we could but see it with the eye, and not merely discern with the mind, as we do, that it is per fectly continuous in quality with the species of radiant heat which we see with the material eye through its affecting the retina with the sense of light. Thus a white hot poker in a room perfectly darkened from all other lights is seen as a brilliant white light gradually becoming reddish and less bright, until it absolutely fades from vision in a dull red glow. Long after it has ceased to be visible to the eye, the fact that heat is being transferred from it to colder bodies all round it, or above it or below it, is proved by our sense of heat in a hand or face held near it on any side or above it or below it. By considering the whole phenomenon of the white hot mass, without much of experimental investi gation, we judge that there is perfect continuity through the whole process, in the first part of which the radiant heat is visible and in the second part invisible to the human eye : and thorough experimental investigation confirms this con clusion. Thus radiant heat is brought under the undula- tory theory of light, which in its turn becomes annexed to heat as a magnificent outlying province of the kinetic theory of heat. 71. In this article we confine ourselves to a practical evaluation of rate of gain or loss of heat across the surface of an isolated solid placed in a medium such as air, and enclosed in a solid surface all at one temperature, as is ap proximately the case with the air and the floor, walls, and ceiling of an ordinary room. A rough approximation to the law of this action, founded on supposing the rate of motion to be in simple proportion to the excess of the tem perature of the isolated solid above the temperature of the surrounding medium and enclosure was used by Fourier in those of his solutions in which surface emissivity or, as he called it, &quot; Conductibilite&quot; exte rieure,&quot; is concerned. Without adopting any hypothesis, we define thermal emissivity as the quantity of heat per unit of time, per unit of surface, per degree of excess of temperature, which the isolated body loses in virtue of the combined effect of radiation and convection by currents of air. This defini tion does not involve the hypothesis of simple proportion ality; and the surface emissivity is simply to be determined by experiment for any given temperature of the enclosure, and any given temperature of the isolated body. Dulong and Petit made elaborate experiments on this subject, but did not give any results in absolute measure. So far as we know the first thoroughly trustworthy ex periments giving emissivities in absolute measure were made in the laboratory of the natural philosophy class in the university of Glasgow by Mr D. Macfarlane, in a series of experiments on the cooling of a copper ball. The results are given in Table VII. The ball experimented on was 4 centimetres in diameter, and was suspended in the interior of a double-walled tin-plate vessel. The space between the double walls of this vessel was filled with water at the temperature of the air, and the interior surface was coated with lampblack. Two thermo-electric junctions, one at the centre of the ball the other in contact with the exterior surface of the enclosure, in circuit with a sensitive mirror galvanometer, served to measure the difference of temperatures between the centre of the ball and the exterior surface of the enclosure. By this arrangement the exterior junction was kept very uniformly at a tempera- XL - 73