Page:The American Cyclopædia (1879) Volume VIII.djvu/585

 HEAT 571 180. It was found by Dulong and Petit that the velocity of cooling in a vacuum for a con- stant excess of temperature increases in a geo- metrical progression when the temperature of the surrounding air increases in an arithmetical progression, and that the ratio of this pro- gression is the same whatever may be the ex- cess of temperature. The experiments of MM. Provostaye and Desains confirm the results of Dulong and Petit. Radiation being the propagation in the lurniniferous ether of un- dulations in consequence of molecular vibra- tions in the radiating body, it would be ex- f jcted that different bodies would have dif- ferent powers of radiation, and experiment shows this to be true. The apparatus used by Sir John Leslie is represented in fig. 6, and is the same that he employed for determining the reflecting powers of bodies. In experi- menting on radiation, the plate d may be re- moved. The cube a has its sides coated with different substances, which may be turned at pleasure toward the mirror &, and the bulb of a differential thermometer may be placed in the focus I. Calling the radiating power of lampblack 100, he found that of other sub- stances as follows : Varnished lead ........... 45 Mercury ................. 20 Polished lead ............. 19 Polished iron ............ 15 vary with the sources of heat. Reflection of Heat. That dark heat rays are capable of re- flection, and that they obey the same laws as the luminous rays may be shown by placing a metallic ball, A, fig. 5, heated below redness, in the focus of a concave mirror, B C, and the Whiting .. 100 Paper 98 .... 95 White glass 90 Tin, gold, silver 12 FIG. 5. bulb of a thermometer, D, in the focus of a j mirror, E F, opposite and at a distance. The temperature indicated by the thermometer will approach that of the ball, but if either ther- ! mometer or ball is removed from its position, the temperature will fall. The following method was employed by Sir John Leslie to determine the heat-reflecting powers of dif- ferent substances. The source of heat, which may be a cubical vessel filled with hot water, or It is commonly supposed that color has much influence on the radiating and absorbing power of bodies, but this is only true of luminous heat. If the cube used in the above experi- ment is filled with hot water, and three of its sides are covered, one with white, another with red, and another with black velvet, all of the same texture, the fourth of polished copper being left uncovered, it will be found that the three velvet sides will radiate alike, the naked side radiating the least. This shows that texture or molecular structure, rather than color, confers radiating power upon surfaces, for obscure heat. The power of a body to absorb heat is precisely proportional to its power of radiation; or in other words, its power of propagating undulations in the ether is equal to its power of accepting motion from the un- dulations of the ether, and is generally pos- sessed in a greater degree by opaque than by transparent bodies, although there are remark- able exceptions, as will be seen by reference to the article DIATHERMANCY. The method em- ployed by Leslie in determining the absorbing powers of bodies was to cover the bulb of the differential thermometer, fig. 6, with the sub- stance to be experimented upon, and place it in the focus, removing the plate d. Tyndall has made elaborate researches upon the radia- ting and absorbing powers of gases, vapors, and flames, and has found them proportional when the same sources of heat were employed, and inversely proportional to their transmitting powers; but he also finds these properties to FIG. 6. a metallic ball, a, fig. 6, is supported by a sliding standard at a distance in front of a concave metallic reflector at 5. The focus of the mir- ror is at c for the divergent rays which come from the source of .heat, but a reflecting plate d reflects them to Z, where the focus is really formed. It is obvious that the heat at the focus I will be in proportion to the reflecting power of the plate d. By using plates of dif- ferent materials he ascertained their reflect- ing power. Calling polished brass 100, he ob- tained the following results : Brass Silver , Tin... Steel. , 100 n Lead 60 Amalgamated tin 10 Glass 10 Lampblack This shows that the metals which are the best reflectors of light are also the best reflectors of heat. Moreover, when it is remembered that white light, which contains all the rays of the