Page:The American Cyclopædia (1879) Volume VIII.djvu/589

 HEAT >75 than at a low temperature. Their results are iown in the following table : Iron Mercury Zinc Antimony Silver the above table it may be seen that the spe- cific heat of all the substances is greater at high than at low temperature, except that of platinum, which remains the same within the limits of the experiment. The reason given for this is that the melting point of platinum is very high, far higher than that of cast iron, and Regnault has found that the increase in its specific heat becomes more rapid as it ap- proaches its melting point. Pouillet, by the method of mixtures, obtained the specific heat of platinum at higher temperatures than those employed by Dulong and Petit, but still very far below the melting point. The following are his results, which differ somewhat from those of Dulong and Petit : MEAN SPECIFIC HEAT OF PLATINUM. Between 82" and 212" F. . 0-0335 82 " 572 0-0343 1292 1832 2192 utrary to the results of Dulong and Petit, Pouillet found there was a variation between 32 and 572, but it will be seen that they agree as to the increase of specific heat with increase of temperature. The specific heat of a solid depends upon its molecular conditions, which may be considerably changed by treat- ment, as by rate of cooling after fusion, by hammering, by compression, or by traction. An increase of density diminishes the specific heat, while expansion increases it; for which reason, probably, it increases with the temper- ature. The following table of specific heats of solids is by Regnault, the range being be- tween 32 and 212 F. : Substances. Sp. heats. Cobalt 0-10696 Zinc 0-09555 Copper 0-09515 Brass 0-09391 Silver 0-05710 Tin 0-05623 Antimony 0-05077 Mercury 0-03332 Gold 0-03244 Platinum 0-03244 Bismuth 0-03084 Specific Heat of Liquids. The specific heat of liquids may be found by the method of cool- ing, by that of mixtures, or by the calorimeter of Lavoisier and Laplace, fig. 10, already de- 397 VOL. vm. 37 Substances. Animal charcoal. Wood charcoal. . . Sulphur. Sp. heats. ... 0-260S5 ... 0-24111 0-20259 Graphite. . 0-20187 Glass Phosphorus ... 0-19763 0-18949 Diamond Grav iron .... Steel. ... 0-146S7 ... 0-129S3 0-11750 Iron. . . Nickel. . . ... 0-11397 .. 0-10863 scribed. Regnault employed the following method : The liquid under experiment is placed in the reservoir a, fig. 12, and this is immersed in a vessel containing water at a certain tem- perature ; a known temperature is therefore given to the liquid in the reservoir by agitating FIG. 12. Eegnault's Method for Liquids. the water in the bath. The stopcock d is then opened, and the fluid is forced into the vessel e, contained in the calorimeter c. The water in the calorimeter, which is cooler than the fluid under experiment, has its temperature raised by the introduction of the latter. The increase is measured by the thermometer, and from this, the weight of the water in the calorimeter and of the fluid under experiment being known, the specific heat of the latter is determined ac- cording to the method given above. Gener- ally, a substance has a greater specific heat when in a liquid than when in a solid state, a fact first observed by Irvine. Thus, the spe- cific heat of ice is only half that of water. The specific heat of liquids also increases with the temperature, but in a greater ratio than that of solids. The following results were obtained by Regnault with water : MEAN SPECIFIC HEAT OF WATER. From 82" to 104 F I'OOIS: " 32 " 176 1-0035 " 82 " 248 1-0067 " 82 " 820 1-0109 " 82 "392 . 1-0160; " 32 " 446 1-0204: It was formerly thought that water had a greater specific heat than any other liquid, but the researches of Dupre and Page indicate that the specific heat of a mixture of water and al- cohol, containing 20 per cent, of the latter, is probably as high as r05. Specific Heat of Oases. The specific heat of a gas at a constant volume differs from that at a constant pressure ; in other words, it takes a greater amount of heat to raise a certain quantity of a gas through a certain number of degrees of temperature if it is allowed to expand than when it is confined. The specific heat under constant pressure ex- ceeds that of constant volume by the amount which would be consumed in producing the ex- pansion. The first important researches on the;