Page:The American Cyclopædia (1879) Volume VIII.djvu/588

 574 HEAT of any substance is readily ascertained. A modification of this apparatus, which gave more accurate results, was devised by Lavoisier and Laplace, and called an ice calorimeter, of which fig. 10 shows a perspective and a sec- tional view. It consists of three concentric FIG. 10. Ice Calorimeter. tin vessels, the inner one for holding the body under experiment, while the two others con- tain pounded ice, that in the outer one to pre- vent external influence, that in the middle one to measure the heat given by the body under experiment. Stopcocks are supplied to each, that connected with the middle ves- sel being for the purpose of drawing off the water which has been produced by the ac- tion of the experimental body. The manner of conducting the experiment is similar to that employed with the block of ice. The principal source of error is the difficulty of estimating the quantity of ice which has melt- ed, as more or less water will adhere to the lumps. Bunsen has devised a calorimeter espe- cially adapted to cases in which only small quantities are experi- mented upon. A test tube, a, fig. 11, which receives the sub- stance to be tested, is fixed in the larger leg of a wide U-shap- ed tube, 6 c, the part &, contain- ing the test tube, being filled with water, and the rest with mercury. A graduated smaller tube, d, open at the top, is ad- justed to the top of the leg c, for the purpose of noting the rise or fall of the mercury in this leg, which it is obvious will be effected by the expansion or contraction of the contents of the leg J. In making the experiment, a is sur- rounded by a freezing mixture and the water frozen. Then the substance under experiment is raised to a certain temperature and placed FIG. 11. Bun- sen's Calori- meter. in the test tube ; it melts a certain quantity of ice, and thereby causes a diminution in vol- ume of the contents of &, and consequently a fall of the mercury in e, and also in the grad- uated tube d. In this way the weight of ice melted may be estimated, and the weight and temperature of the tested substance be- ing known, the specific heat may be readi- ly calculated according to the formula which has been given. 2. The method of mix- tures. An outline of this method was given in defining specific heat; it will now be ap- plied in making determinations. A body is weighed and raised to a certain temperature, and then placed in a vessel containing cold water whose weight and temperature are also known. Let m be the weight of the body, n its temperature, and s its specific heat ; also let w be the weight of cold water, and t its temperature. After a time equilibrium is ob- tained, when the temperature may be repre- sented by e. The quantity of heat which the body has lost will therefore be m s (ne), and that which has been gained by the water will be w (e ), the specific heat of water being unity. Now, as the quantity of heat which is absorbed is equal to that which is given out by the body under experiment, m s (ne) = w (e ), from whence s = w ~. To apply this formula, suppose that three ounces of mer- cury at 212 is mixed with one ounce of water at 32, and that the resulting temperature is 48-2, what is the specific heat of mercury? In this example m = 3, e = 48*2, and n e = 163-8 ; therefore, = =|f = ^- = '033, the specific heat of mercury, which is therefore only about ^ that of water. In accurate experiments corrections are required for er- rors, one of which is caused by the absorp- tion of a small amount of heat by the con- taining vessel. Regnault devised a method of mixtures, using a calorimeter capable of yielding more accurate results, and the elabo- rate experiments which were made by him have been of great value in the arts ; but the method given above sufficiently illustrates the principles involved. 3. The method of cooling. Equal weights of bodies having different spe- cific heats will cool through different degrees of temperature in the same time, the body hav- ing the least specific heat cooling the most rapidly. If two thermometers with blackened bulbs and of the same size are filled, one with mercury and the other with water, and then, at a common temperature, are placed in cool enclosures of the same construction and tem- perature, the mercurial thermometer will cool more than twice as rapidly as the one of wa- ter, the proportion being 30 to 13, because the specific heat of water is 30 times that of mer- cury, while the specific gravity of mercury is 13 times that of water. Specific Heat of Sol- ids. It was found by Dulong and Petit that the specific heat of a solid is greater at a high