Page:The American Cyclopædia (1879) Volume VIII.djvu/590

 576 HEAT specific heat of gases were those of Delaroche and Berard. Their method consisted in passing known volumes of a gas under constant pres- sure and temperature through a spiral tube im- mersed in water, and making their calculations from the increase in its temperature. Re- gnault afterward made more exact experiments with a modification of the apparatus, from which he arrived at the following conclusions : 1. The specific heat of a given weight of a gas which is approximately perfect, or non-conden- sible, does not vary with the temperature of the gas. 2. The specific heat of a given weight of such a gas does not vary with the pressure or density, and therefore the specific heat of a given volume does vary in proportion to the density. 3. The specific heats of equal vol- umes of simple and uncondensible gases and of compound gases which are formed without condensation, such as hydrochloric acid and nitric oxide, are equal. 4. These laws do not hold for condensible gases, either simple or com- pound, as chlorine, bromine, or carbonic acid gas, the specific heat of which increases with the temperature. Specific Heat of Atoms. Before treating of latent heat it will be convenient to consider the law of atomic heat, or the specific heat of atoms, which was discovered by Du- long and Petit in 1819, and which has rendered the knowledge of the specific heats of bodies of so much importance in chemical investiga- tions. This law may be exactly enunciated as follows : The specific heats of elementary bod- ies are inversely proportional to their atomic weights ; in other words, the product of the specific heat of any element into its atomic weight is constant. The following results veri- fying this law are due to Regnault ; only a par- tial list is given : ELEMENTS. Sp. heat. Atomic weight. Product, or p. heat of atoms. Sulphur 0-1776 0*2499 82 24 5-6882 5'9976 Aluminum 0*2148 27-5 5-8932 Zinc 0-0955 65 6'2075 Cadmium 0-0576 112 6-8504 Cobult 0-1070 58-5 6 -2595 Nickel 0-1091 58-5 6-8828 Iron 0-1188 56 6-8728 Manganese 0-1140 55 6-2700 Copper 0-0951 68*5 6-0389 Silver 0-0570 108 6-1560 Gold 0-0824 196 6-8504 0-0508 122 6*1976 Bismuth. . . 0*0808 210 6*4680 Potassium ... 0-1696 89 6-6144 Sodium 0-2984 28 6 '7482 Lithium. . . 0-9408 7 6-5856 Lead 0-0814 207 6*4998 Platinum 0-0324 197 6*8828 Arsenic 0'0814 75 6-1050 Iodine 0-0541 127 6-8707 Bromine (solid) 0-0348 80 6-7740 Mercury (solid) 0-0819 200 6*8800 It will be observed that the products are not exactly the same, but there are the strongest reasons for believing that the variations are owing to differences in physical condition which are unavoidable under the circumstan- ces in which the experiments are made. Assu- ming the theory to be correct, it follows that all elementary atoms, independent of their weight, have the same specific heat, and there- fore that masses of elementary substances con- taining the same number of atoms and under the same physical conditions require the same amount of heat to raise them through an equal number of degrees. Thus, the atomic weight of iron being 56, and that of mercury 200, it will require the same amount of heat to raise 56 pounds of iron or 200 pounds of mercury through the same number of degrees. Neu- mann and Regnault have also found that the specific heats of all compound bodies of similar atomic composition are inversely proportional to their atomic weights. The following are Regnault's results with bichlorides : SUBSTANCES. Chloride of barium, BaCl,. . " strontium, 8rCl a calcium, CaCl 2 .., PbCL, ..... . mercury, HgCl 2 . zinc, ZnCl 2 ...... tin, 8nCl 2 ....... Sp. heat. 0*1199 0-1642 0-1946 0*0664 0-1862 0-1016 At. weight. Product. Ill 95 278 271 18-64 19-00 18-28 18*49 18-46 18-67 19-20 The following results were obtained with car- bonates : SUBSTANCES. Carbonate of lime, CaCO 3 barytes, BaCO,.. strontium, 8rC0 3 . Sp. heat. 0-2086 0*1104 0*1448 0-1934 At. weight. 100 197 147-5 116 Product. 20-86 21-75 21-86 22-43 It will be seen that the numbers in each table agree together more nearly than those of one with the other, but the close agreement in each group justifies the adoption of the law. IV. LATENT HEAT. The doctrine of latent heat was taught by Black in 1762. He was the first to observe that when a body passes from a solid to a liquid state a quantity of heat dis- appears. Thus, if ice at 32 has heat applied to it, and the resulting water as well as the ice is stirred, the temperature will remain at 32 until all the ice is melted. Thus, all the heat which has during this time been absorbed will have disappeared, and was said by Black and his contemporaries to have become latent. According to modern theory, this is not strict- ly true, unless we consider its conversion into another force a latent power which may be again reconverted into heat by the reconver- sion of the water into ice. The energy which manifests itself in heat vibrations is expended in maintaining a different form, or performing a certain amount of internal work, as it is called. Latent Heat of Fusion. If a pound of water at 212 is mixed with a pound of water at 32, the resulting temperature will be a mean, viz., 122 ; but if a pound of ice at 32" is mixed with a pound of water at 212, the result will be two pounds of water at 51.