The heat developed in these combinations, some of which must be regarded as combustions, may proceed from four distinct sources. (a) From the specific heat of the compound being less than the mean of the specific heats of the combining substances (Crawford). In most cases, however, the atoms of simple substances retain their original specific heat when they enter into combination (pp. 248....251). In other cases, on the contrary, combination is attended with an actual increase of specific heat, so that the result would be a production of cold, if heat were not developed from some other cause. 9 236) Thus, 1lb. of hydrogen gas of 3-293 specific heat combines, under the most violent evolution of heat, with 8lb. of oxygen of specific heat 0.236, producing 91b. of water of specific heat 1.000,-whereas calculation gives 3.2938.0·236) =0.576 as the mean of the two specific heats. If then water had a specific heat 0.576, the quantity of sensible heat in hydrogen and oxygen gases together would be exactly sufficient to bring the water formed to the same temperature as that of the gases themselves: but since the actual specific heat of water is 1000, the quantity of sensible heat in the gases is not sufficient for this purpose; and if heat were not developed from some other cause during the combination of oxygen and hydrogen, the water produced would be much colder than the two gases before combination. b. The heat developed may arise from separated heat of fluidity, if, during the combination, gaseous or liquid substances pass to the liquid or solid state. This however will not account satisfactorily for the more intense evolutions of heat in combustions and other combinations: for the latent heat of gases and liquids is small in comparison with such developments of heat. Moreover, in many instances the combination is not attended with condensation; e. g. in the combustion of charcoal and sulphur in oxygen gas, and of hydrogen in chlorine gas; or again, gaseous products are formed from solid bodies, great heat being at the same time evolved, as in the explosion of nitre with charcoal, &c. c. The heat evolved may be a kind of heat different from heat of fluidity, producing no particular state of aggregation, but existing in a state of more intimate chemical combination with the ponderable matter, and set free when one ponderable substance combines with another. d. The heat is formed at the moment of combination by the union of the positive electricity of the one body with the negative electricity of the other. Either the cause adduced under c, or that in d, or both together, must be admitted, in order to explain the development of heat which accompanies the combination of ponderable bodies. (See the observations on the Theory of Combustion under the head of Oxygen.) Cold is produced (1) Principally in some of those chemical combinations in which solid bodies pass to the liquid state,-combinations which are brought about by feeble affinities, the quantity of heat developed being probably far from sufficient to render the dissolving body liquid, so that more heat must be absorbed and rendered latent to supply heat of fluidity: e. g. in the solution of various salts in water and dilute acids, and more especially on mixing these salts, as well as certain acids, crystallized potash, or alcohol, with ice or snow, though the same substances, when mixed with water, evolve heat. (Freezing Mixtures.) Such of these substances as contain water of crystallization must not be deprived of it; otherwise, when mixed with water, they will produce heat instead of cold.-The more finely the substances are pulverized, the more quickly they are mixed, the larger their quantity, and the smaller the conducting power of the containing vessels, the greater will be the degree of cold produced.-To produce the greatest possible degree of cold, the substances of which the freezing mixture is to be made are cooled, before mixing, in another freezing mixture.-With respect to this matter, it must be observed that a solution of common salt in water is completely resolved at 20° into ice and crystallized salts; therefore, common salt and snow cooled to this point no longer act on one another; whereas with chloride of calcium and snow the corresponding point is as low as - 60°, and with sulphuric acid and snow much lower; so that this mixture, when the ingredients have been properly cooled, is capable of producing the most intense degree of cold. (Murray.) One part of water, in dissolving 1 part of nitrate of ammonia, produces a lowering of temperature from +10° to -15.5°; with salammoniac and nitre, from +10° to - 12°; with 1 part of nitrate of ammonia and 1 part of carbonate of soda, from + 10° to 13.8°; with 0.3 sal-ammoniac, 0·1 nitre, and 0.6 chloride of calcium, from +25° to -6°; with sal-ammoniac, nitre, and Glauber's salt, from +10° to 15.5°; 1 part of water likewise produces a considerable degree of cold with sal-ammoniac, nitre, and Glauber's salt; or with salammoniac, 10 nitre, and 16 Glauber's salt. (Walker.) By dissolving 1 part of a salt in 4 parts of water, the following reductions of temperature are obtained: sal-ammoniac 15.19°; nitrate of ammonia 141°; sulphate of potash 2.9°; chloride of potassium 11.81°; nitre 10.6°; Glauber's salt 8·1°; common salt 2.1°; nitrate of soda 9-46'; chloride of barium 4.5°; nitrate of baryta 2.1°; sulphate of magnesia 4.5°; sulphate of zinc 3.1°; nitrate of lead 1.9°; sulphate of copper 2.27°. -If 1 part of a salt be dissolved in 4 parts of a saturated solution of another salt, the following degrees of cold are produced: sal-ammoniac in solution of common salt 8.4°; in solution of nitre 12.6°;-nitre in solntion of sal-ammoniac 9.75°; in solution of common salt 9.4°; of nitrate of soda 7.06°; of nitrate of baryta 9.75°; of nitrate of lead 9.5°;-Glauber's salt in solution of common salt 4.75°;-common salt in solution of sulphate of copper 4.1°; nitrate of soda in solution of sal-ammoniac 9.1°; of nitre 9.2; of common salt 7·81°; of chloride of barium 2·75°; of nitrate of lead 8°;-nitrate of baryta in solution of nitre 0·75°;-sulphate of zinc in solution of sulphate of potash 175°;-sulphate of copper in solution of common salt 4.9°.-The following salts, on the contrary, produce rise of temperature: common salt in solution of sal-ammoniac 4.56°; in solution of Glauber's salt 175°; of nitre 0·75°; of nitrate of soda 38°;-chloride of barium in solution of nitrate of soda 0.64°. (Karsten, Schriften d. Berl. Akad. 1841.) Three parts of crystallized neutral carbonate of soda dissolved in 10 parts of water produce a lowering of temperature amounting to 8.9°, whilst 3 parts of the same salt in the anhydrous state dissolved in 10 parts of water cause a rise of temperature of 12.2°.--3 parts of crystallized Glauber's salt with 10 parts of water produce a fall of 6.7°; on the contrary, 3 parts of dry Glauber's salt with 10 parts of water cause a rise of 2.2°.-3 parts of crystallized sulphate of magnesia with 10 parts of water produce a fall of 31°; and 3 parts of crystallized protosulphate of iron with 10 parts of water also produce a fall of temperature amounting to 31°. (Thomson, Records of Gen. Sc. 1836, July; also Bibl. univ. 5, 182; also J. pr. Chem. 13, 176.) One part of a mixture of 50 parts of oil of vitriol and 55 water, mixed with 1 parts of Glauber's salt, cools from + 10° to -8°.-One part of dilute hydrochloric acid with 13 Glauber's salt, from + 10° to 17.8°. -1 part of dilute nitric acid gives the following reductions of temperature: with 1 sal-ammoniac, nitre, 1 Glauber's salt, from +10° to - 12°; with 14 nitrate of ammonia and 2 phosphate of soda, from +10° to -6°; with 2 phosphate of soda, from +10° to -11°;—and with 14 Glauber's salt, from +10° to - 16°. (Walker.) Cooled mixtures of oil of vitriol and water in different proportions give with Glauber's salt, according to Bischof and Wöllner (Schw. 52, 371), the following reductions of temperature. Six parts of oil of vitriol produce heat with 6 parts of snow, neither heat nor cold with 8 parts, and intense cold with a larger quantity of snow. (Stöchiom. 1, 2, 87.) Cheap freezing mixtures for producing ice in summer, with description of the apparatus: 4 parts of a cooled mixture of 50 oil of vitriol and 55 water, with 5 parts of Glauber's salt, or 9 parts of hydrochloric acid of 15° B. with 14 parts of Glauber's salt. (Decourdemanche, J. Pharm. 11, 584; also N. Tr. 14, 2, 249.)—12 parts of a mixture of 3 oil of vitriol and 2 water with 17.5 parts of Glauber's salt. (Malapert, J. Pharm. 21, 221; also Ann. Pharm. 18, 348.)-3 parts of a mixture of 7 oil of vitriol and 5 water with 4 parts of Glauber's salt. (Boutigny, J. Chim. Méd. 10, 460.) Óne part of snow or pounded ice produces the following degrees of cold: with dilute sulphuric acid (4 oil of vitriol and 1 water) from 0° to 325°;--with 1 dilute sulphuric acid from 7° to 51°. (I have also obtained a considerable degree of cold by mixing the crystallized compound of 49 oil of vitriol and 9 water with snow); -with dilute nitric acid, from — 23° to 49°;-with 1 dilute nitric acid, from -17.8° to 43°;-with 14 crystallized potash, from 0° to - 28°;-with common salt, from 17.8° to - 20.5°;-with 1 common salt, from 0° to — 17·8°; with common salt and nitrate of ammonia, from - 27.8° to 317°;-with chloride of calcium, from 9° to 42.5°;—with 14 chlo -- 12 ride of calcium, from 0° to 49°;-with 1 chloride of calcium, from 0° to 27.8°, and from 7° to - 47°;-with 2 chloride of calcium, from 17.8 to 54.4°, and with 3 chloride of calcium, from 40° to 58°; and, according to Tralles (Gilb. 38, 365), with absolute alcohol, from 0° to 36.9, with highly rectified spirit from, 0° to 30°. Orioli (Nuov. Collez. di Op. Scient. 1823, 104; also Fersasac Bull. des Sc. Math. Phys. et Chim. 1825, 117) obtained, on mixing solid amalgam of lead with solid amalgam of bismuth-the two substances at the same time becoming liquid-a reduction of temperature amounting to 22°. According to Döbereiner (Schw. 42, 182; also Kastn. Arch. 3, 90), 204 parts of lead-amalgam (consisting of 103 lead and 101 mercury) mixed with 172 bismuth-amalgam (71 bismuth+ 101 mercury) cool from +20° to -1°; if to this mixture there be also added 202 parts of mercury, the temperature falls to -8. When a finely-divided mixture of 59 tin, 103-5 lead, and 182 bismuth is dissolved in 808 parts of mercury, the temperature falls from + 17.5° to -10°. 2. The mixture of liquids is in some few instances attended with reduction of temperature, condensation taking place at the same time. This effect is perhaps due to the greater specific heat of the mixture.— Thus, on mixing 44 parts of a concentrated aqueous solution of nitrate of ammonia, sp. gr. = 1.302, with 34 parts of water at 16° C. the temperature falls 5, and the mixture has a specific gravity of 1159, the mean sp. gr. being 1.151. Similar effects are produced on mixing several other saline solutions with water; but the reduction of temperature is not so great. With the same quantity of water, a saturated solution of chloride of potassium gives a reduction of temperature of 0.75; that of common salt, of 0.56°. A saturated solution of nitre gives with a saturated solution of nitrate of soda a rise of 0.06°; but an equal weight of water added to this mixture produces a fall of 1.25°. Saturated solutions of the following salts produce, when mixed in equal quantities, the following degrees of cold: sal-ammoniac and nitre 0.62; sulphate of potash and nitre 0·44°; chloride of barium and sulphate of zinc (accompanied by precipitation) 2°; sal-ammoniac with excess of sulphate of copper 1.6°,-whereas when the sal-ammoniac is in excess, a rise of 1.6 takes place. (Karsten, (Schriften d. Berl. Akad. 1841.) 2. Development and Absorption of Heat from Mechanical Causes. A. Development of Heat accompanying Adhesion-Phenomena. When any liquid substance penetrates finely divided bodies by capillary attraction, no chemical combination taking place between the two, a rise of temperature takes place, amounting in the case of inorganic solid bodies to between and, but with organic bodies-probably because they are more porous and therefore present a larger surface-to between 1° and 10°. (Pouillet.) Pouillet's experiments were made with water, alcohol, acetic ether, and oils; the solid inorganic bodies into which he caused these liquids to penetrate were the filings of different metals, the powders of sulphur, glass, porcelain clay, various earths, and heavy metallic oxides; and the organic substances, charcoal, wood-shavings, cotton, paper, roots, seeds, flour, hair, wool, animal skins, &c. The development of heat from this cause has been confirmed by Regnault. (Ann. Chim. Phys. 76, 133.) B. Development of Heat, produced by Mechanical Alteration of Density. Every mechanical compression or condensation of a body, even if it does not produce a change in the state of aggregation, is attended with evolution of heat; every expansion of the body, on the contrary, though unattended with actual change of aggregation, gives rise to absorption of heat. The development of heat by compression is probably due chiefly to diminution of specific heat in consequence of increase of density; the absorption of heat by expansion to increased capacity for beat,-and the more so, since the development of heat is greater in proportion to the degree of condensation. Air when suddenly compressed evolves a considerable degree of heat, sufficient to ignite German tinder (Fire-syringe). According to Thénard, (Ann. Chim. Phys. 44, 181; also Pogg. 19, 442) paper, oiled paper, and wood may also be ignited by sudden compression in oxygen gas, and oiled paper in chlorine gas; also fulminating silver may be made to detonate in hydrogen, nitrogen, or carbonic acid gas.-The great reduction of temperature which takes place in air under the receiver of the airpump on sudden expansion may be strikingly shown by means of the delicate thermoscope of Breguet.-Liquids, which are but slightly compressible, evolve but little heat when subjected to pressure. Under a pressure of 40 atmospheres, water exhibits scarcely any rise of temperature, alcohol only 1°, and ether 4° or 5°. (Colladon & Sturm, Pogg. 12, 161.)—Metals become hot and even red-hot by hammering, their density at the same time increasing. In the stamping of coin, the development of heat produced by the first blow is greater than that produced by the second or third: the increase of density is likewise greatest at the first blow. Copper coins become more heated than silver, and silver than gold; the specific gravity also increases most in the copper and least in the gold. (Berthollet, Pictet, Biot.) In the boring of cannon with iron borers great heat is evolved, whether the operation be performed in ordinary air, rarefied air, or water. (Rumford.) When an iron rod is broken by hanging weights to it, it lengthens considerably before breaking and becomes very hot. (Barlow, Ann. Phil. 10, 311.) An alloy of 1 part of iron and 2 of antimony emits sparks when filed. (Becquerel.) On a revolving grindstone 74 feet in diameter an iron nail becomes white-hot in minute, brass red-hot in minute; a glass tube becomes red-hot, melts and flies off. (Heinrich.) Agate rubbed on the same grindstone gives off sparks which travel with the grindstone for a little distance the agate also becomes brightly red-hot. The glowing fragments which fly off from glass rubbed on the grindstone set fire to gunpowder (according to Wedgewood). Two pieces of wood take fire when rubbed hard together. When rough glass is rubbed on smooth, the former becomes more strongly heated than the latter; similarly, rough cork on smooth; when white satin is rubbed on black, the former is most strongly heated; on rubbing together smooth glass and cork, the rise of temperature in the two is as 34 : 5; with ground glass and cork as 40: 7; silver and cork as 50: 12; caoutchouc and cork as 29: 11. (Becquerel, Ann. Chim. Phys. 70, 239.) III. INFLUENCE OF HEAT ON THE CHEMICAL COMBINATIONS AND DECOMPOSITIONS OF PONDERABLE BODIES. The influence of heat on the chemical combinations of ponderable bodies in which it acts partly as the principle of fluidity, partly in a manner unknown, has been considered (pp. 36 and 37). Of decompositions of ponderable substances produced by heat, an account has been given pp. 119...122, and 132...166. The most remarkable instance of decomposition by heat is the resolution of water into its constituent gases by the agency of incandescent platinum, lately discovered by Mr. Grove. (Phil. Mag. J. 30, 58; 31, 96.) A platinum wire is sealed into the closed extremity of a kind of tube-retort, having its neck narrowed close above the wire. The tube is filled with pure water, and the current of a voltaic battery made |