vacuum in the instrument above the mercury. Vid. also Von Yelin (Kastn. Archiv. 3, 109), Kämtz (Schw. 40, 200), Egen (Pogg. 11, 276, 335 and 517; 13, 33), Legrand (Ann. Chim. Phys. 63, 368), Despretz (Ann. Chim. Phys. 64, 312; also Pogg. 41, 58), Rudberg (Pogg. 40, 39, and 162), and Henrici. (Pogg. 50, 251). Person has found that when thermometers are exposed to temperatures above 300° or thereabout, the shifting of the zero point is much greater than has been hitherto supposed. Despretz had found it to amount to half a degree in 4 or 5 years under ordinary circumstances. Person finds that at the temperatures just mentioned it sometimes reaches from 12° to 17° in a few hours. (Pogg. 65, 370.) ¶. In the common Air-thermometer, the air is enclosed in a glass bulb having a tube attached to it, and the tube is closed with a drop of some liquid not easily volatile, such as oil of vitriol. (Gay-Lussac, Pogg. 27, 435; Pouillet, Pogg. 41, 144.)-In the Differential Thermometer or Photometer the air is contained in two glass bulbs, connected by a tube bent like a U; a small quantity of liquid contained in the tube is driven backwards and forwards accordingly as one or the other of the bulbs is more strongly heated, and thus shows differences of temperature, but no exact degrees. If one bulb is covered with lamp-black. and the other with gold leaf, the former becomes more strongly heated by exposure to light than the latter, and thus the instrument serves to measure the intensity of light. (Leslie.) The differential thermometer possesses still greater delicacy when filled with vapour of alcohol in contact with excess of alcohol instead of air. (Howard.)-The Air-pyrometer is a hollow sphere of platinum fitted with an escape-tube. The hotter the fire to which the platinum vessel is exposed, the greater is the quantity of air driven out of it, and this is received over water and measured. (Pouillet, Pogg. 39, 367; also Elémens de Physique et de Météorologie, 3me Ed. tom I. P. 351.) -The mercurial thermometer serves for temperatures between + 350° and 38; the spirit-thermometer from +70° to the greatest known degree of cold: for alcohol has never yet been frozen.-In Breguet's Metallic Thermometer, three very fine strips of platinum, gold, and silver are laid on one another, and wound into a spiral, which becomes twisted by the unequal expansion and contraction of these metals arising from changes of temperature, and gives motion to an index: this instrument serves not so much for exact measurement of temperatures as, on account of its thinness, for the detection of very transient changes of temperature (Ann. Chim. Phys. 5, 312; more shortly in Schw. 20, 465.)—The Matal Pyrometer consists of a bar of silver, or for higher temperature of platinum, contained in a tube of porcelain, or of clay mixed with black lead. The metallic bar expands more strongly when heated than the clay, and gives motion to an index traversing a graduated arc. (Vid. Daniell, Qu. J. of Sc. 12, 309; abstr. Schw. 32, 497.) Reaumur divides the interval between the temperatures of melting ice and boiling water into 80, Celsius into 100*, Delisle into 150, and Fahrenheit into 180 equal parts. The first two place the zero at the temperature of melting ice, Delisle at the boiling point of water, Fahrenheit 32 below the melting point of ice: 9° F. = 7·5° D. = 5° C. = 4° R. A correction must be made for the different positions of the zero. The temperatures given in this work (except when otherwise specially mentioned) sur to the Centigrade scale. 288 320 400 752 240 300 572 220 428 = How many degrees Fah. 273° Cels.? According to the table, 270° C. 518° F.; the 3° C. over are equal by d. to 5.4° Fah.; and these added give 518° 54° = 523 4 Fah.-How many degrees of Cels. =676° Fah.? By the table, 671° F. = 355° C.; and by d. 5° F. = 2.78° C., therefore together 671° F. 355° + 2·78° 357.78 C. = Wedgewood's Pyrometer depends upon the contraction of cylinders of clay at high temperatures. The first degree W. corresponds, according to Wedgewood, to 598° C., and each degree W. is equal, according to the same authority, to 72° C. According to Guyton-Morveau, on the other hand, the first degree W. corresponds to 270° C., and each degree W. is equal to only 34° C. This pyrometer appears to give but very uncertain indications, the inaccuracy arising chiefly from this circumstance that the clay cylinders contract as much at a low red heat continued for some time as at a more powerful heat sustained but for a short time. Prinsep (Ann. Chim. Phys. 41, 247) makes alloys of silver and gold, ten parts of which contain 1, 2, 3, 4, 5, 6, 7, 8, or 9 parts of gold;-and for very high temperatures, alloys of gold and platinum containing 99, 98, 97, &c. per cent. of gold; they are made into flattened buttons. These alloys he places in separate cupels in the fire whose strength is to be determined, and ascertains which of them are fused. From a compa rative estimation with an air-pyrometer made of gold, it appears that silver melts at 999° C.; 9 parts of silver and 1 of gold at 1049; 8 silver and 2 gold at 1070°; 6 silver with 4 gold at 1099°; and 3 silver with 7 gold at 1379. An alloy containing 30 parts gold and 70 platinum is infusible even in the strongest blast furnace. 7. Heat imparts to many ponderable bodies particular colours which vary according to the quantity of heat contained in the bodies. Whenever such bodies are heated, they assume a colour different from that which distinguishes them in the cold; but on cooling again, the original colour reappears. This change of colour is not accompanied by any chemical change. This appearance is presented by the following liquids, and by the under-mentioned solids in the state of powder. Sulphur, which at ordinary temperatures is pale pellow, acquires a brownish yellow colour when heated just below its melting point.Hyponitric acid is colourless at 20°, pale yellow at 0°, orange yellow at +20°, and its vapour becomes darker the hotter it is.-Titanic, tantalic and molybdic acids, which are white at common temperatures, become of a lemon-yellow colour when they are heated; lemon-yellow tungstic acid becomes orange-yellow when heated,-green oxide of chromium becomes brown,-orange-coloured chromic acid, red,―pale grey anhydrous tersulphate of chromium, peach-blossom colour,-lemon-yellow neutral chromate of potash or soda, aurora-coloured,-orange-coloured bisulphuret of arsenic, red brown,-lemon-yellow tersulphurct of arsenic, of a colour varying from orange to red brown,-white oxide of antimony and white antimonious acid, lemon-yellow,-pale yellow antimonic acid, brownish yellow,-lemon-yellow oxide of bismuth, of a colour varying from orange to red brown,-very pale yellow oxide of zinc, lemon-yellow,-orange yellow sulphuret of cadmiuni, first brownish, then crimson red,-yellowish white peroxide of tin, orange-yellow,-yellow oxide of lead, brown-red,— scarlet-red minium, violet-coloured,-yellow chromate of lead, brownish, brown-red peroxide of iron, dark brown,-colourless aqueous solution of acid pernitrate of iron, reddish yellow,-red sub-oxide of copper, brownish grey,-brownish black protoxide of copper, deep black,-tile-red oxide of mercury, brownish black,-scarlet cinnabar, carmine-red,-white protosulphate of mercury, first yellow, then red,-yellow basic protonitrate of mercury, red, and yellow di-iodide of mercury, red. Elevation of temperature then always imparts a darker colour, and generally yellow or brown. Schönbein (Pogg. 45, 263) suggests that heat may produce an incipient decomposition, which however does not go so far as the separation of any of the elements; thus red oxide of mercury may when heated assume the brown-black colour of the suboxide from losing a part of its oxygen, which however is retained in a peculiar manner in the mass, and so forth. It is not however every change of colour that will accord with this hypothesis. 8. The heat which diffuses itself through ponderable bodies accumulates in them in quantities which differ according to their peculiar nature, whether we compare them with regard to weight or volume. Different bodies require different quantities of heat to raise their temperature equally, and disengage unequal quantities of heat in cooling through the same number of degrees of temperature. This different Capacity of bodies for heat is called Specific Heat when the bodies are compared with regard to their weight, and Relative Heat when they are compared with regard to volume. the relative heat. When bodies of different temperatures and different capacities for heat are mixed together, the temperature of the mixture is not the mean between the temperatures of the individual substances. Equal weights of bodies equally heated or equally cooled, but of different capacities for heat, raise or lower the temperature of a given quantity of water through different numbers of degrees. Or they melt unequal quantities of ice at 0°. Spheres of equal size and equally heated, but of substances having different capacities for heat, require different times to cool to the same point in the same medium-the radiating power of the surfaces being either accounted for or made the same in all. Gases enclosed in a manometer placed in a warmer medium require different times to produce in them the same amount of expansion by heat. The first of these methods was adopted by Wilke, Crawford, Kirwan, Dalton and Potter; the second by Delaroche & Bérard, Avogadro, Neumann, Regnault; the third particularly by Lavoisier & Laplace with their calorimeter; the fourth by Mayer, Böckmann, Petit & Dulong, Hermann, De la Rive & Marcet; the fifth by De la Rive & Marcet,-for the determination of the specific heats of bodies.-Moreover, with regard to the specific heats of gases, Dulong availed himself of the velocity of sound, by causing the gases to blow into a flute and determining the pitch of the sound, and Suermann, of the cooling produced by the evaporation of water in a stream of the gas. The specific heat multiplied into the specific gravity gives Specific Heat of Elastic Fluids at the ordinary Pressure of the Atmosphere. * This number cannot be right; for 1.0293.0.9757 = 1·0043. All gases, according to Haykraft, and all simple gases at least, according to De la Rive & Marcet, have the same relative heat: the greater relative heat which Haykraft found in olefiant gas he attributes to the mixture of ether vapour with the gas. But according to all other observations, this view first promulgated (Gilb. 45, 321) but afterwards retracted (Gilb. 48, 392) by Gay-Lussac, is very doubtful. = If the quantity of heat required to raise by 1° the temperature of air enclosed in a vessel with rigid sides be assumed 1.000, the quantity required to produce the same rise of temperature in an equal quantity of air confined under the same pressure, in such a manner that while the pressure remains constant it can expand freely when heated, will be 1·421; and if it be again reduced by pressure to its former bulk, this quantity 1-421 of heat corresponding to the increase of volume must be set free. A distinction must therefore be made between (a) Relative Heat under constant volume, and (b) Relative Heat under constant pressure (Dulong). [The relative heats given in the table refer to b.] Air, oxygen, hydrogen, and carbonic oxide gas have the same relative heat under constant pressure; hence it may be surmised that they have likewise the same relative heat under constant volume, and therefore that they evolve the same quantity of heat when subjected to the same pressure. Carbonic acid gas under the same pressure shows a rise of temperature of only 0·337°, nitrous oxide gas of 0.343, and olefiant gas of 0.240°. If it be assumed that all gases when equally compressed evolve the same quantity of heat, these last three gases must be supposed to have greater relative heat with reference to a given volume; and this may be reckoned (assuming that of air = = 1) for carbonic acid gas, 0.337 : 0·421 = 1 : x = 1.249; for nitrous oxide gas = 1.227, and for olefiant gas = = 1754. To find the relative heat of carbonic acid gas with reference to a constant pressure, we have the proportion, 1.421 : 1·249 + 0·4211:x=1·175; and so on. According to these suppositions, all gases under the same pressure, |