the specific heat of a fixed gas at constant pressure to the sp. ht. at constant volume is given as follows by different writers (Eng'g, July 12, 1889): Regnault, 1.3953; Moll and Beck, 1.4085; Szathmari, 1.4027; J. Macfarlane Gray, 1.4. The first three are obtained from the velocity of sound in air. The fourth is derived from theory. Prof. Wood says: The value of the ratio for air, as found in the days of La Place, was 1.41, and we have 0.2377 +1.41 = 0.1686, the value used by Clausius, Hanssen, and many others. But this ratio is not definitely known. Rankine in his later writings used 1.408, and Tait in a recent work gives 1.404, while some experiments gives less than 1.4 and others more than 1.41. Prof. Wood uses 1.406. Specific Heat of Gases.-Experiments by Mallard and Le Chatelier indicate a continuous increase in the specific heat at constant volume of steam, CO2, and even of the perfect gases, with rise of temperature. The variation is inappreciable at 100° C., but increases rapidly at the high temperatures of the gas-engine cylinder. (Robinson's Gas and Petroleum Engines.) Specific Heat and Latent Heat of Fusion of Iron and Steel. (H. H. Campbell, Trans. A. I. M. E., xix. 181.) Åkerman. Troilius. Heating from 0 to 1800° C.. Hence probable value solid rail steel.. 318 330 calories per kilo. 325 calories per kilo. .1175 .1081 .1125 66 06 melted rail steel.. Latent heat of fusion, pig iron, calories per kilo.. 46 66 66 From which we may assume that the truth is about: Steel, 20; pig iron, 30. EXPANSION BY HEAT. In the centigrade scale the coefficient of expansion of air per degree is 0.003665 = 1/273; that is, the pressure being constant, the volume of a perfect gas increases 1/273 of its volume at 0° C. for every increase in temperature of 1° C. In Fahrenheit units it increases 1/491.2 = .002036 of its volume at 32° F. for every increase of 1° F. Expansion of Gases by Heat from 32° to 212° F. (Regnault.) If the volume is kept constant, the pressure varies directly as the absolute temperature. Lineal Expansion of Solids at Ordinary Temperatures. (British Board of Trade; from CLARK.) Aluminum (cast)... 66 plate.. Brick. .00000627 .00001129 .001129 .001083 .00000957 .00001722.001722 .001868 Bronze (Copper, 17; Tin, 22; Zinc 1). .00000986 .00001774 .001774 .00000975.00001755 .001755 .001392 .00000594 .00001070.001070 .00000887 .00001596 .001596 .001718 .00004278.00007700 007700 .00000451 .00000812.000812 .00000499.00000897.000897 .00000438 .00000789.000789 .00000498.00000897 .000897 .00000648 .00001166 .001166 .001235 66 cast Lead.. Magnesium .002694 Marbles, various { Masonry, brick from .00000256 .00000460.000460 to... .00000494.00000890.000890 Cubical expansion, or expansion of volume = linear expansion X 3. Absolute Temperature-Absolute Zero.-The absolute zero of a gas is a theoretical consequence of the law of expansion by heat, assuming that it is possible to continue the cooling of a perfect gas until its volume is diminished to nothing. If the volume of a perfect gas increases 1/273 of its volume at 0° C. for every increase of temperature of 1° C., and decreases 1/273 of its volume for every decrease of temperature of 1° C., then at 273° C. the volume of the imaginary gas would be reduced to nothing. This point 273° C., or 491.2° F. below the melting-point of ice on the air thermometer, or 492.66° F. below on a perfect gas thermometer = - 459.2° F. (or - 460.66°), is called the absolute zero; and absolute temperatures are temperatures measured, on either the Fahrenheit or centigrade scale, from this zero. The freezing point, 32° F., corresponds to 491.2° F. absolute. If po be the pressure and o the volume of a gas at the temperature of 32° F. = 491.2° on the absolute scale = To, and p the pressure, and v the volume of the same quantity of gas at any other absolute temperature T', then pv T t+459.2 pv Povo To = 491.2 = T Povo The value of pov。 To for air is 53.37, and pv = 53.37T, calculated as follows by Prof. Wood: 1 080728 A cubic foot of dry air at 32° F. at the sea-level weighs 0.080728 lb. The volume of one pound is v。 = 12.387 cubic feet. The pressure per square foot is 2116.2 lbs. = Povo 2116.2 X 12.387 26214 = To 491.13 = 491.13 = 53.37. The figure 491.13 is the number of degrees that the absolute zero is below the melting-point of ice, by the air thermometer. On the absolute scale, whose divisions would be indicated by a perfect gas thermometer, the cal culated value approximately is 492.66, which would make pv = 53.217. Prof. Thomson considers that 273.1° C., 459.4° F., is the most probable value of the absolute zero. See Heat in Ency. Brit. Expansion of Liquids from 32° to 212° F.-Apparent expansion in glass (Clark). * Volume at 212°, volume at 32° being 1: Water.... Water saturated with salt.. Mercury.. Alcohol Olive and linseed oils.. Turpentine and ether Hydrochlor. and sulphuric acids 1.06 For water at various temperatures, see Water. For air at various temperatures, see Air. LATENT HEATS OF FUSION AND EVAPORATION. Latent Heat means a quantity of heat which has disappeared, having been employed to produce some change other than elevation of temperature. By exactly reversing that change, the quantity of heat which has disappeared is reproduced. Maxwell defines it as the quantity of heat which must be communicated to a body in a given state in order to convert it into another state without changing its temperature. Latent Heat of Fusion.-When a body passes from the solid to the liquid state, its temperature remains stationary, or nearly stationary, at a certain melting point during the whole operation of melting; and in order to make that operation go on, a quantity of heat must be transferred to the substance melted, being a certain amount for each unit of weight of the substance. This quantity is called the latent heat of fusion. When a body passes from the liquid to the solid state, its temperature remains stationary or nearly stationary during the whole operation of freezing; a quantity of heat equal to the latent heat of fusion is produced in the body and rejected into the atmosphere or other surrounding bodies. The following are examples in British thermal units per pound, as given in Landolt & Börnstein's Physikalische-Chemische Tabellen (Berlin, 1894). Prof. Wood considers 144 heat units as the most reliable value for the latent heat of fusion of ice. Person gives 142.65. Latent Heat of Evaporation.-When a body passes from the solid or liquid to the gaseous state, its temperature during the operation remains stationary at a certain boiling point, depending on the pressure of the vapor produced; and in order to make the evaporation go on, a quantity of heat must be transferred to the substance evaporated, whose amount for each unit of weight of the substance evaporated depends on the temperature. That heat does not raise the temperature of the substance, but disappears in causing it to assume the gaseous state, and it is called the latent heat of evaporation. When a body passes from the gaseous state to the liquid or solid state, its temperature remains stationary, during that operation, at the boiling-point corresponding to the pressure of the vapor: a quantity of heat equal to the latent heat of evaporation at that temperature is produced in the body; and in order that the operation of condensation may go on, that heat must be transferred from the body condensed to some other body. The following are examples of the latent heat of evaporation in British thermal units, of one pound of certain substances, when the pressure of the vapor is one atmosphere of 14.7 lbs. on the square inch: Water Ether. Substance. Bisulphide of carbon.... Boiling-point under 212.0 172.2 95.0 114.8 The latent heat of evaporation of water at a series of boiling-points extending from a few degrees below its freezing-point up to about 375 degrees Fahrenheit has been determined experimentally by M. Regnault. The results of those experiments are represented approximately by the formula. in British thermal units per pound, I nearly 1091.7 0.7(t - 32°) = 965.7 0.7(t- 212°). The Total Heat of Evaporation is the sum of the heat which disappears in evaporating one pound of a given substance at a given tem. perature (or latent heat of evaporation) and of the heat required to raise its temperature, before evaporation, from some fixed temperature up to the temperature of evaporation. The latter part of the total heat is called the sensible heat. In the case of water, the experiments of M. Regnault show that the total heat of steam from the temperature of melting ice increases at a uniform rate as the temperature of evaporation rises. The following is the formula in British thermal units per pound: For the total heat, latent heat, etc., of steam at different pressures, see table of the Properties of Saturated Steam. For tables of total heat, latent heat, and other properties of steams of ether, alcohol, acetone, chloroform, chloride of carbon, and bisulphide of carbon, see Rontgen's Thermodynamics (Dubois's translation.) For ammonia and sulphur dioxide, see Wood's Thermodynamics; also, tables under Refrigerating Machinery, in this book. EVAPORATION AND DRYING. In evaporation, the formation of vapor takes place on the surface; in boiling, within the liquid: the former is a slow, the latter a quick, method of evaporation. If we bring an open vessel with water under the receiver of an air-pump and exhaust the air the water in the vessel will commence to boil, and if we keep up the vacuum the water will actually boil near its freezing-point. The formation of steam in this case is due to the heat which the water takes out of the surroundings. Steam formed under pressure has the same temperature as the liquid in which it was formed, provided the steam is kept under the same pressure. By properly cooling the rising steam from boiling water, as in the multipleeffect evaporating systems, we can regulate the pressure so that the water boils at low temperatures. Evaporation of Water in Reservoirs.-Experiments at the Mount Hope Reservoir, Rochester, N. Y., in 1891, gave the following results: Mean temperature of air in shade.... "water in reservoir.. July. Aug. Sept. Oct. 68.2 70.5 70.3 68.7 53.3 54.4 75.2 74.7 ........ 5.59 4.93 4.05 3.23 2.16 66 humidity of air, per cent... Evaporation in inches during month.. Rainfall in inches during month.............. Evaporation of Water from Open Channels. (Flynn's Irrigation Canals and Flow of Water.)-Experiments from 1881 to 1885 in Tulare County, California, showed an evaporation from a pan in the river equal to an average depth of one eighth of an inch per day throughout the year. When the pan was in the air the average evaporation was less than 3/16 of an inch per day. The average for the month of August was 1/3 inch per day, and for March and April 1/12 of an inch per day. Experiments in Colorado show that evaporation ranges from .088 to .16 of an inch per day during the irrigating season. In Northern Italy the evaporation was from 1/12 to 1/9 inch per day, while in the south, under the influence of hot winds, it was from 1/6 to 1/5 inch per day. In the hot season in Northern India, with a decidedly hot wind blowing, the average evaporation was 1⁄2 inch per day. The evaporation increases with the temperature of the water. Evaporation by the Multiple System.-A multiple effect is a series of evaporating vessels each having a steam chamber, so connected that the heat of the steam or vapor produced in the first vessel heats the second, the vapor or steam produced in the second heats the third, and so The vapor from the last vessel is condensed in a condenser. Three vessels are generally used, in which case the apparatus is called a Triple Effect. In evaporating in a triple effect the vacuum is graduated so that the liquid is boiled at a constant and low temperature. on. Resistance to Boiling.-Brine. (Rankine.)-The presence in a liquid of a substance dissolved in it (as salt in water) resists ebullition, and raises the temperature at which the liquid boils, under a given pressure; but unless the dissolved substance enters into the composition of the vapor, the relation between the temperature and pressure of saturation of the vapor remains unchanged. A resistance to ebullition is also offered by a vessel of a material which attracts the liquid (as when water boils in a glass vessel). and the boiling take place by starts. To avoid the errors which causes of this kind produce in the measurement of boiling-points, it is advisable to place the thermometer, not in the liquid, but in the vapor, which shows the true boiling-point, freed from the disturbing effect of the attractive nature of the vessel. The boiling-point of saturated brine under one atmosphere is 226° Fahr., and that of weaker brine is higher than the boiling-point of pure water by 1.2° Fahr., for each 1/32 of salt that the water contains. Average sea-water contains 1/32; and the brine in marine boilers is not suffered to contain more than from 2/32 to 3/32. Methods of Evaporation Employed in the Manufacture of Salt. (F. E. Engelhardt, Chemist Onondaga Salt Springs; Report for 1889.)-1. Solar heat-solar evaporation. 2. Direct fire, applied to the heat. ing surface of the vessels containing brine-kettle and pan methods. 3. The steam-grainer system-steam-pans, steam-kettles, etc. 4. Use of steam and a reduction of the atmospheric pressure over the boiling brine-vacuum system. When a saturated salt solution boils, it is immaterial whether it is done under ordinary atmospheric pressure at 228° F., or under four atmospheres with a temperature of 320° F., or in a vacuum under 1/10 atmosphere, the result will always be a fine-grained salt. The fuel consumption is stated to be as follows: By the kettle method, 40 to 45 bu. of salt evaporated per ton of fuel, anthracite dust burned on perforated grates; evaporation, 5.53 lbs. of water per pound of coal. By the pan method, 70 to 75 bu. per ton of fuel. By vacuum pans, single effect, 86 bu. per ton of anthracite dust (2000 lbs.). With a double effect nearly double that amount can be produced. |