Page:The American Cyclopædia (1879) Volume VIII.djvu/591

 HEAT 577 There is thus a difference in the heat of the two mixtures of 71, and since the temperature of one of the constituents in each mixture, viz., boiling water, was the same, this dif- ference of 71 must represent the heat which is required to liquefy one pound of ice, and which is the same as that required to raise two pounds of water through a range of 71, or one pound of water through 142, or 142 pounds of water through 1. If we take as a unit of heat that quantity which is necessary to raise one pound of water through 1, the latent heat of water will be represented by 142 on Fahren- heit's scale, and by 78*88 on the centigrade scale. The experiment may be varied by min- gling a pound of ice at 32 with a pound of water at 174, when the resulting temperature the fusion of the ice will be found to remain 32, showing as before the expenditure of ", which is the latent heat of water. Ac- ling to the experiments of M. Person, the latent heat of water is more nearly 142'65, or on the centigrade scale 79*25. The following are his results with other liquids, calling the latent heat of water a thermal unit : TABLE OF LATENT HEAT8. SUBSTANCES. W=l. In deg. F. In deg. C. __ hur Nitrate of soda. . . . Nitrate of potash . . Tin Bismuth Lead Zinc ... Cadmium Silver , Mercury . . . 28- 18-660 21-070 2-880 Latent Heat of Vaporization. Liquids in ing into a state of vapor absorb a vast ount of heat. The conversion into vapor y be rapid, as in boiling, or it may be slow, when water evaporates in the open air at mon temperatures. In either case disap- arance of heat in proportion to the quantity evaporated is the result. If a flask of cold water is placed over a lamp, the temperature will continue to rise until it reaches 212 F., when ebullition will commence ; but the tem- perature will remain at 212 until the water has all boiled away. If the water at the com- mencement of the operation is at 32, and the supply of heat is uniform, the time occupied in evaporating it will be about 5 times that which is occupied in raising it to the boiling point, although the temperature has not risen above 212 ; therefore 5 times as much heat is absorbed in evaporating a given quantity of water as in raising it through 180. The la- tent heat of steam is therefore about 5 times 180, or 990 F. If the steam is reconverted to the liquid form, precisely this amount of heat reappears; in other words, the energy into which the heat was converted to maintain state of vapor is reconverted into heat when the steam is reconverted into water. This is shown in the method of Despretz for determin- ing the latent heat of vapors, which consists in condensing them in a worm immersed in water, and estimating the quantity of heat imparted to the latter. The retort C, fig. 13, heated by FIG. 13. Desprctz's Apparatus for Latent Heat of Vaporization. a lamp, contains the liquid whose vapor is the subject of experiment. The vapor in passing through the worm S is condensed, imparting its latent heat to the water in the vessel R, and being collected in a vessel placed under the stopcock r, its weight can be found ; and that of R, or the calorimeter, and its contents being known, and also their temperature, the in- crease of the latter furnishes the data for cal- . 14. Faraday's Apparatus for Liquefying Gases. culating the latent heat of the vapor. Re- gnault used more elaborate apparatus, and his results were rather more exact. If pressure^ applied to a gas confined in an enclosure, its temperature will be raised, and if the pressure is immediately removed, the gas will return to