PHYSICS OF THE EIGHTEENTH CENTURY. 289 



simplest idea of the subject. The method employed by Black in his 

 own determinations of specific heats will be better understood after 

 an explanation of another cognate discovery of his, which was an- 

 nounced shortly after the observations on specific heat had been made 

 public. If two precisely similar vessels filled, one with ice-cold water 

 at 32 F., and the other with actual solid ice, be brought into a warm 

 room, the thermometric indications, though originally the same (i.e., 

 32), soon exhibit this difference : the temperature of the cold water 

 begins at once to rise, and in a short time (say an hour) the water may 

 have attained nearly the same temperature as the surrounding atmo- 

 sphere : while, on the other hand, no rise of temperature will be per- 

 ceptible in the vessel containing the ice so long as any of the ice re- 

 mains immelted, and the thermometer will constantly record 32 for 

 several hours. As the vessels are similar and equal, they must receive 

 equal quantities of heat in the same time ; yet, while the thermometer 

 in the water-vessel has risen perhaps 20, that in the ice-vessel gives 

 no indication whatever of the heat which has been communicated to 

 that vessel. What, asked Black, becomes of the heat which has been 

 all along given to the vessel of ice ? His reply was that the heat had 

 in some way been expended in converting the ice into water : of this 

 heat the thermometer gives no indication ; but, although it has not 

 shown itself as temperature, it is nevertheless existent in a latent form. 

 For if, when the whole has become changed into ice-cold water at 32, 

 the vessel is exposed to cold again, the water at 3 2 is not immediately 

 reconverted into ice, but must first part with all its latent heat. The 

 amount of this heat which becomes latent when ice is converted into 

 water may be measured. If i Ib. of dry pounded ice at 32 be mixed 

 with i Ib. of water at 175 F., the result will be 2 Ibs. of water at 32; 

 that is. the pound of ice will, in this case, be converted into water with- 

 out any gain of temperature, but the heat which becomes latent in the 

 process is abstracted from the pound of water originally at 175, the 

 temperature of which is thereby reduced to 32. The heat, therefore, 

 which is required to convert i Ib. of ice at 32 into water also at 32, 

 would, if applied to i Ib. of water, raise its temperature 143; and as 

 the quantity of heat required to raise i Ib. of water i is taken as the 

 unit-quantity, the latent heat of ice is 143 heat-units. Not 143 degrees, 

 be it noted, but so much heat as would raise 143 Ibs. of water i F., 

 or so much as would raise i Ib. of water 143 F. 



Black found that in all general cases, whenever a solid body is con- 

 verted into a liquid, a certain quantity of heat becomes latent. If a 

 crucible containing a quantity of tin filings be gradually heated, a 

 thermometer placed amidst the tin will gradually and steadily rise 

 until it marks 442 F., and then, although the crucible receives heat 

 constantly, the thermometer will become stationary at 442 until all 

 the tin has melted. The process of melting begins the moment the 

 thermometer marks 442, and no further rise of the thermometer takes 



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