ON THE MEASUREir AWD THE NATURE OF HEAT. 651 



perimcnts,in some such cases, that the capacity of the same substance is actually 

 greater in a liquid than in a solid state, and in a state of vapour, than in cither; 

 and both Dr. Irvine and Dr. Crawford have attempted to deduce, from a 

 comparison of the proportional capacities of water and ice, with the quantity 

 of heat extricated during congelation, a measure of the whole heat which is 

 contained in these substances, and an estimation of the place which the 

 absolute privation of heat, or the natural zero, ought to occupy in the scale 

 of the thermometer. Thus, when a pound of ice, at 32°, is mixed with a 

 pound of water at 172° of Fahrenheit, the whole excess of 140° is absorbed 

 in the conversion of the ice into water, and the mixture is reduced to the 

 temi)€rature of 32°; and, on the other hand, when a pound of ice freezes, a 

 certain quantity of heat is evolved which is probably capable of raising the tem- 

 perature of a pound of water 140°, or that of 140 pounds a single degree. 

 Pr. Crawford found, by means of other experiments, that a quantity of heat 

 capable of raising the temperature of water 9° would raise that of ice as much 

 as 10°; hence he inferred that the capacity of ice wasT?,- as great as that of 

 water, and that if this capacity, instead of being reduced to -^, had been 

 Avholly destroyed, the quantity of heat extricated would have been 10 times, 

 as great, or about 1400°, which has, therefore, been considered as tl>e whole 

 quantity of heat contained in a pound of water at 32°, and the beginning of 

 the, natural scale has been placed about 13^8° below the zero of Fahren- 

 lielt. ; Dr. Irvine makes tl>e capacity of ice still less considerable, and places- 

 the natural zero about 900- degrees below tliat of Fahrenheit. 



If direct experiments on the quantities of heat, required for producing 

 certain elevations of temperature, in different states of the same substance, 

 compared in this, maimer with the emission or absorption of heat which takfes 

 place while those changes are performed, agreed with similai- experiment* 

 made on different substances, there could be no objection to the mode of 

 representation. But if it should appear that such compaiTsous frequently 

 present us with contradictory results, we could no longer consider the theory 

 of capacities for heat as sufficient to explain the phenomena. With respect to the 

 simple changes constituting congelation and liquefaction, comlensationandeva- 

 poration, and compression and rarefaction, there appears to be at present noevi- 

 dcnce of the insufficiency of this theory ; it has not perhaps yet been shown that the 

 heat absorbed in any one cliange is always precisely equalto that which is emitted 



