HEAT. 



35 



Fig. 13. 



According to Lavoisier and La Place, 

 as much heat is absorbed by one pound 

 of ice in melting, as would raise the 

 temperature of the same weight of water 

 135 of Fahrenheit's scale. By numer- 

 ous experiments with this apparatus, 

 they ascertained the comparative quan- 

 tities of caloric evolved by different 

 bodies in passing from one temperature 

 to another, by which the comparative 

 quantities contained were also made 

 known. 



When a liquid was operated upon, it 

 was put into a glass mattrass, allow- 

 ance being made for the effects of 

 the glass, and suspended in the wire 

 cage ; and when gases were subjected to 

 experiment, they were made to pass 

 through a spiral tube enclosed within 

 the wire cage, the tube being furnished 

 with a thermometer at each end. 



To prevent the contents of the outer 

 vessel from being frozen in experiments 

 made with this apparatus, it is neces- 

 sary that the temperature of the exter- 

 nal air should be a little above 32 ; for, 

 if the outer vessel should be cooled be- 

 low that point, it would absorb heat 

 from the middle vessel, and thus inter- 



fere with the result of the experiment ; 

 but the temperature should not be more 

 than ten or twelve degrees higher than 

 the freezing point ; for, in that case, the 

 air within the apparatus being heavier 

 than the external air, it would descend 

 and escape by the water-tube at the bot- 

 tom, occasioning a current of air to 

 pass through the instrument, which 

 would melt a portion of the ice, inde- 

 pendently of the heat intended to melt 

 it. 



Mr. Wedgewood pointed out 'two 

 sources of inaccuracy in the use of this 

 apparatus ; one of which arises from a 

 portion of the water produced by the 

 melting of the ice being retained by ca- 

 pillary attraction, among the pores of 

 the unmelted ice, making the quantity 

 obtained less than it ought to be. 

 Endeavouring to remedy this source 

 of error, Lavoisier and La Place ex- 

 posed the pounded ice, for some time, 

 to the atmosphere, before the experi- 

 ment began, that the pores might be 

 filled with water. It was also shown 

 by Wedgewood, that while the process 

 of thawing is going on in the upper 

 part of the middle vessel, the water 

 D 2 



