QUANTITY OF HEAT. SPECIFIC HEAT. 



73 



freezing mixture, is passed through the test-tube A until a cap of ice ia 

 formed round it in the vessel B. The calorimeter is now placed in melt- 

 ing ice or snow, and left, preferably for some days, till the ice cap is all 

 at 0. A layer of wool is placed at the bottom of A to prevent breakage, 

 and some distilled water is added. When all is at 0, the calorimeter 

 still surrounded by melting ice is ready for use. It is better, as 

 pointed out by Professor Boys, Phil. Mag., xxiv., 1887, p. 214, to sur- 

 round the calorimeter by an air-jacket by enclosing it in an outer tube, 

 which is immersed in the melt- 

 ing ice. A small piece of the 

 substance to be experimented 

 on is raised to any desired 

 temperature, and then dropped 

 into A, giving its heat to the 

 water. But as the water rises 

 in temperature it becomes more 

 dense, and therefore remains at 

 the bottom of the tube round 

 the substance. Gradually heat 

 is conducted out into B, where 

 some of the ice is melted by 

 it, and the melting continues 

 till all is again at 0. The 

 tube C having been calibrated, 

 the recession of the mercury 

 along it gives the weight of 

 ice melted, a contraction of 

 1-09082 -1-00012 = -0907 cc., 

 corresponding to the melting 

 of 1 gramme. For comparative 

 measurements it is only neces- 

 sary to observe the contraction 

 in different cases, but for the 

 determination of the amount 

 of heat actually yielded, it is 

 necessary to know the latent 

 heat of water or the number of 

 calories taken up by 1 gramme 

 of ice at in melting to 

 water at 0. By using a known 

 weight of water at a known 



boiling temperature, that is, by adding a known number of calories 

 to A, Bunsen determined the latent heat of water as 80 - 025.* The 

 calory, in terms of which this is expressed, is y^ of the heat required 

 to raise 1 gramme of water from to 100. It will be observed that 

 in this method the radiation correction disappears, for the substance 

 taking in the heat always remains at the same temperature, and is always 

 in equilibrium with its surroundings. 



Joly's Steam Calorimeter. Professor Joly has introduced and 



* In Ostwald's Physico- Chemical Measurements, p. 136, will be found a description 

 of modifications of the calorimeter to make it more convenient in use. 



FIG. 56. Bunsen's Ice Calorimeter. 



