72 HEAT. 



conveniently be made C. by suiTounding the outside of the enclosure 

 with melting ice. In some experiments the space between the calorimeter 

 and the enclosure has been exhausted. The quantity of heat given out 

 by the surface of the containing vessel in a given time depends not on 

 the nature of its contents, but on the temperature of its surface alone. 

 If, for example, in two different cases, the temperature is observed to 

 fall at twice the rate in one case that it falls in the other when the 

 mean temperature is the same, the heat given out in the same time is 

 the same in the two cases, therefore the capacity of the calorimeter and 

 its contents in the first case is only half as great as in the second case. 

 Hence we have 



W + TWjSj = 



where w is the water equivalent of the calorimeter and thermometer, and 

 m^rn^s^ are respectively the masses and specific heats of the liquids in 

 the two experiments. 



The method was originally applied both to solids and liquids, but it 

 was found that with solids it did not give good results, owing to vari- 

 ations of temperature within the solids. The circulation in liquids 

 during their cooling maintains their temperature more nearly uniform 

 at each instant, and so the objection is much less in their case. 



The Melting Ice Method Bunsen's Ice Calorimeter. In this 

 method the heat given out by a body in cooling down from some higher 

 temperature to 0. is measured by the quantity of ice which it will 

 melt. It was first used by Black and afterwards by Lavoisier and La- 

 place. They collected and weighed the water resulting from the melting. 

 But as it was practically impossible to collect the whole of the water, the 

 method failed to give very good results. It was, however, modified by 

 Bunsen in such a way as to make it of very great service. In his calori- 

 meter, instead of collecting the water resulting from the melting, the 

 contraction which takes place in the change from solid to liquid is ob- 

 served. The amount of this change was measured by a separate experi- 

 ment in which a known weight of ice at was contained in a bulb, the 

 rest of the space being filled with mercury. The ice was then melted 

 to water at 0, mercury being drawn into the bulb to occupy the space 

 left by the ice in melting ; the additional weight of mercury gave the 

 contraction. He found that a gramme of ice at contracted from 

 1-09082 cc. to 1-00012 cc. of water at 0.* 



The construction of the calorimeter is illustrated by Fig. 56. 



A is a test-tube fused into the glass vessel B, which is continued into 

 the narrow tube 0. B is nearly filled with water which has been 

 previously deprived of air by boiling, and the remaining space is 

 occupied by mercury, which also fills the tube round the bend and 

 some way along the horizontal part which lies against a graduated scale. 

 To prepare the calorimeter for use, a stream of alcohol, cooled by a 



* Barnes (Physical Constants of Ice, Trans. Roy. Soc. Canada, Ser. III., 

 vol. iii., Sect. III., 1909-10) gives a summary of the work on Ice, and concludes 

 that the best value for the density of natural ice is - 91704, while that of 

 artificial ice is 0-91676. He takes as the best value of the latent heat 

 of fusion a determination by Professor A. W. Smith in 1903 corrected to 

 79-818. 



