THE CALORIMETER SYSTEM AND MEASUREMENT OF HEAT. 151 



The number of grams of water removed by the absorbers multiplied 

 by the above factor gives the number of calories of heat escaping from 

 the chamber in water vapor. To obtain the total quantity of heat 

 involved in the vaporization of water, however, it is necessary to make 

 allowance for the latent heat of water vapor still remaining in the 

 chamber, the correction being added or subtracted according to whether 

 the amount of residual vapor at the end is larger or smaller than that 

 present at the beginning of the experimental period. 



SENSIBLE HEAT REMOVED IN THE WATER CURRENT. 

 UNIT OF HEAT. 



The ordinary definition of the large calorie is the quantity of heat 

 required to raise the temperature of i kg. of water i C. This, how- 

 ever, is only approximately correct, because the specific heat of water 

 varies with the temperature. 1 It is therefore necessary to define the 

 unit of heat somewhat more exactly. 



In experiments the temperature of the calorimeter chamber is main- 

 tained very constant at 20 ; hence the specific heat of water at 20 is 

 taken as the standard ; and the calorie here used is the quantity of heat 

 necessary to raise a kilogram of water from 19.5 to 20.5. 



CALCULATION OF THE QUANTITY OF HEAT MEASURED. 



The weight of the water determined by the water-meter, multiplied 

 b} 1 the difference between the temperature of the water as it enters and 

 that as it leaves the chamber, gives the quantity of heat as measured at 

 the mean between the temperatures of the ingoing and outgoing water. 

 According to the explanation given above, however, this must be cor- 

 rected for the difference between the specific heat of the water at this 

 mean temperature and that at 20 . The latter value is designated as heat 

 measured in terms of C 20 ; the former value is designated as heat measured 

 in terms of C t , in which / is any temperature other than that of 20. 



In finding the true value of C t , it is necessary to know the mean 

 specific heat of water for the range of temperature employed in any 

 given period. The temperature of the ingoing w T ater is sometimes as 

 low as i ; that of the outgoing water is rarely above 15, and more 

 frequently not far from 10. If, for example, the water enters the cal- 

 orimeter at 2, a condition that is very common during the hard- work 



1 The results of a large number of experiments on the specific heat of water at 

 different temperatures have been discussed in considerable detail in another publi- 

 cation (U. S. Dept. of Agr., Office of Experiment Stations Bull. 63, p. 55). From a 

 table there given showing the specific heat of water at different temperatures and 

 referring to that at 20 as a unit, it is seen that the specific heat of water at o is 

 1.0090; at .5, 1.0056; at 10, 1.0029; at J 5. i.ooio. The difference between C. i9 

 and C is one of nearly i per cent. 



