VI. CALOUIMETRIC MEASUREMENTS 179 



may be regarded as constant. When the specific heat changes with 

 changes in temperature, the heat quantity may be expressed by the 

 more general equation: 



AQ = m f^l^ cr clT (7) 



The specific heat (Ct) in this ease is not a constant but is a function 

 of the temperature. 



Usually heat is measured in calories (cal.). In thermodynamics 

 one uses as a rule the 15 degree calorie {11). This calorie is the 

 amount of heat that raises the temperature of 1 gram of water from 

 15 to 16°C. It is almost exactly equal to the "mean calorie," 

 nameh^ the amount of heat that raises the temperature of 0.01 gram 

 of w^ater from the freezing point to the boiUng point at 1 atmosphere 

 pressure. 



Because electrical units can be standardized more easily than the 

 heat capacity of water, the calorie is now defined on the basis of the 

 international joule. 1 cal. = 4.1833 international joules. The joule 

 in turn is defined as volt times coulomb. The volt is 1/1.0183 of the 

 electromotive force of the normal Weston cell at 20°C., and the cou- 

 lomb is the amount of electricity that deposits 0.001118 gram of silver 

 in an electroh^ic cell. This is 1/96494 of the electricity carried by 1 

 gram equivalent or the charge of 6.24 X 10^^ electrons. Some au- 

 thors express heat directly in terms of joules (J). It might be advan- 

 tageous to maintain calories for expressing heat and use joules when 

 other forms of energy, particularly chemical energy or radiant energy, 

 are measured calorimetrically. 



In much biophysical work heat quantities are given in kilocalories 

 (kcal.). One kilocalorie equals one thousand calories. Terms such 

 as "kilogram calories" and especially "large calories" are obsolete 

 and should be avoided. 



4. Latent Heat 



The final temperature of a mixture of cold and warm substances 

 may be the same as the initial temperature of one component. If, 

 for example, 100 g. of water vnth a temperature of 8° are poured into 

 100 g. of water which contains a considerable amount of ice and has a 

 temperature of 0°, the final temperature of the mixture is 0°. The 

 100 g. of the warmer water being cooled from 8 to 0° lost 800 cal. of 

 heat. Instead of raising the temperature of the colder water this 



