222 NUTRITION OF FARM ANIMALS 



ries. A calorimeter constructed after this principle is a water 

 calorimeter. Such calorimeters have been devised in a great 

 variety of forms according to the special purpose in view. The 

 two essential requirements are that any escape of heat by con- 

 duction or radiation shall be either preventable or measurable 

 and that the temperature increase be accurately determined. 



In the second method the heat is caused to expend itself in 

 changing the physical state of some substance as, for example, 

 in melting ice or in evaporating some volatile liquid. The ice 



FIG. 31. Lavoisier's ice calorimeter. (Schaefer, Text Book of Physiology.) 



calorimeter is one of the oldest forms of calorimeter and has been 

 extensively used for certain classes of work. Figure 31 shows a 

 simple form of ice calorimeter used by Lavoisier. The source 

 of heat is placed in the central vessel and imparts its heat to 

 the surrounding ice, while the access of any extraneous heat is 

 prevented by the outside jacket of ice. 



In Lavoisier's calorimeter the amount of ice melted was determined 

 by collecting and weighing the resulting water, but a much more 

 accurate method of measurement is based upon the contraction which 

 ice undergoes when converted into water. 



