n8 THE CHEMISTRY OF CATTLE FEEDING 



Calorimetry. The true measure of the potential energy 

 of the food, according to definition (p. 114), is the amount of 

 heat developed by the oxidation. This is determined by 

 means of an apparatus called a calorimeter. 



Several different forms of calorimeter are in use ; but they 

 all consist essentially of a platinum shell or bomb submerged 



in water. The sub- 

 stance to be tested is 

 introduced into this 

 bomb along with com- 

 pressed oxygen gas and 

 ignited by means of 

 electric sparks. The 

 heat produced by the 

 combustion is com- 

 municated to the water 

 outside, and the rise of 

 temperature is noted. 

 A sectional drawing 

 of the instrument, as 

 actually used, is shown 

 in the illustration l 

 (Fig. 7). The mass 

 and specific heat of 

 the apparatus, as well 

 as the mass of the 

 water and of the sub- 

 stance burned, must, 

 of course, be taken 

 into account in an 

 experiment of this 

 kind. 



The results are generally expressed in terms of the 

 amount of heat required to raise the temperature of i gram of 

 water 1 C. This unit, called a calorie, is too small for physio- 

 logical investigations, and the kilo-calorie is often used instead. 



1 Bulletin No. 21, Office of Experiment Stations, U.S. Department of 

 Agriculture. 



FiG. 7. Atwater's Calorimeter. 



