236 NUTRITION OF FARM ANIMALS 



measuring heat previously described (309). Water calorim- 

 eters may be subdivided into those in which the heat is im- 

 parted to a stationary mass of water and those called flow 

 calorimeters, in which it is taken up by a current of water. 



Latent heat calorimeters make use of the second method of 

 heat measurement, viz., causing it to effect a change in the 

 physical state of the calorimetric substance. Thus Lavoisier 

 employed an ice calorimeter in his experiments upon the re- 

 lations between respiration and heat production. This type of 

 calorimeter, however, is not well suited to experiments with 

 animals and has been but little used. 



Emission calorimeters may be said not to be in a strict sense 

 calorimeters at all, i.e., they do not serve directly to measure 

 quantities of heat but only to compare the rate of heat pro- 

 duction by different sources, but they may be used indirectly 

 to measure quantities. The principle of the emission calorim- 

 eter may be illustrated as follows : If a known source of heat 

 (an electric resistance, for example) be placed in a closed recepta- 

 cle located in a room kept at constant temperature, it will tend 

 to heat the walls of the container. As the temperature of the 

 walls rises, however, heat will be radiated from them with in- 

 creasing rapidity until a balance is established between heat 

 radiation and heat production and the temperature of the 

 walls remains constant. If, now, a second source of heat, 

 an animal, for example, be substituted for the first one, 

 keeping the external conditions the same, and if it appears that, 

 when an equilibrium is reached, the temperature of the walls is 

 the same as in the first case, it is concluded that the rate of 

 heat radiation is the same as in the first case, and that the 

 animal is producing heat at the same rate as was the electric 

 resistance, so that the amount of heat produced by the animal 

 in a unit of time is thus indirectly measured. 



The respiration calorimeter. All animal calorimeters used 

 for experiments of any length must necessarily be provided 

 with ventilation. To prevent a loss of heat in the air current, 

 it is introduced at the same temperature as that at which it 

 leaves the apparatus. The ventilating air current, however, 

 tends to remove water vapor from the chamber and the evapo- 

 ration of this water, of course, absorbs a corresponding amount 

 of heat as the so-called " latent heat of evaporation " of water. 



