57 



A MANUAL OF PHYSIOLOGY 



temperature are measured, and the heat production can then be 

 calculated. The apparatus consists of a chamber in which a human 

 being can live for several days and nights. A stream of air is supplied 

 and the chemical changes produced in this are investigated in the 

 manner already described (p. 241). 



Air calorimeters have sometimes been used for physiological 

 purposes. A diagram of one is shown in Fig. 196. Such calori- 

 meters are really thermometers with an immense radiating surface, 

 for only a small proportion of the heat given off by the animal goes 

 to heat the measuring substance. The heat required to raise the 



FIG. 196. AIR CALORIMETER. 



(/.), cross-section ; (//.), longitudinal section ; A, cavity of 

 calorimeter for animal ; B, copper cylinder corrugated so as to 

 increase the radiating surface ; C, air space enclosed between B 

 and a concentric copper cylinder F ; C is air-tight, and is connected by the tube 2 

 with the manometer M. The other end of the manometer is connected with an 

 exactly similar calorimeter, in which a hydrogen flame is burnt in the space cor- 

 responding to A, or in which the air in A is heated by a coil of wire traversed by 

 an electrical current. The flame or current is regulated so as to keep the coloured 

 petroleum or mercury in the manometer M at the same level in both limbs ; the 

 amount of heat given off to the one calorimeter by the flame or current is then 

 equal to that given off by the animal to the other. D is an external cylinder 

 of copper or tin perforated by holes (6, 7) at intervals. The purpose of it is to 

 prevent draughts from affecting the loss of heat from F ; 4, 5, are tubes through 

 which thermometers can be introduced into C ; i is the terminal of a spiral tube, 

 which is coiled in the end portion of the air space C. The sections of the coils 

 are indicated by small circles. The other end of the spiral tube is 3 ; through 

 this tube air is sucked out, and so the proper ventilation of the animal is kept up. 

 The object of the spiral arrangement is that the air aspirated out of A may give 

 up its heat to the air in C before passing out. E is a door with double glass 

 walls. 



temperature of a litre of air by i is very small in comparison 

 with that required to raise the temperature of a litre of water 

 by the same amount. Hence a given quantity of heat raises 

 the temperature of an air calorimeter much more than that of a 

 water calorimeter of the same dimensions ; and the loss of heat to 

 the surroundings being proportional to the elevation of temperature, 

 in the water calorimeter the chief part of the heat is actually retained 

 in the water, while in an air calorimeter the greater portion passes 

 through the air space, and is radiated away. When the amount of 

 heat lost by the calorimeter becomes equal to that gained from the 

 animal, the ' steady ' reading of the instrument is taken, and from 

 this the heat production can be deduced by an experimental 



