332 A TEXTBOOK OF PHYSIOLOGY 



form of air calorimeter. In Fig. 186 is shown a self-registering appar- 

 atus, suitable for the study of small animals. It consists of a Dewar's 

 flask, with the thermo-electric junctions upon the inlet and exit water 

 tubes. These junctions are connected with a galvanometer, the 

 deflection of which can be registered. 



The direct method is especially applicable to man, since the amount 

 of work done can be measured, and the amount of heat lost and the 

 respiratory exchange calculated, at the same time. Special double- 

 walled calorimeters adequately furnished with bed, chairs, etc., and 

 devices for performing mechanical work, have been built in some 

 laboratories. Within the calorimeter is a coil of water-pipes, fitted with 

 special metal discs, which quickly takes up any heat liberated in the 

 chamber. Cool water is made to flow through this coil and the 

 volume and temperature of the water entering and leaving it is 

 accurately measured, and thus the heat output is calculated. Almost 

 the whole of the heat given off by the body is taken up by the 

 circulating water, for any loss of heat to the outside air is checked 

 by maintaining equality of temperature between the inner and outer 

 walls. This is insured by means of (1) thermo-electric couples con- 

 nected with a galvanometer which detects any inequality ; (2) electric 

 furnaces; (3) water coils placed between the outer and inner walls, 

 by means of which any difference in temperature is compensated. 

 Adequate ventilation is maintained by some form of pump, and the 

 carbon dioxide output and oxygen intake obtained after the fashion 

 already described for determining the respiratory exchange; pre- 

 cautions have to be taken to secure the measurement of heat given 

 off in the ventilation air (Fig. 187). 



For the performance of mechanical work a bicycle is used; the 

 hind-wheel is replaced by a metal disc which revolves against a 

 strap, the tension of which is measured by a spring balance. By 

 means of friction the work done is thus converted into heat. The 

 revolutions of the bicycle and the force required to turn the bicycle 

 can be measured, and the work calculated from these data. 



How accurately such appliances can work is shown by the following: 

 In several series of experiments extending over forty-five days, the 

 measurement of the amount of heat produced by the animal in the 

 calorimeter equalled 99-53 per cent, of the heat calculated to have 

 arisen from the combustion of food and body tissues. 



The following is an example of a five-day experiment on a fasting 

 dog of 4-5 kilos weight : 



Calculated indirectly from the nitrogen and carbon ex- 

 creted = 259-3. 

 Calculated directly by the calorimeter met hod = 261-0. 



