THERMOMETRY AND CALORIMETRY 



679 



although in general with less accuracy, from the amount of carbon 

 dioxide given off. With careful technique, the results obtained by 

 indirect agree closely with those obtained by direct calorimetry, which 

 is tantamount to saying that the law of conservation of energy holds 

 in the animal body (Rubner). 



Air calorimeters have sometimes been used for physiological pur- 

 poses, but at present they have little more than historical interest. 

 A diagram of one is shown in Fig. 223. Such calorimeters 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 temperature of a litre of air 

 by i is very small in comparison with that required to raise the tem- 



tx 6 9 f 



(a.) 



Fig. 223. Air Calorimeter: (/.) Cross-Section; (//.), Longi- 

 tudinal 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 corresponding 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. 



perature 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 tem- 

 perature, 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 graduation of 

 the apparatus. The one advantage of an air calorimeter is that it 

 follows more closely rapid variations in th heat-production of the 



