ii THE THEKMIC ECONOMY OF THE OEGANISM 61 



the needle of the meter passed the zero. The electric current then acted on 

 an electric magnet which set a pen in motion. 



The principle underlying calorimeters of the second type consists in the 

 method of measuring the temperature, which is not done with the thermometer, 

 but by making use of the expansion of gases ; the variations in volume under 

 constant pressure are directly proportional to the alterations in temperature. 

 Of apparatus of this type we may mention the air calorimeters used by 

 d'Arsonval for man and animals. 



The former, as will be seen by Fig. 10, is made of two metal cylinders, one 

 over the other, containing two cavities : an inner one in which the subject of 

 the experiment is placed, and in which there circulates a current of air in the 

 direction indicated by the arrow ; and an outer one, hermetically closed, 

 containing the quantity of air which serves as the calorimetric medium. 



FIG. 10. D'Arsonval's air calorimeter. 



The variations in the volume of this air, due to the alterations in temperature 

 set up by the source of heat, are registered by the (J -shaped water manometer, 

 of which one limb is in direct communication with the air in the calorimeter 

 and the other with air contained in a large vessel placed in the same room 

 as the calorimeter for the purpose of neutralising the effects of the variations 

 of temperature which are not due to the subject of the experiment. 



The same purpose is served by the arrangement of the calorimeter for 

 animals shown in Fig. 11, in which one limb of the recording manometer is 

 connected with the circular space between the two metal sides of the calori- 

 meter in which the animal is placed, while the other limb communicates 

 with the space of another calorimeter of the same size, but empty. 



It is clear that this arrangement gives a real differential thermometer, 

 which constantly indicates how much higher the internal temperature is 

 than that of the external surroundings. 



Both calorimeters are in direct communication with the air contained in 

 two extremely light metal cylinders, which dip into the water filling the larger 

 outer cylinders. The movable cylinders are connected with the two arms 



