CH. XL.] 



CALORlMEtRY 



601 



in the amount of moisture in the atmosphere, and in the amount of 

 work done, would considerably alter the above figures. 



Calorimetry. Calorimeters employed in chemical operations are 

 not suitable for experiments on living animals. An animal sur- 

 rounded by ice or mercury, the melting and expansion of which 

 respectively are measures of the amount of heat evolved, would be 

 under such abnormal conditions that the results would be valueless. 

 Lavoisier, however, used an ice calorimeter in his experiments on 

 animals. 



The apparatus often employed is the water calorimeter. This 

 was first used by Crawford (1788). Dulong's instrument is shown in 

 fig. 443. The animal is placed in a metal chamber, surrounded by a 



Fin. 443. Dulong's Calorimeter : C, calorimeter, consisting of a vessel of cold water in which the 

 chamber holding the animal is placed ; G', gasometer from which air is expelled by a stream of 

 water. The air enters the respiratory chamber. G, gasometer receiving the gases of expiration 

 and the excess of air. t, t', thermometers ; a, a wheel for agitating the water. Observe the 

 delivery-tube on the left is much twisted in the water-chamber, so as to give off its heat to the 

 surrounding water. (From McKendrick's "Physiology.") 



water-jacket. There are also tubes for the entrance and exit of the 

 inspired and expired gases respectively. The heat given out by the 

 animal warms the water in the jacket, and is measured by the rise 

 of temperature observed in the water, of which the volume is also 

 known. The air which passes out from the chamber goes through a 

 long spiral tube, passing through the water-jacket, and thus the heat 

 is abstracted from it and measured. 



Air-Calorimeters are now, however, generally used. Fig. 444 is an 

 outline sketch of the one which has been most used in this country. 



It consists of two precisely similar chambers made of thin sheet 

 copper. Each chamber has two walls between which is an air space ; 

 and the outer is covered by a thick casing of felt (F) to prevent 

 fluctuations in the temperature of the surroundings from affecting 

 the air in the air-space. The chambers are made perfectly air-tight, 

 except for the ventilating tubes AA, A 'A'. By means of these, the 



