804 CALORIMETERS. [BOOK n. 



calculating the heat produced by the whole body, are subject 

 to many sources of error. 



The calorimeters usually employed in chemical operations, in 

 measuring for instance the heat given out in chemical changes, are 

 unsuitable for experiments on living animals. Such are the mercury- 

 calorimeter, in which the chemical action to be studied is made to take 

 place in the midst of a mass of mercury, from the consequent expansion 

 of which through the heat taken up the amount of heat given out is 

 calculated, or the ice-calorimeter in which in a similar way the heat 

 given out is calculated from the amount of ice melted. The latter has 

 been used for physiological purposes, but an animal surrounded by ice 

 is under such abnormal conditions that the results are of little value. 

 The methods usually adopted by physiologists are as follows. 



In one method, the water-calorimeter, the animal is placed in a 

 metal chamber surrounded by a jacket filled with water. The heat 

 given out by the animal warms the water in the jacket, and the amount 

 given out is calculated upon the increase of the temperature of the 

 water. By supplying the animal with air through a long spiral tube 

 passing through the water-jacket, the heat given out in the expired air 

 is prevented from being lost. 



This method may be employed in a simpler form, when the heat 

 given out by a part of the body, the arm or leg for instance, is all that 

 has to be determined. The part is then merely placed in a bath of 

 water, from the changes of temperature of which the amount given out 

 is calculated. And this modification of the method may with due 

 precautions be employed for the whole body. 



In Rosenthal's calorimeter the chamber in which the body or part 

 of the body is placed is surrounded by, not a water-jacket, but an air- 

 jacket, which thus serves as an air-calorimeter. The instrument 

 consists essentially of three concentric copper cylinders ; the inner one 

 contains the animal (or other source of heat) ; the outer one serves 

 merely as a casing to protect those inside from changes of temperature 

 due to currents of air and the like ; and the middle one encloses an air 

 space between itself and the inner one. There are special arrange- 

 ments for closing the cylinders after the introduction of the animal, 

 and for supplying the animal with air for breathing purposes. With 

 the air-jacket, or space between the inner and middle cylinders, are 

 connected a manometer and a thermometer. When an animal (or 

 other source of heat) is placed in the inner cylinder, the temperature 

 and the pressure of the air in the air-jacket are increased ; and from 

 the amounts of increase measured by the thermometer and the mano- 

 meter the amount of heat given out from the animal is calculated. 



The calorimeters of D'Arsonval and Rubner are constructed on 

 very similar principles. 



Various attempts have been made to ascertain the amount of 

 heat given out by the body in an indirect manner, as for instance 

 by calculating the heat given out by the oxidation of the food. 

 As trustworthy as any is the plan of simply subtracting the 

 normal daily mechanical expenditure from the normal daily 

 income. Thus, 150,000 k.-m. subtracted from one million k.-m. 



