CHAP, v.] NUTRITION. 635 



rimeter, and from the data thus gained calculating the heat 

 produced by the whole body, are subject to additional sources 

 of error. Improved methods, however, especially of recent 

 years, have so far eliminated many sources of error that the 

 results obtained by observations on the whole body may be 

 received with increasing confidence. 



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 mer- 

 cury-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 instru- 

 ment 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 arrangements for closing the cylinders after the intro- 

 duction 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 thermome- 

 ter. 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 manometer the amount of heat given 

 out from the animal is calculated. 



The calorimeters of D'Arsonval and Rubner are constructed on 

 very similar principles. 



