494 NUTRITION. 



action, unaccompanied by any muscular manifestations, the most energetic 

 action of the heart or of the bowels, with the slight exceptions mentioned 

 above, the busiest activity of the secreting or metabolic tissues, all these end 

 simply in augmenting the expenditure in the form of heat. 



A normal daily expenditure in the way of mechanical labor can be easily 

 determined by observation. Whether the work take on the form of walk- 

 ing, or of driving a machine, or of any kind of muscular toil, a good day's 

 work may be put down at about 150,000 kilogramme-metres. 



The normal daily expenditure in the way of heat cannot be so readily 

 determined. Direct calorimetric observations on the whole body are attended 

 with so many difficulties, except in the case of small animals, that their value 

 is uncertain ; and observations made by placing a part only of the body, an 

 arm or leg for example, in the calorimeter, and from the data thus gained 

 calculating the heat produced by the whole body, are subject to many ad- 

 ditional 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 by the amount of ice melted. The latter has been used for physio- 

 logical 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 tempera- 

 ture of which the amount given out is calculated. And this modification or the 

 method may with due precautions be employed for the whole body. 



In Rqsenthal'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 

 other one serves merely as a casing to protect those inside from changes of temper- 

 ature 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 introduction of the animal, and for supplying the animal with 

 air for breathing purposes. With the air-jacket, or space between the inner or 

 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 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. 



Various attempts have been made to acertain 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 kilogramme-metres subtracted from 

 one million kilogramme-metres gives 850,000 kilogramme-metres as the daily 



