CHAP, v.] NUTRITION. 841 



A normal daily expenditure in the way of mechanical labour 

 can be easily determined by observation. Whether the work take 

 on the form of walking, 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-meters. 



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

 readily determined. Direct calorimetric observations on living 

 organisms are in all cases attended with many difficulties, and 

 subject to many sources of error. These are very great when the 

 observations are made on the whole body, even in the case of small 

 animals ; 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 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 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, riot 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, 



F. ii. 54 



