634 



THE CONSERVATION OF ENERGY 



[CH. XLII. 



an electro-magnet by the turning of the pedals, which thus enables 

 the rider to perform a measurable quantity of mechanical work. 



The calorimeter is also supplied with a bed, a table, a chair, and 

 a double window, through which food of known weight and com- 

 position can be supplied, so that an experiment may continue over 

 two or three days, and the effect of work, sleep, various diets, etc., 

 can be studied. 



Of the heat produced in the body, it is estimated by Helmholtz 

 that about 7 per cent, is represented by external mechanical work, 

 and that of the remainder about four-fifths are discharged by radia- 

 tion, conduction, and evaporation from the skin, and the remaining 

 fifth by the lungs and excreta. This is only an average estimate, 

 subject to much variation, especially in the amount of work done. 



The following table exhibits the relation between the production 

 and discharge of energy in twenty-four hours in the human organism 

 at rest, estimated in calories.* The table conveniently takes the form 

 of a balance-sheet in which production and discharge of heat are com- 

 pared ; to keep the body- temperature normal these must be equal. 

 The basis of the table in the left-hand (income) side is the same as 

 Voit's diet (see p. 480) : 



Production of heat. 



Metabolism of 



Protein (120 gr.) . 120 x 4000 = 

 Fat(100gr.) . 100x9400 = 

 Carbohydrates -. , 

 ( = 333 gr. starch)) ' 



Calories. 

 480,000 

 940,000 



** 9sn 

 ,385,280 



2,805,280 



Discharge of heat. 



Warming water in food, 



2 -6 kilos x 25 C. = 65,000 

 Warming air in respiration, 



16 kilos x 25 x 0-24= 96,000 

 Evaporation in lungs, 



630 gr. x582= 366,660 

 Radiation, evaporation, etc., 

 at surface, plus the thermal 

 equivalent of mechanical 

 work done accounts for the 

 remainder. . . . 2,277,620 



2,805,280 



The figures under the heading Production are obtained by multi- 

 plying the weight of food by its physiological heat-value. The 

 figures on the other side of the balance-sheet are obtained as follows : 

 The water in the food is reckoned as weighing 2 '6 kilos. This is 

 supposed to be at the temperature of the air, taken as 12 C. ; it has 

 to be raised to the temperature of the body, 37 C., that is, through 

 25 C. Hence the weight of water multiplied by 25 gives the number 

 of calories expended in heating it. The weight of air is taken as 



* The calorie we are taking is sometimes called the small calorie ; by some the 

 word calorie is used to denote the amount of heat necessary to raise 1 kilogramme 

 of water 1 C. This is called the large calorie. 



