THE ENERGY EQUATION. 



liberated and external work done. In this connection it may 

 be well to recall some relations first pointed out by Hirn, in 

 which a comparison is drawn between the work of man and 

 an engine. In both cases the work is accomplished through 

 the expenditure of the potential energy stored up in carbonace- 

 ous substances, food in the one instance, coal in the other. As 

 the illustrations above show the heat from the food is prac- 

 tically constant, whether it be evolved through oxidation in 

 the body or in a calorimeter. Imagine now a small steam 

 engine burning a constant amount of coal inside a calorimeter. 

 In one case let no work be done by the piston ; the heat of the 

 steam is not employed in expansion, but is totally absorbed by 

 the water of the calorimeter, which takes up a certain number 

 of calories that may be accurately noted. In a second case 

 allow the same amount of coal to be burned under the boiler 

 of the small engine in the same time, but let the engine do 

 work outside the calorimeter which may be accomplished, for 

 example, by means of a small shaft passing through the walls 

 of the calorimeter in such a way as to convey no appreciable 

 amount of heat. It will now be found that the gain in tem- 

 perature in the water of the calorimeter is less than before 

 for the same coal consumption, and that the difference is meas- 

 ured by the external work alone. One calorie less in the 

 calorimeter heat corresponds to 423.5 gram-meters of work 

 done through the agency of the shaft. 



\Yith an animal the case is different. The doing of exter- 

 nal work necessitates always the burning of more food than is 

 the case with the fasting metabolism, when the energy require- 

 ment is for doing internal work, as will be explained below. 

 In the engine the amount of coal burned may be constant 

 whether work is done or not. However, with the animal this 

 result is noticed : While at rest assume that for every liter of 

 oxygen absorbed 5 large calories of heat are liberated; it does 

 not follow that when work is done and 2 liters of oxygen are 

 absorbed in the same time that now 10 Cal. of heat will be 

 liberated. From the 10 Cal. we must subtracMheTieat 



