GENERAL METABOLISM. 



625 



the body. The amount of heat energy required for its evaporation must be 

 added to the amount of heat energy carried away by the water current. 

 Atwater measured the heat equivalent of the external work performed in 

 a very interesting manner. He provided a bicycle which was connected 

 with a dynamo. The electric current produced was led through an incan- 

 descent lamp, and there transformed into heat energy, which can be directly 

 measured as such. From the duration of the experiment, and the amount of 

 electric current produced, the amount of work performed can be estimated. 



According to this method of investigation, the kinetic energy of the 

 body is computed from three factors. First, there is the total amount of 

 heat carried away as heat, including that carried away by the air current ; 

 second, the latent heat of the water vapor which is carried away from the 

 body ; and third, there is the heat equivalent of the external work. The 

 first amount of heat has several sources. It comprises the heat given off 

 by radiation and conduction from the skin, that obtained from the excreta 

 on their being cooled to the room temperature, and further there is 

 the cooling of the expired air (carbon dioxide and water vapor) to the 

 room temperature. 



By means of such an apparatus not only the influence of different kinds 

 of nourishment upon metabolism can be determined exactly, but also 

 their relations to the external work. As we have already seen, 1 it is by 

 the help of such experiments that we have been able to prove that the 

 fats, in order to be utilized for muscular work, need not necessarily be 

 transformed into carbohydrate, but that their calorific value may be 

 applied directly. 



See Lecture XV, p. 338 et seq. 



