THE USE AND ABUSE OF FOOD. 335 



work upon, its energies are wasted or used to the injury of 

 the machine itself. The supply of water, and its continual 

 use (in the form of steam) in the propulsion of the engine, 

 are the processes corresponding to the continual exhaustion 

 and renewal of the muscles and nerves of the human frame, 

 And the comparison may be carried yet further. We see 

 that in the case of the engine the amount of smoke, or 

 rather of carbonic acid, thrown out by the blast-pipe is a 

 measure of the vital energy (so to speak) within the engine ; 

 but the amount of work done by the engine is measured 

 rather by the quantity of steam which is thrown out, because 

 the elastic force of every particle of steam has been exerted 

 in the propulsion of the engine before being thrown out 

 through the blast-pipe. In a manner precisely correspond- 

 ing to this, the amount of carbonic acid gas exhaled by a 

 man is a measure of the rate at which mere existence is pro- 

 ceeding; but the amount of work, mental or muscular, 

 which the man achieves, is measured by the amount of used- 

 up brain-material and muscle-material which is daily thrown 

 off by the body. I shall presently show in what way this 

 amount is estimated. 



In the composition of the muscles there is a material 

 called fibrine, and in the composition of the nerves there is 

 a material called albumen. These are the substances * which 

 are exhausted during mental and bodily labour, and which 

 have to be renewed if we are to continue working with our 

 head or with our hands. Nay more, life itself involves 

 work ; the heart, the lungs, the liver, each internal organ of 

 the body, performs its share of work, just as a certain pro- 

 portion of the power of a steam-engine is expended in merely 

 moving the machinery which sets it in action. If the waste 

 of material involved in this form of work is not compensated 



* Fibrine and albumen are identical in composition. Caseine, 

 which is the coagulable portion of milk, is composed in the same man- 

 ner. The chief distinction between the three substances consists in 

 their mode of coagulation ; fibrine coagulating spontaneously, albumen 

 under the action of heat, and caseine by the action of acetic acid. 



