364 FRAGMENTS OF SCIENCE 



among machines ? When I lift a weight, or throw a stone, 

 or climb a mountain, or wrestle with my comrade, am I 

 not conscious of actually creating and expending force? 

 Let us look at the antecedents of this force. We derive 

 the muscle and fat of our bodies from what we eat. Ani- 

 mal heat you know to be due to the slow combustion of 

 this fuel. My arm is now inactive, and the ordinary slow 

 combustion of my blood and tissue is going on. For every 

 grain of fuel thus burned a perfectly definite amount of 

 heat has been produced. I now contract my biceps muscle 

 without causing it to perform external work. The com- 

 bustion is quickened, and the heat is increased; this addi- 

 tional heat being liberated in the muscle itself. I lay hold 

 of a 56-lb. weight, and by the contraction of my biceps 

 lift it through the vertical space of a foot. The blood and 

 tissue consumed during this contraction have not devel- 

 oped in the muscle their due amount of heat. A quantity 

 of heat is at this moment missing in my muscle which 

 would raise the temperature of an ounce of water some- 

 what more than one degree Fahrenheit. I liberate the 

 weight: it falls to the earth, and by its collision generates 

 the precise amount of heat missing in the muscle. My 

 muscular heat is thus transferred from its local hearth to 

 external space. The fuel is consumed in my body, but 

 the heat of combustion is produced outside my body. 

 The case is substantially the same as that of the Voltaic 

 battery when it performs external work, or produces ex- 

 ternal heat. All this points to the conclusion that the 

 force we employ in muscular exertion is the force of burn- 

 ing fuel and not of creative will. In the light of these 

 facts the body is seen to be as incapable of generating 

 energy without expenditure, as the solids and liquids of 



