BCIENCE AND MAN. 605 
No engine, however subtly devised, can evade this law of 
equivalence, or perform on its own account the smallest 
modicum of work. The machine distributes, but it cannot 
create. Is the animal body, then, to be classed 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. Animal 
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 
additional heat being liberated in the muscle itself. I lay 
hold of a 56-lb. weight, and by the contraction of rny biceps 
lift it through the vertical space of a foot. The blood and 
tissue consumed during this contraction have not developed 
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 somewhat 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 mus- 
cular 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 external heat. 
All this points to the conclusion that the force we employ 
in muscular exertion is the force of burning 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 the voltaic bat- 
tery. The body, in other words, falls into the category of 
machines. 
We can do with the body all that we have already done 
with the battery heat platinum wires, decompose water, 
magnetize iron, and deflect a magnetic needle. The com- 
bustion of muscle may be made to produce all these effects, 
as the combustion of zinc may be caused to produce them. 
