346 FRAGMENTS OF SCIENCE. 



amining the battery which works the engine we find 

 that the zinc consumed does not yield its full amount 

 of heat. The quantity of heat thus missing within is 

 the exact thermal equivalent of the mechanical work 

 performed without. Let the water fall again to the 

 lower level; it is warmed by the fall. Add the heat 

 thus produced to that generated by the friction, me- 

 chanical and magnetical, of the engine; we thus obtain 

 the precise amount of heat missing in the battery. All 

 the effects obtained from the machine are thus strictly 

 paid for; this ' payment for results ' being, I would 

 repeat, the inexorable method of nature. 



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 burnt a perfectly definite amount of heat has be'en 

 produced. I now contract my biceps muscle without 

 causing it to perform external work. The combustion 

 is quickened, and the heat is increased; this additional 

 heat being liberated in the muscle itself. I lay hold 

 of a 56 Ib. 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 

 developed in the muscle their due amount of heat. A 

 quantity of heat is at this moment missing in my muscle 



