HERMANN VON HELMHOLTZ 295 



there would remain only a single motion of water, namely, the tides, 

 which are produced by the attraction of the sun and moon. 



How is it now, with the motions and the work of organic beings? 

 To the builders of the automata of the last century, men and animals 

 appeared as clockwork which was never wound up, and created the 

 force which they exerted out of nothing. They did not know how to 

 establish a connection between the nutriment consumed and the work 

 generated. Since, however, we have learned to discern in the steam- 

 engine this origin of mechanical force, we must inquire whether some- 

 thing similar does not hold good with regard to men. Indeed, the 

 continuation of life is dependent on the consumption of nutritive ma- 

 terials : these are combustible substances, which, after digestion and 

 being passed into the blood, actually undergo a slow combustion, and 

 finally enter into almost the same combinations with the oxygen of 

 the atmosphere that are produced in an open fire. As the quantity of 

 heat generated by combustion is independent of the duration of the 

 combustion and the steps in which it occurs, we can calculate from 

 the mass of the consumed material how much heat, or its equivalent 

 work is thereby generated in an animal body. Unfortunately, the 

 difficulty of the experiments is still very great ; but within those limits 

 of accuracy which have been as yet attainable, the experiments show 

 that the heat generated in the animal body corresponds to the amount 

 which would be generated by the chemical processes. The animal 

 body therefore does not dififer from the steam-engine, as regards the 

 manner in which it obtains heat and force, but does differ from it 

 in the manner in which the force gained is to be made use of. The 

 body is, besides, more limited than the machine in the choice of its 

 fuel ; the latter could be heated with sugar, with starch-flour, and 

 butter, just as well as with coal or wood; the animal body must dis- 

 solve its materials artificially, and distribute them through its sys- 

 tem ; it must, further, perpetually renew the used-up materials of its 

 organs, and as it cannot itself create the matter necessary for this, 

 the matter must come from without. Liebig was the first to point 

 out these various uses of the consumed nutriment. As material for 

 the perpetual renewal of the body, it seems that certain definite albumi- 

 nous substances which appear in plants, and form the chief mass 

 of the animal body, can alone be used. They form only a portion 

 of the mass of nutriment taken daily; the remainder, sugar, starch, 



