Prof. Helmholtz on the Interaction of Natural Forces. 509 



carried to the coast of Newfoundland and produces raw cold. 

 Further, by the heat of the sun a portion of the water is con- 

 verted into vapour, which rises in the atmosphere, is condensed 

 to clouds, or falls in rain and snow upon the earth, collects in 

 the form of springs, brooks and rivers, and finally reaches the 

 sea again, after having gnawed the rocks, carried away light 

 earth, and thus performed its part in the geologic changes of the 

 earth ; perhaps besides all this it has driven our water-mill upon 

 its way. If the heat of the sun were withdrawn, there would 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 connexion 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 something similar 

 does not hold good with regard to men. Indeed, the continua- 

 tion of life is dependent on the consumption of nutritive mate- 

 rials : these are combustible substances, which, after digestion 

 and being passed into the blood, actually undergo a slow com- 

 bustion, 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 inde- 

 pendent of the duration of the combustion and the steps in which 

 it occurs, we can calculate from the mass of the consumed ma- 

 terial how much heat, or its equivalent work, is thereby generated 

 in an animal body. Unfortunately, the difficulty of the experi- 

 ments 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 differ 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 dissolve its materials artificially, and distri- 

 bute them through its system ; 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 con- 

 sumed nutriment. As material for the perpetual renewal of the 



