162 ON THE INTERACTION OF NATURAL FORCES. 



raw cold. Further, by the heat of the sun a portion of the 

 water is converted 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 with- 

 drawn, 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 con- 

 tinuation of life is dependent on the consumption of nutritive 

 materials : these are combustible substances, which, after diges- 

 tion 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 inde- 

 pendent of the duration of the combustion and the steps in 

 which it occurs, we can calculate from the mass of the con- 

 sumed material how much heat, or its equivalent work, is 

 thereby generated in an animal body. Unfortunately, the diffi- 

 culty of the experiments is still very great ; but within those 

 limits of accuracy which have been as yet attainable, the ex- 

 periments 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 



