SOURCES OF TERRESTRIAL ENERGY 



cause the rhythmic rise and fall of the piston, 

 and drive the running-gear of the machine 

 shop or propel the railway train. In such wise 

 it may be shown that the various agencies which 

 man makes subservient to industrial purposes 

 are nothing but variously differentiated sun- 

 beams. The windmill is driven by atmospheric 

 currents which the sun set in motion. The 

 water-wheel is kept whirling by streams raised 

 by the sun to the heights from which they are 

 rushing down. And the steam-engine derives 

 its energy from modern or from ancient sun- 

 beams, according as its fires are fed by wood or 

 by coal. 



But the solar energy stored up by vegetables 

 is given out not only in such mechanical pro- 

 cesses, but also in the vital activities of the hu- 

 man beings whose needs such processes supply. 

 The absolute dependence of animal upon vegetal 

 life is illustrated in the familiar fact that animals 

 cannot directly assimilate inorganic compounds. 

 The inorganic water which we drink is necessary 

 to the maintenance of life ; but it percolates 

 untransformed through the tissues and blood- 

 vessels, and it quits the organism in the same 

 chemical condition in which it entered it. And 

 although minute quantities of the salt which we 

 daily eat, and of the carbonates and iodides of 

 iron which we sometimes take as tonics, may 

 perhaps undergo transformation in the tissues, 



331 



