JOSEPH HENRY, LL.D. 289 



water in films ; on the origin of mechanical power, 

 and the nature of vital force. 



Henry says : 



"The mechanical power exerted by animals is 

 due to the passage of organized matter in the 

 stomach, from an unstable to a stable equilibrium ; 

 or, as it were, from the combustion of the food. It 

 therefore follows that animal power is referable to 

 the same source as that from the combustion of 

 .fuel namely, developed power of the sun's 

 beams. But, according to this view, what is 

 vitality? It is that mysterious principle not 

 mechanical power which determines the form 

 and arranges the atoms of organized matter, em- 

 ploying for this purpose the power which is derived 

 from the food. . . . 



" Suppose a vegetable organism impregnated with 

 a germ (a potato, for instance) is planted below the 

 surface of the ground, in damp soil, under a temper- 

 ature sufficient for vegetation. If we examine it 

 from time to time, we find it sending down rootlets 

 into the earth, and stems and leaves upward into 

 the air. After the leaves have been fully expanded 

 we shall find the tuber entirely exhausted, nothing 

 but a skin remaining. The same effect will take 

 place if the potato be placed in a warm cellar ; it 

 will continue to grow until all the starch and 

 gluten are exhausted, when it will cease to in- 

 crease. If, however, we now place it in the light, 

 it will commence to grow again, and increase in 

 size and weight. If we weigh the potato previous 



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