396 THE CONNECTION AND EQUIVALENCE OF FORCES. 



Without the light of the sun plants cannot grow. The 

 living germ, the green leaf, owe to the sun their power of 

 transforming earthy elements into living, vigorous structures. 

 The germ may, indeed, be evolved under ground without the 

 action of light, but only when it breaks through the surface 

 of the soil does it first acquire the power, by the sun's rays, 

 of converting inorganic elements into its own structure. The 

 illuminating and heating rays of the sun, in thus bestowing 

 life, lose their own light and heat. Their power now becomes 

 latent in the new products of the frame, which have been 

 produced under their influence from carbonic acid, water, and 

 ammonia. The light and heat with which our dwellings are 

 illuminated and warmed are but those bestowed by the sun. 



The food of men and animals consists of two classes of 

 materials, which differ totally in their nature. One class is 

 destined to the production of blood and the maintenance of 

 the structure of the body ; the other is similar in composition 

 to ordinary materials for combustion. Sugar, starch, the 

 gum of bread, may be regarded as transformed woody fibre, 

 for we can prepare them from this substance. Fat, in its 

 amount of carbon, resembles closely mineral coal. We heat 

 our bodies as we do stoves, by combustibles which possess the 

 same elements as wood and coal, but which differ essentially 

 from them by being soluble in the fluids of the body. 



The elements of nutrition from which the temperature of 

 the body is derived, evidently produce no mechanical power ; 

 because power is but converted heat, and the heat which 

 maintains and elevates the temperature of the body does not 

 produce any other effect than that of warmth. 



All those mechanical operations constantly taking place in 

 the living body, in the movement of organs and limbs, are 

 dependent on an accompanying change in the composition 

 and properties of those highly complex sulphur and nitrogen 

 constituents of the muscles, which, though furnished by the 

 blood, are in the first instance derived directly from the food 



