MUTUAL RELATION OF PLANTS AND ANIMALS. 181 



plants. Hence if an entire animal is burnt to ashes, 

 the residue will differ little from the ashes of beans, 

 lentils, and peas. 



In bread and flesh, therefore, man consumes the 

 ash-constituents of seed, or of seed-constituents which 

 the farmer has obtained from his fields in the form of 

 flesh. 



Of the large amount of mineral substances which 

 man consumes in his food during a lifetime, but a small 

 fraction remains in his body. The body of an adult 

 does not increase in weight from day to day, which 

 proves that all the constituents of his food must com- 

 pletely pass out again from his system. 



Chemical analysis demonstrates that the excrements 

 of man contain the ash-constituents of bread and flesh 

 very nearly in the same quantity as they exist in the 

 food, which in the body undergoes a change similar to 

 that which would take place in a furnace. 



The urine contains the soluble, the solid excrements 

 the insoluble ash-constituents of food : the stinking sub- 

 stances are the smoke and soot of an imperfect combus- 

 tion. "With these are mixed up the undigested and the 

 indigestible remains of food. 



The dung of swine fed on potatoes contains the ash- 

 constituents of the potato ; that of the horse, the ash- 

 constituents of hay and oats ; that of cattle, the ashes 

 of turnips, clover, and other plants which have served 

 them as food. Farm-yard manure comprises a mixture 

 of all these excrements. 



That farm-yard manure will completely restore the 

 fertility of a field exhausted by cultivation is a fact 

 fully established by the experience of a thousand years. 



Farm-yard manure supplies to the field a certain 

 quantity ^of organic, i. e. combustible substances, to- 

 gether with the ash-constituents of the food consumed. 

 We must now consider what part is taken, in the 

 restoration of fertility, by the combustible and incom- 

 bustible constituents of the manure. 



The most superficial examination of a cultivated 

 field shows that all the combustible constituents of the 



