lW» on manure. 



When bread, flesh, potatoes, hay, or oats, are burned in a com- 

 mon fire-place, with an ordinary draught, but perfectly exposed to 

 the entrance of the air, the carbon of these substances is con- 

 verted into carbonic acid, their hydrogen into water, their nitro- 

 gen is set at liberty in the form of ammonia, and their sulphur 

 assumes the form of sulphuric acid, so that at last nothing re- 

 mains except the mineral ingredients of these substances in the 

 form of ashes. In the form of volatile products, we obtain car- 

 bonic acid, carbonate of ammonia, and water, and besides these 

 (if the combustion be incomplete), smoke or soot ; in the incom- 

 bustible residue we obtain all the salts contained in the food. 



When water is poured over these ashes, the alkalies dissolve, 

 and also the soluble phosphates, common salt, and sulphates ; 

 the residue, insoluble in water, contains salts of lime and magne- 

 sia, and silica, if the substance burned contained the latter sub- 

 stance. 



Exactly the same process ensues in the bodies of animals. 

 Through the skin, and by means of the lungs, the carbon and 

 hydrogen of the food are expelled in their final form of carbonic 

 acid and water ; all the nitrogen of the food is collected in the 

 urinary bladder in the form of urea, which by the simple union of 

 the elements of water is converted into carbonate of ammonia. 



When the body regains its original weight, exactly as much 

 carbon, hydrogen, and nitrogen, as it took in the food, must have 

 been expelled from it. It is only in youth, and in the process of 

 fattening, that an increase in weight takes place, and that, there- 

 fore, part of the constituents of the food remains in the body : in 

 old age, on the contrary, the weight decreases, that is, more is 

 separated from the body than enters into it. 



The nitrogen of the food is, therefore, daily expelled by the 

 urine in the form of urea and of compounds of ammonia. The 

 faeces contain the unburned substances of the food, such as the 

 woody fibre, chlorophyl,* and wax, which have suffered no 

 change in the organism ; the carbon, hydrogen, and nitrogen of 

 these substances are very small in quantity, in comparison with 



• Chlorophyl is the green coloring matter of the leaves and other part* 

 of plants. 



