20 THE SCIENTIFIC PRINCIPLES 



direct experimental evidence bearing upon the 

 rationale of rotation has been acquired, and it 

 will be my duty to lay it before you in some 

 detail. 



Treating of manure, Liebig laid the greatest stress 

 on the return by it of the potass and the phosphates 

 removed by the crops. But, in his first edition, he 

 also insisted on the importance of the nitrogen, 

 especially that in the liquid excretions of animals, 

 and condemned the methods of treatment of animal 

 manures by which the ammonia was allowed to be 

 lost by evaporation. It is curious, and significant, 

 however, that some of the passages in that edition, 

 in which he the most forcibly urges the value of the 

 nitrogen of animal manures, are omitted in the third 

 and fourth editions. 



In his second work, that on Animal Chemistry, 

 published in 1842, Liebig discussed two subjects of 

 much interest and practical importance to the agri- 

 culturist : — namely, the sources in the food of the fat 

 stored up in the animal body, and the characteristic 

 food requirements of the animal organism induced by 

 the exercise of force. 



To render the points here at issue intelligible, it is 

 necessary to remind you that the constituents of 

 food, both vegetable and animal, may broadly be 

 classed into those containing nitrogen, and those not 

 containing it — in other words, into the nitrogenous, 

 and the non-nitrogenous constituents. 



From the nitrogenous constituents of the food, the 

 nitrogenous constituents of the animal body — the 

 membranes and cellular tissue, the nerves and brain, 

 cartilage, and the organic part of bones — must be 

 derived. It is admitted that by oxidation and 

 transformation within the bodv, some of these nitro- 



