OF AGRICULTURE. 11 



produced. He states that its composition varies 

 according to the access of oxygen and water ; that 

 it absorbs oxygen, and gives out carbonic acid, which 

 supplies nourishment to plants ; also, that it yields 

 soluble extractive matter again and again by time 

 and exposure. He further states that the more the 

 decomposition has proceeded, the more refractory, and 

 the less active and useful for vegetation, is the residue. 



With the exception of the discourses of Sir 

 Humphrey Davy, and the writings of Thaer, the 

 subject seems to have received comparatively little 

 attention, nor was any other addition of importance 

 made to our knowledge in regard to it, during a 

 period of more than twenty years, from the date of 

 the appearance of De Saussure's work in 1 804. 



From about 1825 to 1840, Dr. Carl Sprengel, for- 

 merly Professor of Agriculture at Brunswick, pub- 

 lished a series of about thirty papers in connection 

 with agriculture, and agricultural chemistry. These 

 memoirs covered a wide range of subjects, and re- 

 corded the results of numerous investigations by the 

 author himself. Among them may be mentioned in- 

 vestigations on humus, humic acid, and humates ; on 

 the constituents of some surface soils and subsoils ; 

 on the composition of various kinds of straw, and 

 their value as food and litter ; on the amount of 

 potash in granites, and other rocks ; on the ash- 

 constituents of cereal grains, &c. Lastly, he published 

 a general treatise on manures, including a chapter on 

 animal manures, in which he gives the results of the 

 analysis of the solid and liquid excretions of the 

 various animals of the farm, and, among other points, 

 insists upon the value as manure of the ammonia 

 yielded by such materials. 



Professor Schiibler, of Tubingen, also published 



