USE OF BURNED CLAY AS A MANURE. 55 



much ammonia in the soil as 0250 lbs. of horse's urine* would 

 yield to it, even on the supposition that all the nitrogen of the 

 urea and hippuric acid were absorbed by the plants without the 

 smallest loss, in the form of carbonate of ammonia. If we 

 furnish to a field 40 lbs. of gypsum, and if we suppose that the 

 tenth part of this enters into plants in the form of sulphate of 

 ammonia, we would actually supply nitrogen sufficient for 100 

 lbs. of hay, 50 lbs. of wheat, or 60 lbs. of clover. 



Water is absolutely necessary to effect the decomposition of 

 the gypsum, on account of its difficult solubility (1 part of gyp- 

 sum requires 400 parts of water for solution), and also to assist 

 in the absorption of the sulphate of ammonia by the plants : 

 hence it happens, that the influence of gypsum is not observable 

 on dry fields and meadows ; whi ? e the gaseous carbonate of 

 ammonia formed by the decay of animal manures on such fields, 

 on the other hand, does not fail in producing a favorable effect. 



The decomposition of gypsum by carbonate of ammonia does 

 not take place instantaneously ; on the contrary, it proceeds very 

 gradually ; and this explains why the action of the gypsum lasts 

 for several years. 



The well-known advantage derived by manuring fields with 

 burnt clay, and the fertility of ferruginous soils, may be ex- 

 plained in an equally simple manner. The favorable effects 

 produced by these causes have been ascribed to the great attrac- 

 tion for water exerted by dry clay and ferruginous earth ; but 

 common dry arable land possesses this property in as great a 

 degree ; and besides, what influence can be ascribed to a hun- 

 dred pounds of water spread over a field, in a condition in which 

 it cannot be made available either by the roots or leaves 1 The 

 true cause is this : — 



Peroxide of iron and alumina are distinguished from all other 

 metallic oxides by their power of forming solid compounds with 



* The urine of the horse contains, according to Fourcroy and Vauquelin, 

 in 100) parts, 



Urea . . . 7 parts. 



Hippurate of soda . 14 " 



Salts and water . 979 " 



1 000 parts. ( See Appendix. ) 



