REVIEW OF PRECEDING THEORIES. 201 



congelation, where, owing to the absence of all vegetation, no ani. 

 mal, not even a bird, can now find nourishment. 



Cheese must be formed from the plants upon which cows feed. 

 The meadows of Holland must, of course, obtain their nitrogen 

 from the air. The milch cows in Holland remain on the fields 

 both day and night; all the salts contained in their fodder 

 must remain upon the fields in the form of urine and of solid 

 excrements, a small quantity proportionately being removed in 

 the cheese. 



The condition of fertility of these meadows can change as little 

 as that of our fields, which, although not grazed upon, receive, in 

 the form of manure, the greatest part of the ingredients removed 

 from them. 



In the cheese districts of Holland, these ingredients remain on 

 the meadows ; while in our system of farming, they are collected 

 at home, and carried, from time to time, to our fields. The ni- 

 trogen of the urine, and that of the solid excrements of the cow, 

 are obtained in Holland from the air ; and from the same source 

 must be obtained the quantity of that element contained in all 

 the kinds of cheese prepared in Holland, Switzerland, and other 

 countries. 



The meadows in Holland, for centuries, have produced millions 

 of cwts. of cheese : there are annually exported from this country 

 thousands of cwts. of this substance ; and yet this exportation 

 does not in any way diminish the productiveness of their meadows, 

 although they have never received from the hand of man more 

 nitrogen than they originally contained. 



Hence it is quite certain, that in our fields, the amount of nitro- 

 gen in the crops is not at all in proportion to the quantity supplied 

 in the manure, and that the soil cannot be exhausted by the ex- 

 portation of products containing nitrogen (unless these products 

 contain at the same time a large amount of mineral ingredients), 

 because the nitrogen of vegetation is furnished by the atmosphere, 

 and not by the soil. Hence also we cannot augment the fertility 

 of our fields, or their powers of production, by supplying them 

 with manures rich in nitrogen, or with ammoniacal salts alone. 

 The crops on a field diminish or increase in exact proportion to' 

 10* ?.•- • i 



