CHAP, n.] HORSE-DUNG. 25 



substances which those plants require for food, 

 it is necessary to supply the deficiency by arti- 

 ficial means ; and this is in fact the theory of 

 manures, for any substance may become a 

 manure if its addition is required to render the 

 soil suitable to the plants which are to be grown 

 in it. As, however, certain elements are oftener 

 required by plants than any other, those sub- 

 stances in which these elements abound are so 

 much the most frequently used as manures, 

 that few people think of any others. I shall, 

 however, say a few words of all the kinds in 

 common use, dividing them into the three heads 

 of animal, vegetable, and mineral. 



Animal Manures. — The most common ani- 

 mal manure, and that which is most useful, is 

 undoubtedly horse-dung, as it is not only rich 

 in all the elements required for vegetable food, 

 but it evolves so much heat in decaying, that it 

 can be made to supply an artificial climate, 

 when necessary, to those plants which are na- 

 tives of climates warmer than our own. During 

 the process of decay, it also throws off a 

 quantity of ammoniacal gas, of so caustic 

 a nature as to destroy any plants subjected to its 

 influence. This the gardeners call burning the 

 plants ; and in fact the leaves look black and 

 shrivelled, as though they had been burnt by 

 fire. To avoid the danger of injuring plants in 

 this manner, horse-dun^ - , when procured fresh 

 from a stable, must be laid in a heap, and 

 turned over several times to allow the caustic 

 gases to escape, till its fermentation is suffi- 

 ciently abated to allow of its being safely 

 applied to the plants. As, however, some of 



