340 APPLICATION OF PRINCIPLES. 



obtained by mixing together equal parts of salt and 

 lime ; or by, supplying the plant with soluble carbon 

 and nitrogen. It is in proportion to their power of 

 furnishing these principles, and to the length of time 

 during which they continue to do so, that manures are 

 active or sluggish, and durable or ephemeral in their 

 operation. Carbonic acid, when decomposed, fur- 

 nishes an essential part of the starch and other sub- 

 stances secreted by plants ; and nitrogen seems, from 

 its great abundance in their system, at least when 

 young, to be indispensable to their existence : the 

 first is a fact of universal notoriety, the latter has been 

 ascertained by modern chemists to be also apparently 

 a constant phenomenon, (fiee Introduction to Botany, 

 Sd ed. p. 370, 379, &c, and Appendix.) For these 

 reasons every description of putrefying animal or 

 vegetable matter, from putrid yeast and malt-dust to 

 horses' hoofs and feathers, have been used for the 

 purpose of fertilising land, the nature of whose differ- 

 ent actions constitutes a study of itself, very obscure, 

 but of the highest degree of importance. 



In the more delicate of horticultural operations, 

 liquid manure, prepared by steeping dung in water, 

 and drawing it off when clear ar.i of the colour of 

 porter, is most generally now employed, and is 

 undoubtedly the best form in which it can be admi- 

 nistered, in consequence of its concentration, the faci- 

 lity of its administration in any quantity, and its con- 

 taining nothing but soluble matter. It was first used 

 by Mr. Knight, who not only applied it with great 

 advantage to Fruit trees, but also to the Heaths and 



