SOILS AND MANUKES. 357 



the surface soil, a few drops of hydrochloric acid should be 

 poured upon a specimen of it; if this produces an efferves- 

 cence, it is a sure sign of the presence of carbonate of 

 lime, and the earth probably belongs to the useful kinds 

 of marl, which may then readily be ascertained more ex- 

 actly by a chemical examination." 



497. Volatile Substances in Manures. There are some 

 valuable substances in some manures which are volatile, 

 and the skill of the farmer is called in requisition to pre- 

 vent their flying off, or to fix them, as it is expressed. If 

 he carelessly leave his manure heaps to putrefaction, he 

 will lose a large part of some of their most valuable mate- 

 rial. He will lose, for example, much of the ammonia. It 

 will pass off into the air, and so will be lost to him, though 

 it will not be lost to the earth, for it will be brought down 

 by the rain. By losing it he will unwittingly benefit oth- 

 er farmers over a wide extent of territory, for the volatile 

 matter will be largely diffused. There are means of fixing 

 the ammonia, which are applied sometimes in the manure 

 heap, and sometimes in the field with the manure as it is 

 scattered. These means will be noticed hereafter. 



498. Animal Manures. These are of two kinds the sub- 

 stances composing the body of the animal, and the excre- 

 tions. They are generally the most valuable manures that 

 we have, for they contain, besides other ingredients, a con- 

 siderable amount of that very important element, nitrogen. 

 The excretions of different animals vary much according 

 to the kinds of food upon which they live. This is of 

 course to be taken into consideration by the farmer in the 

 application of these manures, and in the mixture of other 

 manures with them. 



499. Guano. This is the manure of sea-birds, which has 

 been accumulated during a long period of time in deep 

 layers upon uninhabited islands and rocks. There are im- 



