472 



'ron and alumina are distinguished from all other metallic oxides by 

 their power of forming solid compounds with ammonia. Minerals con- 

 taining alumina or oxide of iron also possess in an eminent degree the 

 remarkable property of attracting ammonia from the atmosphere and 

 of retaining it." — p. 144. 



Powdered chai'coal is another element which powerfully absorbs am- 

 monia ; and will take up ninety times its volume of ammoniacal gas, 

 which it gives out upon being wet with water. Decayed wpod resembles 

 charcoal in this property, absorbing seventy-two times its own volume. 

 This explains further the operation of humus, which supplies not only 

 carbonic acid, but likewise nitrogen, to the growing plants. 



Liebig concludes this chapter with a beautiful reflection : 



" Carbonic acid, water and ammonia contain the elements necessary 

 for the support of animals and vegetables. The same substances are 

 the ultimate products of the chemical process of decay and putrifaction. 

 All the innumerable products of vitality resume, after death, the origin- 

 al form from which they sprang. And thus death, — the complete disso- 

 lution of an existing generation, — becomes the source of life for a new 

 one." — p. 147. 



The next subject of discussion with Liebig relates to the inorganic 

 constituents of plants. These are potash, soda, lime, magnesia, oxide 

 of iron, manganese, silica, and other substances. The plants cannot be 

 perfected without them. Alkalies of one kind may often be substituted 

 for those of another ; but they are always found in equivalent propor- 

 tions. These inorganic substances are admitted to the plants in combi- 

 nation with some acid. They exist independently of the plant, and are 

 not the product of vital action. They are found in different soils, and 

 are the result of the decomposition of various rocks. Potash is an im- 

 portant constituent of most felspars. Some of the salts are evaporated 

 in sea-water, and in that way carried far into the interior, and after be- 

 ing spread upon the earth, are carried down by the rains. They are 

 returned to the soil in decayed vegetable and animal matter, and in the 

 excrements of animals. They are found in the ashes of plants, in the 

 form of carbonates ; and by careful analysis their amounts in different 

 cases have been accurately ascertained. The amount of alkaline sub- 

 stances required by plants is very minute. But that amount is requisite 

 to the perfection of the vegetation. It is easy to conceive how small an 

 amount is required in the soil, when it is understood that sea-water con- 

 tains only 1-12400 of its weight of carbonate of lime, and yet that is 



