SALTS, HUMUS, DECAY. 15 



They have been named Salts, from their resemblance 

 to common table salt, though their properties are usually 

 very different. 



48. Carbonic acid, for example, intimately combined 

 with lime, forms a salt called carbonate of lime, which 

 is chalk or limestone. Sulphuric acid, combined with 

 lime, forms sulphate of lime, or plaster of Paris. Nitric 

 acid, chemically combined with potash, forms nitrate of 

 potash, or saltpetre. All these are salts of great impor 

 tance in agriculture. 



49. Oxygen is also continually combining with wood 

 and other vegetable substances. The decay of the fallen 

 leaves is produced by oxygen slowly combining with the 

 carbon of the leaves. Moisture and warmth are favorable 

 to this combination, or oxidation, and heat is always pro 

 duced by it. A heap of leaves, decaying, grows warm 

 and continues warm till they are all turned into leaf 

 mould, geine or humus. So the very gradual decay 

 of trunks of old dead trees, and of every thing made of 

 wood, is principally owing to the combination of oxygen 

 with the carbon in the wood. 



Nearly all decay is produced by oxygen. It is oxida 

 tion. During the process of decay of vegetable substances, 

 not only carbonic acid, but, previously, humic acid, (from 

 humus, earth,) and ulmic acid, (from ulmus, an elm,) 

 are formed. Both these are made of carbon, hydrogen 

 and oxygen, and both are elements of the food of plants. 



50. Humus, or geine, in all its states, is a compound 

 of carbon, with the elements of water, oxygen and hydro 

 gen. When decay has just begun, the decaying substance 

 is called ulmin; with a little more oxygen, it becomes 

 ulmic acid. In both these, there is more hydrogen than 

 is necessary to form, with the oxygen, water. 



