MANURES. 5d 



Although there are various sources from which plants may de- 

 rive their hydrogen, yet water may be considered as the chief one. 

 Of this substance it constitutes two parts in three ; and being the 

 only medium by which they can receive their food, it is constantly 

 passing through their organs, and is there decomposed, furnishing 

 both hydrogen and oxygen to them. Ammonia is a compound of 

 hydrogen and nitrogen, and may have a part to perform in fur- 

 nishing this gas. 



Plants are surrounded with the atmosphere containing oxygen, 

 are constantly supplied with water containing it, and are always 

 absorbing carbonic acid of which it is a constituent ; and therefore 

 no one will be at a loss to find the source from which plants pro- 

 cure this gas. 



Although nitrogen forms a large part of the atmosphere and a 

 very small part of plants, it does not appear that they obtain this 

 substance from this source. 



We have just stated that ammonia consists of hydrogen and ni- 

 trogen, and it is an abundant product of the decay of animal and 

 vegetable substances. This is the substance commonly called 

 hartshorn. It is known to exist in the juices of some plants 

 already formed, and is absorbed in large quantities by water. It 

 exists in the atmosphere in very small quantities at all times, and 

 being washed down by rains, is brought in contact with the roots. 

 Thus passing into the circulation, it is decomposed and its nitrogen 

 appropriated. 



Another source is nitric acid (aquafortis.) This is also one of 

 the products of decaying animals and vegetables. Besides this, it 

 is produced during thimder showers, by the effect of the lightning 

 upon the air, causing its elements to enter into a chemical union, 

 and being absorbed by the rain is brought down and taken up by 

 plants. The effect of some salts of this acid as manures, depends, 

 without doubt, upon thissubstance. 



We have thus examined as fully as our space will allow, the con- 

 stitution of plants, and their relation to the soil. It yet remains, 

 before entering directly upon the subject of manures, to consider 

 some circumstances w^hich modify their action. This will be done 

 in the opening of the next number. 



