206 NATURAL HISTORY. 



nutrition solely from the earth which forms the basis and 

 furnishes facilities for the firm planting and outspreading 

 of its roots ; this is the office assigned to it by nature, but 

 nothing more. The manner in which the nutritive mat- 

 ter is received by the plant is as follows : Hydrogen and 

 oxygen are received through the water, which is indis- 

 pensably necessary to maintain its vitality, and, either in 

 the form of rain or vapor, penetrates the earth ; in this a 

 quantity of carbonic acid gas is dissolved, which the 

 plant absorbs. Nitrogen is received partly by the atmos- 

 pheric air, which is a compound of oxygen, nitrogen, and 

 watery vapor, and partly that, during a thunder-storm, 

 much nitric acid is found in the water, which, always 

 holding in solution a greater or less quantity, finds its 

 way to the roots of the plants. In the meantime few 

 plants, subsisting barely on inorganic matter, attain to 

 full, luxuriant growth, but require, in order to a more 

 perfect development, the presence of organic matter, 

 which, in combination with the elementary substances 

 above named, tends to the promotion and improvement 

 of the vegetable classes. This organized matter, well 

 known as manure or compost, is formed of decayed ani- 

 mal or vegetable substances ; and as these decompositions 

 serve for a healthy nourishment to plants, we must infer 

 that the subjects of the vegetable kingdom possess the 

 faculty to perform the important work of purifying the 

 atmosphere of such impure gases as would prove dele- 

 terious to man and beast. It is well known how soon 

 the air of a room in which men or animals are confined 

 becomes impure ; and, remaining unventilated, causes 

 death. This is because the oxygen inhaled with the at- 

 mospheric air is exhausted, and the carbonic acid thrown 

 off in breathing poisons the air, soon making inspiration 



