42 SCIENTIFIC AGRICULTURE. 



part of the weight of water : it does not occur in combination 

 witli any of the mineral masses of the globe, except coal, and 

 this is itself of vegetable origin. This gas burns with a pale 

 yellow flame, its combustion is attended by the formation of 

 water. 



Plants do not grow in this gas, but gradually wither and 

 die. Its specific gravity is 0.0687: 100 gallons of water 

 absorb 1-i- gallons of this gas. It is the gas used' for inflating 

 balloons. Hydrogen is readily obtained by the action of sul- 

 phuric acid on zinc or iron. It is necessary to the growth of 

 vegetation, but not in a free or uncombined state. The com- 

 bining number of hydrogen is 1. 



CARBON ITS PROPERTIES AND RELATIONS. 



Carbon exists in a pure and crystaline form in the diamond ; 

 graphite, or black lead, and common charcoal, are examples of 

 carbon of impure varieties. It constitutes a large proportion 

 of all animal and vegetable substances: nearly all plants in a 

 dried state, contain from 40 to 50 per cent, by weight, of 

 carbon. This substance is of great importance in the art of 

 culture, on account of its power of absorbing large quantities 

 of the gases and vapors of the atmosphere, this is especially 

 true of charcoal, or carbon in a light and porous form. Char- 

 coal is used for filtering impure water, which it cleanses f om 

 decayed animal or vegetable substances, and coloring matters 

 which are held in solution: it is used also in clarifying syrups 

 and oils: it has the power of absorbing noxious vapors and 

 gases, which result from the decomposition of animal and 

 vegetable matters, and of preventing or retarding the decay 

 of all organic substances. 



The gases and moisture which are arrested and retained by 

 carbon in the soil, are again readily yielded up to the roots of 

 plants, during the process of growth. Several important ends 

 are subserved by carbon in the soil : it purifies impure air and 



