GEOLOGY OF AGRICULTURE. 78 



takes the color and form of yellow cubes, resembling 

 gold, for which reason it has received the name, as be- 

 fore mentioned, of fooVs gold. 



* 



117. Sulpkuret of Hydrogen (HS). — This is a gas, 

 having the fetid smell of spoiled eggs. It cannot be 

 regarded as a permanent ingredient of soils, but in 

 richly-manured lands, it is formed in the soil, and 

 may have something to do with the growth of plants. 



118. Light Carburetted Hydrogen (CH'^). — This is the 

 gas which rises and floats in bubbles on the surface 

 of water, in which vegetable matter is decaying. It is 

 formed also in soils in which there is vegetable matter 

 far below the surface. Vegetable matter decaying in 

 the air, produces carbonic acid ; but when decaying 

 with the exclusion of air, it gives off carburetted hy- 

 drogen. 



119. Heavy carburetted hydrogen (C'H') is not known 

 to possess any relations to agriculture. This is the 

 gas used for purposes of lighting. 



120. Ammonia (NH') (see Table I., 20) is a most 

 valuable, though not a permanent ingredient of soils. 

 In conjunction with carbonic acid, it exists in the air, 

 in exceedingly minute quantities, and rain-water and 

 snow are always impregnated with it. More will be 

 said of its relations to the growth of crops hereafter. 



121. From what has now been stated, it appears 

 that the mineral pai*t of soils is made up essentially 



4 



