CHEMISTRY OF AGRICULTURE. 403 



only source of the carbon of plants, and as carbon constitutes 

 about one-half of the weight of any vegetable matter, it per- 

 forms the most important part in the nutrition of plants. It is 

 an invisible gas, pervading the atmosphere and constituting 

 about y oVo^ P^^'^ ^^ ^^^ weight. Combined with Lime, forming 

 limestone, marble, or chalk, (all carbonates of lime,) and in 

 various other combinations it enters largely into the construc- 

 tion of soils and rocks. The leaves of plants absorb this gas 

 from the atmosphere and readily decompose it, returning its 

 oxygen to the air to reperform its office of uniting with carbon, 

 and appropriating its carbon to its own uses. 



The amount of carbonic acid in the atmosphere is compara- 

 tively very small ; still it is amply sufficient for the demands of 

 plants, and it is constantly being resupplied. Were the formation 

 of this gas to cease, the stock on hand would last our present 

 vegetation but about seven years ; but many sources of 

 reproduction are constantly active in supplying so imperative 

 a demand. Factory furnaces are its wholesale manufactories, 

 (because the coal therein consumed consists mainly of carbon, 

 which unites with the oxygen of the air, supplied by the draft). 

 Every cottage fire is continually producing a new supply, and 

 the blue smoke issuing from the cottage chimney — described 

 by so many poets — possesses a new charm when we reflect that, 

 besides denoting a cheerful fire on the hearth, it contains the 

 material for forming food for the cottager's table, and new faggots 

 for his fire. The wick of every burning lamp draws up the 

 carbon of the oil to be made into carbonic acid by the flame. 

 All matters in process of combustion, putrefaction or decay, 

 return to the atmosphere in that operation those constituents 

 which they derived from it. The respiration of animals — 

 being a union of the carbon of the blood, derived from food, 

 with the oxygen of the inhaled air — is an important source of 

 the generation of this form of matter, and the decay of the 

 animal's body after death continues the process. 



Carbon is never permanent in any of its forms ; it may exist 

 to-day in the atmosphere, and to-morrow in the plant; and 

 from them it may be transformed into animal muscle. That in 

 the air is always ready to assume the vegetable form, and the 

 inmiense deposits in coal beds only await the assistance of com- 



