INTO CARBONIC ACID AND WATER. 153 



have already been considered, — the light carburetted hydrogen to under- 

 go a further change, by which it also is resolved into carbonic acid and 

 water. Thus, if to 



1 of Light Carburetted Hydrogen = CH2 we add 



4 of Oxygen = O4 



Carbonic Acid. Water. 



We have CH2 O, or CO2 + 2 HO 



Or one of this gas with 4 of oxygen may be changed into 1 of carbonic 

 acid and 2 of water. 



Now, when this gas escapes into the air it becomes diffused through a 

 large excess of oxygen, and is thus ready, at any instant, to be decom- 

 posed. Through the atmosphere streams of electricity are continually 

 flowing, and every wandering spark that passes athwart a portion of 

 this mixture decomposes so much of the light gas, and produces in its 

 stead the equivalent proportions of carbonic acid and watery vapour. 

 Thus it happens that of the vast quantity of this and other combustible 

 gases which are continually escaping into the air, so few traces are dis- 

 cernible even by the aid of the most refined processes of art. By a wise 

 provision of nature such substances as are void of use to either animals 

 or plants, if not speedily removed from the air altogether, are there con- 

 verted into such new forms of matter as are fitted to minister to the ne- 

 cessities of living beings. 



Though therefore in the natural decay of vegetable matter in the pre- 

 sence of air and moisture, a certain portion of its carbon escapes into the 

 air in the form of light carburetted hydrogen, this compound is but a 

 step towards the final change into carbonic acid and water. In the soil 

 the vegetable matter is continually undergoing decay, various sub- 

 stances are produced in greater or less quantity, some solid, some liquid, 

 and some gaseous like the light gas of which we have been speaking, — 

 but all of them, like this gas, are only hastening — some by one road, so 

 to speak, and some by another — towards that final destination which 

 sooner or later they are all fated to reach ; when in the form of carbonic 

 acid and water they shall be in a condition to minister again to the nour- 

 ishment of all plants. 



While in the soil some part of this vegetable matter assumes forms 

 which are capable of entering again into the roots of lixing plants, and, 

 without further resolution in the air, of being converted by the living 

 plant into portions of its own substance. The nature and composition 

 of these forms of matter, so far as they are known, will be considered in 

 a subsenuent lecture. — [See Part 11., Lectures Xl.-XIIl., " On the 

 constitution of soils. ^^] 



It is upon the ^naZ result of this natural decay to which all vegetable 

 matter is subject, that the carbonic acid of the atmosphere depends for 

 its largest supplies. The rapidity with which organized bodies perish, 

 and become resolved into gaseous compounds, depends partly upon the 

 climate and partly on the nature of the substances themselves, — but all 

 hurry forward to the same end, and it is with difficulty that w^e are able 

 for a time to arrest or even to retard their steps. It is by this perpetual 

 and active obedience of all dead matter to one fixed law that the exist- 

 ing condition of things is maintained ; — and thus it happens that either 

 by the respiration of the animals which live upon it, by tlie process of 



