LAW OF THE DECAY OF VEGETABLE MATTER. 15J 



111 connection with this subject, I must draw your attention to one in- 

 teresting, as well as important, fact. I have spoken of coal as a sub- 

 stance of vegetable origin, and there is no doubt that all the carbon it 

 contains once floated in the air in the form of carbonic acid. But the 

 period when it was so mixed with the atmosphere is remote almost be- 

 yond conception. When, therefore, we raise coal from its ancient bed 

 and burn it on the earth's surface, we add to the carbon of the air a por- 

 tion which has not previously existed in the atmosphere of our time. 



The coal consumed in Great Britain alone is estimated at 20 millions 

 of tons, containing on an average at least 70 per cent., or 14 millions of 

 tons of carbon. But if the annual produce of an acre of cultivated land 

 contain half a ton (p. 147) of carbon derived from the air, the coal con- 

 sumed in this country would supply carbonic acid to the crops grown 

 upon '28 millions of acres. Or, since in Great Britain about 34 millions 

 of acres are in cultivation (p. 12), the coal we annually consume produces 

 a quantity of carbonic acid which is alone sufficient to supply food to the 

 crops that grow upon seven-eighths of the arable land of this country. 



IH. PRODUCTION OF CARBONIC ACID BY THE NATURAL DECAY OF VEGE- 

 TABLE MATTER. LAW OF THIS DECAY. 



Over large tracts of country in every part of the globe, the vegetable 

 productions of the soil are never cropped or gathered, but either accumu- 

 late — as occasionally in our peat bogs; or decay and gradually disappear 

 — as in the jungles of India or in the tropical forests of Africa and South- 

 ern America. 



^ he final results of this decay are the same as those which attend 

 upon ordinary combustion, but the conditions under which it takes place 

 being different, the immediate results are to a certain extent different 

 also. 



When a vegetable substance is burned in the air, the oxygen of the at- 

 mosphere is the only material agent in effecting the decomposition. 

 The carbon of the burning body unites directly with this oxygen and 

 forms carbonic acid. 



In the natural process of decay, however, at the ordinary temperature 

 of the atmosphere, vegetable matter is exposed to the action of both air 

 and water ; these both co-operate in inducing and carrying on the decom- 

 position, and hence carbonic acid is not, as in the case of combustion, the 

 chief or immediate result. 



A detail of all the steps through which vegetable matter is known to 

 pass before it is finally resolved into carbonic acid and water, would be 

 difficult for you to understand, and is here unnecessary. A general 

 view of the way in which by the united agency of air and water, the 

 decay of organic substances is effected and promoted, may be made 

 very intelligible, and will sufficiently illustrate the subject for our pre- 

 sent purpose. 



In combustion, as we have seen, the whole of the vegetable substance 

 is resolved directly into carbonic acid and water, at the expense of the 

 oxygen of the atmosphere. In natural decay a small and variable por- 

 tion only is so changed, but to the extent to which this change does take 

 place carbonic acid is directly formed and sent up into the air. Suppose 

 such a change — a slow corl ^ustiou in reality — to take place to a certain 



