[ 399 ] 



XXXVIII. The Chemical and Geological History of the 

 Atmosphere. By John" Stevenson, M.A., F.I.C.* 



[Continued from p. 323.] 



A NOTHER line of inquiry that has some bearing on our 

 Jl\. subject is provided by studying the rate of growth of 

 vegetation. Sixty years ago Liebig made or obtained estimates 

 regarding the amount of dry wood produced per acre per 

 annum on average forest-land, of dry hay on meadow-land, 

 of straw and corn on arable land, and also of beetroot on 

 arable land. The results were very similar in all cases when 

 reduced to dry substance, and were nearly equivalent to 

 2 tons of dry wood, or dry hay, &c. per English acre per 

 annum. Lord Kelvin, in his address on the Age of the 

 Earih, thinks this is too high an estimate to take as an average 

 for the whole earth — he thinks it is " much higher than the 

 average activity of vegetable growth on land and sea/' But 

 if we call the average rate over the land-surface of the earth 

 only half of ihe above estimate of Liebig's, and leave the 

 sea out of account altogether, we shall probably not err on 

 the side of making too high an estimate, for though a large 

 proportion of the earth's surface is covered by deserts, there 

 are large areas in tropical regions where the luxuriance is 

 very much greater than that of the above estimate, and 

 besides there is a very considerable growth of vegetable 

 matter in the sea which we are leaving out of account. Let 

 us also suppose that the dry wood, hay, &c. contain on the 

 average 40 per cent, of carbon, and that the oxygen and 

 hydrogen constituting the bulk of the remaining 60 per cent. 

 are chemically equivalent to each other, that is to say, that 

 they are present in the proportions' which would form water. 

 At this rate of growth (viz. 1 ton of dry wood &c. per acre), 

 there would be 0'4 ton of carbon per acre per annum separated 

 from atmospheric carbonic acid, or 256 tons per square mile 

 per annum. If we now take the land-surface of' the earth 

 at 50,000,000 square miles, this makes a total quantity of 

 12,800,000,000 (nearly 13 thousand million) tons of carbon 

 per annum. If we next assume that all this carbon is 

 deposited and preserved in the form of coal, and that the 

 process goes on at the same rate year after year, it is easy to 

 see that in rather less than 40,000 years we should have the 

 500,000.000,000,000 tons of coal that are equivalent to the 

 whole of our free oxygen. But of course we know very well 

 that by far the most of the vegetable matter now grown on 

 the earth is soon re-oxidized into carbonic acid and water, 



* Communicated by the Author. 



