TRANSACTIONS OF SECTION B. 517 



•wliich it has been coucluded tliat plants do not even utilise tlie carbonic acid which 

 they may take up from the soil by their roots. However this may be, we may 

 safely conclude that practically the whole of the carbon which it is the object of 

 the cultivator to force the plants he grows to take up is derived from the atmo- 

 sphere, in which it exists in such extremely small proportion, but nevertheless lar"-e 

 actual, and constantly renewed amount. 



Judging from the more recent researches on the point, it would seem probable 

 that the estimate of one part of carbon, as carbonic acid, in 10,000 of air, is more 

 probably too high than too low as an estimate of the average quantity in the 

 ambient atmosphere of our globe. And, although this would correspond to several 

 times more in the column of air resting over an acre of land than the vegetation of 

 that area can annually take iip, it represents an extremely small amount at any 

 one time in contact with the growing plants, and it could only suffice on the suppo- 

 sition of a very rapid renewal, accomplished as the result, on the one hand of a con- 

 stant return of carbonic acid to the atmosphere by combustion and the respiration 

 of animals, and on the other of a constant interchange and equalisation among the 

 constituents of the atmosphere. 



It_ will convey a more definite idea of what is accomplished by vegetation in the 

 assimilation of carbon from the atmosphere if I give, in round numbers, the results 

 of some direct experiments made at Kothamsted, instead of making general state- 

 ments merely. 



In a field which has now grown wheat for thirty-seven years in succession, there 

 are some plots to which not an ounce of carbon has been returned during the whole 

 of that period. Yet, with purely mineral manure, an average of about 1000 pounds 

 of carbon is annually removed from the land ; and where a given amount of nitro- 

 genous manure is employed with the mineral manure, an average of about 1500 pounds 

 per acre per annum more is obtained ; in all an average of about 2500 pounds of 

 carbon annually assimilated over an acre of land without any return of carbonaceous 

 manure to it. 



In a field in which barley has been grown for twenty-nine years in succession, 

 quite accordant results have been obtained. There, smaller amounts of nitrogenous 

 manure have been employed with the mineral manure than in the experiments with 

 wheat above cited : but the increase in ihe assimilation of carbon for a given amount 

 of nitrogen supplied in the manure is greater in the case of the barley than of the 

 wheat. 



With sugar-beet, again, larger amounts of carbon have been annually accumu- 

 lated without the supply of any to the soil, but under the influence of a liberal 

 provision of both nitrogenous and mineral manure, than by either wheat or barley. 

 Lastly, with grass, still larger amounts of carbon have been annually accumu- 

 lated, without any supply of it by manure. 



:i(:»!i Many experiments have been made, in Germany and elsewhere, to determine the 

 amount of the ditferent constituents taken up at different periods of the growth of 

 various plants. But we may refer to some made at Kothamsted long ago to illus- 

 trate the rapidity with which the carbon of our crops may be withdrawn from the 

 atmosphere. 



In 1847, we carefully took samples from a growing wheat crop at difierent stages 

 of its progress, commencing on June 21, and in these samples the dry matter, the 

 mineral matter, the nitrogen, &c., were determined. On each occasion the produce 

 of two separate eighths or sixteenths of an acre was cut and weighed, so that the 

 data were provided to calculate the amounts of the several constituents which had 

 been accumulated per acre at each period. The result was that, whilst during 

 little more than five weeks from June 21, there was comparatively little increase 

 in the amount of nitrogen accumulated over a given area, more than half the total 

 carbon of the crop was accumulated during that period. 



Niunerous experiments of a somewhat similar kind, made in another season, 1850, 

 concurred in showing that, whilst the carbon of the crop was more than doubled 

 after the middle of June, its nitrogen increased in a much less degree over the same 

 period. 



Similar experiments were also made, in 1854 and in 185G, with beans. The 



