WEIGHT OF CARBON JN THE ATMOSPHERE. 147 



siderable portion of the carbon of the manure and of the soil would 

 naturally disappear during the four years' cropping above-mentioned, 

 and that, therefore, the proportion of carbon derived from the air in 

 Boussingault's experiments, nmst have been really considerably greater 

 than is indicated by the numerical results. 



Let two-thirds of the entire quantity of carbon contained in a series of 

 crops be taken as the average proportion, [Lecture II., p. 31,] which, on 

 cultivated land in our climate, must be derived from the air in the form 

 of carbonic acid — and let the average weight of the dry crop reaped be 

 estimated at a ton and a half per acre. Then, if the crop contain half 

 its weight of carbon,* the plants grown on each acre must annually ex- 

 tract from the air 10 cwt. or 1120 lbs. of carbon in the form of carbonic acid. 



• 



§ 2. Of the relation which the quantity of carbon extracted by plants from 

 the air, bears to the whole quantity contained in the atmosphere. 



But the question will here at once suggest itself to you — does not the 

 quantity thus extracted from the air really form a very' large proportion 

 of the whole weight of carbon which is contained in the atmosphere ? A 

 simple calculation will give us clear ideas in regard to this interesting 

 point. 



We have already seen tliat, by the results of De Saussure, the aver- 

 age quantity of carbonic acid in the atmosphere of our globe may be 

 estimated at o-^o V'^^^ ^^ i^^ entire bulk. This is equal very nearly 

 to T^(nro ^^ i^^ weight. f Or taking the whole weight of the atmosphere 

 at 15 lbs. on the square inch — that of the carbonic acid will be 0*009 lbs. 

 or 63 grs. per square inch. But as carbonic acid contains only 27§ per 

 cent, of its weight of carbon, the weight of this element which presses 

 on each square inch of the earth's surface is only 17f (17*39) grs. Upon 

 an acre this amounts to 7 tons.} 



But if the crop on each acre of cultivated land annually extracts from 

 the air half a ton of carbon, the whole of the carbonic acid in the atmos- 

 phere would sustain such a vegetation over the entire globe for 14 years 

 only. And if we even suppose such a vegetation to extentl over one 

 hundredth part of the earth's surface only, it still appears sufficient to 

 exhaust the carbonic acid of the air in 1400 years. 



* Boussingault states, that of all the plants usually cultivated for food— so far as his experi- 

 ments have gone— the Jerusalem artichoke draws the largest portion of its sustenance from 

 the air — or yields the greatest weight of food from tlie smallest weight of manure. It is true 

 generally indeed that all those plants, "which, like the Jerusalem artichoke and the white 

 carrot, grow freely on sandy soiis containing little vegetable matter and with the addition 

 of little manure, extract the greatest proportion of their sustenance from the air. Such 

 plants, therefore, are likely to prove the most profitable articles of culture where such soils 

 and a scarcity of manure simultaneously prevail. 



t The mean of 225 experiments made by De Saussure between 1827 and 1829 gave as 

 above stated about 4-10000 or l-2500th part for the mean bulk of the carbonic acid in the air, 

 which is nearly 6-lOOOths of its whole weight. Among these observations the maximum 

 was 5-8 ten-thousandths, the minimum 3-15. If we take the maximum bulk at 6-10000tb3 

 of the air— the maximum weight of the carbonic acid is nearly 9-lOOOOtlis of that of the at- 

 mosphere. In elementary works it is generally stated in round numbers at l-lOOOth of the 

 weight of the air, but if the best experimental results we possess are to be any guide to us, 

 this is at least one-third too high. 



It is also of consequence to remark, that this estimate of the whole weight of the carbonic 

 acid in the air is founded on the supposition that, in the highest regions of the atmosphere: 

 the carbonic acid is present in a proportion nearly equal to that in which it is found imme 

 diately above the eartli's surface— which is by no means established. 



t 15-583 lbs.— an acre being 4840 square yards, containing each 1296 square inchesj 



