FOOD OP PLANTS IN GENERAL. 485 



tire district of Brandenburg, whose soil consists entirely of sea and 

 down-sand. It is still in many places composed of a loose and pure quick- 

 sand of 100 feet deep, and so moveable that it does not, as I have had 

 an opportunity of witnessing in the neighbourhood of Berlin, require 

 any very high wind to change entirely the configuration of the surface. 

 Such spots are found likewise between Charlottenburg and Grunewald, 

 and between Berlin and Tegel. Young pines are sometimes found stand- 

 ing with their first branches buried in the soil, and after eight days with 

 a naked stem, three feet in length, and the roots so exposed that one could 

 creep through them. This soil, as is seen in the Spree w aid, so far as it 

 is moistened by the rivers Spree and Havel, produces vigorous pine vege- 

 tation, which most certainly cannot draw all its carbon from sources fur- 

 nished by the soil, for it has never possessed it, nor has it been furnished 

 to it by artificial processes. The older standing forests have obtained 

 by the fall of the leaf and the action of wind upon the trees so much 

 organic substance as to become in a measure suited for arable land, 

 though such land yield very poor crops of corn, because it needs the 

 essential physical and chemical qualities, which only can be supplied by 

 active culture, and the addition of the salts required by the Cerealia. 



In the cases adduced, we find a production of carbon in organic com- 

 pounds which clearly could not arise from the elements of the soil, be- 

 cause it either contained none originally, or would soon become exhausted 

 of that which it contained ; and yet it becomes continually richer in 

 carbon, even though the decay of vegetation continually carries it off, 

 and with astonishing rapidity under the tropics. The substance of the 

 soil, then, is assuredly not the source from whence plants derive their 

 carbon, and no other is left but carbonic acid. Hence it can be easily 

 understood that the carbonic acid of the soil may contribute to the food 

 of plants, as well as carbonic acid derived from any other source. 



It becomes a question whether there is enough carbonic acid existing 

 to supply the necessities of vegetation upon the whole earth. 



Supposing the part of the earth which is covered with vegetation to be 

 one-fifth of the entire surface, that will give a space of two millions of 

 square miles, or of 43,124 millions of acres ; upon each acre we may allow 

 an annual produce of 2000 Ibs. of carbon, which, on an average, is certainly 

 not too little ; we have to provide for a yearly demand of about 300 

 billion Ibs. of carbonic acid, the source of at least a third part of which 

 we have seen above. How near this is to the truth, we may ascertain by 

 considering some secondary facts. North America alone produces (accord- 

 ing to the North American Almanac for 1843) annually 219,163,319 

 Ibs. of tobacco, which being burned, would yield about 340 million Ibs. 

 of carbonic acid, so that the yearly produce of carbonic acid from tobacco 

 smoking alone cannot be estimated at less than 1000 million Ibs. Other 

 very extensive processes involving the formation of carbonic acid are 

 not alluded to in the above computations. Calculations have been made 

 respecting the disengagement of this gas from the lungs ; but from the 

 want of data none have been made upon the production of it from the 

 skin, which is no less important. In the same way, many processes of 

 combustion, which are carried on in various methods of culture or in other 

 cases, are entirely overlooked in our calculations. In the whole of 

 Northern Germany, the burning of moors is a very common practice ; 

 below Ems it is annually done on the largest scale. So in Corsica, the 

 makis, or evergreen shrubs, are cut down once in three years, and burned 

 upon the soil. In North and South America the breaking up of new 



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