NOT INDISPENSABLE FOR PLANTS. 79 



to possess the power of condensing gases within its 

 pores, and particularly carbonic acid. And it is by 

 virtue of this power that the roots of plants are sup- 

 plied in charcoal, exactly as in humus, with an at- 

 mosphere of carbonic acid and air, which is renewed 

 as quickly as it is abstracted. 



In charcoal powder, which had been used for this 

 purpose by Lukas for several years, Buchner found a 

 brown substance soluble in alkalies. This substance 

 was evidently due to the secretions from the roots 

 of the plants which grew in it. 



A plant placed in a closed vessel in which the air, 

 and therefore the carbonic acid, cannot be renewed, 

 dies exactly as it would do in the vacuum of an air- 

 pump, or in an atmosphere of nitrogen or carbonic 

 acid, even tholigh its roots be fixed in the richest 

 mould.* 



Plants do not, however, attain maturity, under or- 

 dinary circumstances, in charcoal powder, when they 

 are moistened with pure distilled water instead of 

 rain or river water. Rain water must, therefore, con- 

 tain within it one of the essentials of vegetable life ; 

 and it will be shown, that this is the presence of a 

 compound containing nitrogen, the exclusion of which 

 entirely deprives humus and charcoal of their influ- 

 ence upon vegetation. 



* A few years since I had an opportunity of observing a striking in- 

 stance of the effect of carbonic acid upon vegetation in the volcanic 

 island of St. Michael (Azores). The gas issued from a fissure in the 

 base of a hill of trachyte and tuffa from v^rhich a level field of some 

 acres extended. This field, at the time of my visit, was in part covered 

 with Indian corn. The corn at the distance of ten or fifteen yards from 

 the fissure, was nearly full grown, and of the usual height, but the 

 height regularly diminished until within five or six feet of the hill, 

 where it attained but a few inches. This effect was owing to the great 

 specific gravity of the carbonic acid, and its spreading upon the ground, 

 but as the distance increased, and it became more and more mingled 

 with atmospheric air, it had produced less and less effect. — IV. 



