SECOND REPLY. 163 



position, not to be nourished on that humus, which 

 increases instead of decreasing with every decaying genera- 

 tion, but by its means to be supplied with nourishment 

 from the atmosphere. 



Boussingault, in his " Economic Rurale,"* brings forward 

 a still more interesting example. In his first sojourn in 

 America, he visited a spot in the neighbourhood of 

 La Vega da Supia, which, during his stay, was converted 

 by an earthquake into a waste surface of fragments of 

 porphyry, whereby all vegetation was destroyed, and buried 

 many fathoms deep beneath the rocks. When he returned 

 to the same spot, after an absence of ten years, the wild 

 and bare masses of rock were already clothed with a young, 

 luxuriantly- vegetating Acacia grove. And without doubt, 

 the rock-islands arising by volcanic force from beneath the 

 floods of the ancient ocean, at a period which lies hundred, 

 thousands of years beyond the human history of our globe, 

 were gradually clothed in a similar manner with vegetation 

 until, in favourable places, those masses of humus were 

 at last accumulated, which serve as the luxuriant sub- 

 stratum of the inexhaustible vegetable life of the primeval 

 forests of the tropics. In this physical property of humus, 

 and not in its chemical constituents, we have to seek the 

 reason why a more luxuriant vegetation thrives upon a soil 

 rich in humus, than on one in which an admixture of this 

 substance is wanting. 



But how now ? If carbonic acid, ammonia and water 

 form the sole food of plants, if these matters are already 

 present in sufficient quantity in the vast reservoir of the 

 aerial ocean, if even without humus these matters may be 



* " Rural Economy, in its relations with Chemistry, Physics and 

 Meteorology," by J. B. Boussingault, translated into English. Second 

 Edition, 8vo. London, 1846. 



II* 



