148 WHAT DOES MAN LIVE UPON? 



the nutrition of the plant includes, therefore, the inquiry 

 into the sources of carbon and nitrogen ; oxygen and 

 hydrogen being sufficiently provided by water and atmo- 

 spheric air. The notion which has hitherto been generally 

 received, is, that the plant extracts its carbon and nitrogen 

 from manure, or from the humus of the soil. 



All animal and vegetable bodies, so soon as they are 

 dead, pass over into a state of decomposition, by means of 

 which they are dissipated, sooner or later, in the atmo- 

 sphere, being changed into carbonic acid, ammonia and 

 water. So long as this process is incomplete, a residue, 

 itself much altered, of a brownish or black colour, remains, 

 which at the commencement of the decomposition is called 

 manure, and toward its close, humus or vegetable mould. 

 It is a complex mixture of very manifold products of 

 decomposition. Now it was argued thus : carbon and 

 nitrogen are abundant in humus ; in a soil that is rich in 

 humus or is well manured, plants thrive better than in one 

 which is poor in humus ; consequently, humus is the source 

 of the carbon and nitrogen of plants. But this reasoning 

 is altogether inconclusive. 



There was a period of our earth's existence when yet no 

 vegetation clothed its solid crust, in which no animal lived, in 

 which no humus could possibly be present. From this soil, 

 devoid of humus, gradually developed vegetation, in such vast 

 quantity, in such gigantic luxuriance, that the same, buried 

 and preserved for us by subsequent revolutions, assumes a 

 most essential place in human economy in the present day ; 

 I mean the vegetation of one of the oldest geognostic forma- 

 tions the coal period. The annual consumption of coal in 

 Europe amounts to more than 33,875,000 tons, and 

 geognosy shows that, even if the consumption of coal 

 should increase, the store will certainly last for five him- 



