78 ON THE ORIGIN AND ACTION OF HUMUS. 



titute of carbonaceous matter; and the extensive 

 prairies of the Western Continent show that the car- 

 bon necessary for the sustenance of a plant may be 

 entirely extracted from the atmosphere. Again, in 

 the most dry and barren sand, where it is impossible 

 for nourishment to be obtained through the roots, we 

 see the milky-juiced plants attain complete perfec- 

 tion. The moisture necessary for the nutrition of 

 these plants is derived from the atmosphere, and 

 when assimilated is secured from evaporation by the 

 nature of the juice itself. Caoutchouc and wax:, 

 which are formed in these plants, surround the water, 

 as in oily emulsions, with an impenetrable envelope 

 by which the fluid is retained, in the same manner as 

 milk is prevented from evaporating by the skin 

 which forms upon it. These plants, therefore, be- 

 come turgid with their juices. 



Particular examples might be cited of plants, which 

 have been brought to maturity, upon a small scale, 

 without the assistance of mould ; but fresh proofs 

 of the accuracy of our theory respecting the origin 

 of carbon would be superfluous and useless, and 

 could not render more striking, or more convincing, 

 the arguments already adduced. It must not, how- 

 ever, be left unmentioned, that common wood char- 

 coal, by virtue merely of its ordinary well-known 

 properties, can completely replace vegetable mould 

 or humus. The experiments of Lukas, which are 

 appended to this work, spare me all further remarks 

 upon its eflicacy. 



Plants thrive in powdered charcoal, and may be 

 brought to blossom and bear fruit if exposed to the 

 influence of the rain and the atmosphere ; the char- 

 coal may be previously heated to redness. Charcoal 

 is the most " indifferent " and most unchangeable 

 substance known ; it may be kept for centuries with- 

 out change, and is therefore not subject to decompo- 

 sition. The only substances which it can yield to 

 plants are some salts, which it contains, amongst 

 which is silicate of potash. It is known, however, 



