184 
liebig's organic chemistry. 
woody fibre, by allowing a more free access of ox)'gen of 
the air. 
Hydrogen and oxygen also enter into the composition of 
vegetables. Pure woody fibre contains these elements in 
the proportion to form water; but other parts may contain 
one of these elements in a greater or lesser proportion. The 
source of these elements is obviously supplied by the de- 
composition of water, and a greater or lesser amount of each 
element is assimilated according to the compound formed. 
Nitrogen exists in many vegetable compounds, and the 
source of its supply is an important point. Plants will grow 
in pure charcoal if moistened with rain water; rain water 
can only contain nitrogen as dissolved atmospheric air, or 
* in the form of ammonia, but nitrogen, when uncombined, is 
a remarkably indifferent substance, while its combinations 
are capable of ready transformation. Experiments have 
placed beyond doubt the existence of ammonia in the atmo- 
spheric air; for it has been detected in rain water and in 
snow. The sources from which it is derived are fully 
pointed out, but our space will not admit of detail, and the 
inferences drawn from this important discovery can only 
be properly appreciated by a perusal of the author's re- 
marks. 
These four elements constitute the principal amount of 
vegetable matter, but other substances are equally necessary 
in the formation of special organs; viz., their inorganic con- 
stituents. These have been supposed to be only accidental- 
ly present, being taken up by the plants because existing in 
the soil, and that they exercise no peculiar influence on the 
growth. Analyses of the ashes of the same kind of vegeta- 
ble productions, grown on different soil, in which unequal 
amounts of the metallic bases were obtained, are adduced in 
proof of this. But "it must be considered as a most remark- 
able accident, that these same analyses furnish proof for the 
very opposite opinion." These bases exist in the form of 
salts, and when the amount of the different metallic oxides 
