ACACIA ARMATA. 
295 
vegetables, that have been naturally produced on the surface of a 
diluvium deposit, which is a mixture of sandy, flinty gravel, 
containing a large quantity of iron, and resting on the bed of the 
London clay. It is reasonable to suppose that the first vegetable 
production of this soil was lichens, which absorb a large portion 
of carbon from the air, and require little from the soil. These 
were succeeded by mosses, which by their decomposition and 
mixture with the sand formed a thin strata of mould, fit for the 
production of other plants, amongst which were ferns. The roots 
of these would penetrate to a great depth in the gravelly soil, 
and collect what alkali it was capable of yielding : and so, by the 
decay of a crop of leaves annually on the surface, a quantity of 
potash would become incorporated with the new-formed earth 
sufficient for the growth of heath, and ultimately leguminous 
plants, such as furze and Acacia. The peat, therefore, which we 
use for potting is a soft, light, spongy substance, holding water by 
capillary attraction ; is composed of woody fibre, white sand, a 
little potash, the oxide of iron imparted to it by the subsoil, and 
dispersed through the whole mass ; as is also the tannin principle 
of the mass which it contains in a considerable degree, and has 
given to it that antiseptic property without which it could never 
have been composed ; because if plants are not imbued with this 
principle, their whole substance, when life is extinct, would pass 
into air, and resume their original elements. 
But the power of this tannin is not only destroyed by lime and 
other earths, but also rapidly by the roots of the plant I am 
writing of. 
If we can place any reliance in Liebeg’s theory, we may rea¬ 
sonably infer that the principle called humic acid exists in peat, 
“ humus being a carbonaceous substance, and containing hydrogen, 
undergoes, when moistened, a process of slow combustion. This 
goes on in the woody fibre from the commencement of its decay 
till it passes into coal of humus. During the intermediate stages 
it constitutes the former description of humus, which, when acted 
upon by an alkali, is converted into humic acid, and combines 
with it.” 
So that, if humic acid is the agent by which the soluble parts 
of soil are converted into the proper food of plants, we see that the 
process is facilitated or retarded by the presence of a large or 
small quantity of alkalies. I may here mention one circumstance 
