178 
OF GARDENING AS A SCIENCE. 
nutrition, will not dissolve ; it will not yield colouring matter to water. Therefore 
we conclude that the plant, under the stimulus of solar light, (which also operates 
on the ground, and is itself absorbed thereby — not extinguished,) decomposes the 
water that exists in the soil, and thus induces an electric current, which in its turn 
decomposes the manure, and converts it to primary raw sap. What this sap may 
be, it is more than possible we shall never be able to determine with precision ; 
but we may be allowed to suggest that the elements of water are thus developed, 
which combine instantly, in the form of the purest water, depositing the carbon 
within the soil *. It is by no means asserted that all the carbon of the manure or 
vegetable earth is deposited ; a portion may be, and very probably is, dissolved at 
the moment of the extrication of both, by an equivalent portion of hydrogen ; 
but what we contend for is this — That water does not act as a direct solvent of 
manure, no such thing as dissolved liquid manure being taken up, nutrimentally, 
by the roots ; and that sap, be it (according to the usual acceptation of the word) 
simply aqueous, or a compound of water, and some hydro-carbon, is the 
secondary result of an electrolytic process, nearly allied to that which we term 
galvanic or voltaic. 
If our views be correct, there will be little difficulty to account for the darker 
hue assumed by ground under crop ; for the carbon, being deposited, remains in 
it ; or if we suppose with Liebig that there is a modification of humus, which 
is perfectly insoluble, called " coal of humus" the same phenomenon will occur. 
This coal, according to the Lecture on Manures by Dr. Daubeny, p. 250 of the 
Journal of the R. Ag. Soc, is derived from common humus by the gradual 
process of decay, but is already advanced to that condition in which alkalies 
or earths can neither convert it into humic acid, nor render it soluble. 
In common with Liebig, Dr. Daubeny believes that humus, " during the whole 
period of its decay, until it has reached that ultimate point at which it ceases to be 
soluble, and has become a kind of caput mortuum^ goes on continually disengaging 
carbonic acid ; so that the roots of plants fixed in humus of this quality are 
surrounded by an atmosphere of the gas in question, which is therefore held in solu- 
tion hy the water taken up by them as sap." 
Where is the proof that carbonic acid is so dissolved ? Is it not far more likely 
that if this aerial acid be produced, it passes at once into the atmosphere, and 
thence is taken into the vegetable organization by the leaves, to be converted into 
specific fluids and woody fibre ? 
Water certainly does act as a solvent, but not of vegetable mould ; as rain^ it 
conveys into the earth with every shower a certain quantity of ammonia^ which is a 
saline substance of extreme utility in vegetation. On this subject Liebig thus ex- 
presses himself : — " Any one may satisfy himself of the presence of ammonia in rain 
by simply adding a little sulphuric or muriatic acid to a quantity of rain-water, and 
evaporating this nearly to dryness in a clean porcelain basin. The ammonia 
* All manure, vegetable mould, &c., contains oxygen, hydrogen, and carbon ; and sometimes nitrogen. 
