REVIEW OF LEIBIG. 
249 
carbonic acid, still are proved by their place of 
abode to be dependent for their food on organic 
matter in a state of decomposition. From this 
series of plants there is but a step toward those 
growing in peat, and heath-soil, &c. I do not 
mean, concludes Dr. Mohl, to prove by these state¬ 
ments that all plants feed on organic substances; 
my object is rather to explain that the introduction 
of organic matter into the system of vegetation is 
not, as Leibig assorts, necessarily detrimental to 
the economy of plants; and to show that it is by 
tar preferable to have recourse to positive and pre¬ 
cise experiments, than to indulge in groundless 
and general remarks and theories. 
In the second chapter ( Origin and Nature of 
Humus) Leibig states, that vegetable matter is 
successively changed by decomposition into humus, 
and that it constantly forms carbonic acid with 
the oxygen of the atmospheric air; all which 
forms a constant source of nourishment for plants, 
which decompose both the carbonic acid taken up 
by the roots, and that obtained by the leaves from 
the air. This, says Dr. Mohl, was known long 
ago. New, however, most certainly, is the argu¬ 
mentation by which Leibig intends to prove (p. 56) 
that humus is unnecessary, and that plants derive 
their food exclusively from the atmosphere. In 
corroboration thereof, he adduces the antediluvian 
and tropical vegetation, and says of the former, 
that its gigantic palms, gramineas (sic !) and ferns, 
could dispense altogether with soil, on account of 
the immense development of their foliage, as they 
are also distinguished from those of the present 
world by their scanty roots. Leibig says further, 
that in hot climates the succulent plants require 
but a slight connexion with the soil, and develop 
themselves without its Co-operation, in proof of 
which he adduces the slender roots of Sedum, Cac¬ 
tus, and Sempervivum. Leibig believes, in fine, 
that in lactescent plants the humidity absorbed 
from the air, and indispensable to their growth, is 
protected from evaporation by the very nature of 
their sap, as humidity is surrounded by caoutchouc, 
and is protected by a sort of impermeable integu¬ 
ment ! Risum teneatis amici —exclaims Mohl, in 
allusion to these opinions. The assertion that an¬ 
tediluvian plants lived on a soil devoid of humus 
is so extraordinary, that Mohl refutes it at some 
length. We know, he says, that monocotyledo- 
nous and cryptogamic plants possess no tap-roots, 
but merely fibres, which, although they are slen¬ 
der, still are very numerous. The assertion, there¬ 
fore, that plants with thick, branchy roots (like 
our trees) obtain their food from the soil, and those 
plants which possess fibrous ones are nourished by 
the air—is untenable. Leibig himself “ considers 
the absorption of inorganic substances to be neces¬ 
sary for the nourishment of plants; those, however, 
can only be absorbed by the roots.” “ The read¬ 
er,” concludes Mohl, “ will, I trust, not expect 
from me a refutation of Leibig’s assertions on trop¬ 
ical vegetation, which are really beneath criticism. 
If the luxuriant vegetation of the tropics, with 
their virgin forests, palms, and arborescent grasses, 
is to be typified by a few Sedums, Cacti, or Sem- 
pervivums, and if lactescent plants are to be look¬ 
ed upon as surrounded by a coat of India-rubber, 
then, certainly, anything may be proved—and, not 
least, the ignorance of the propounder.” 
Professor Leibig’s third chapter, inscribed “ The 
Assimilation of Hydro gen f proves pretty well 
that all which chemistry has made out about 
chemical processes in the interior of plants amounts 
to almost nothing, Leibig states, in the first in¬ 
stance, that woody fibre consists of carbon and the 
component parts of water, or of carbon plus a cer¬ 
tain quantity of hydrogen. Here, therefore, the 
very first proposition in the progress of assimilation 
contains an either and an or. Leibig thinks 
(p. 60) that decomposition of water is the more 
likely to take place, because water is the easier 
of decomposition; and this is plausible enough. 
But what shall we think of the consistency of Lei¬ 
big, if in all other parts of his work the decompo¬ 
sition of carbonic acid is considered as a self-evi¬ 
dent fact (p. 121) ; and leaves are said to possess 
powers of decomposition stronger than that of the 
most powerful agencies, because they can decom¬ 
pose carbonic acid, which resists the strongest gal¬ 
vanic battery! Leibig states further, that the 
formation of acids, of ethereal oils, (having no oxy¬ 
gen,) and of caoutchouc, may he considered as 
combinations of carbonic acid with water; ail, or 
the greater part, of the oxygen having been elim¬ 
inated. This may be true in a chemical point of 
view, but it remains to be proved that these com¬ 
binations are really formed by water and carbonic 
acid, and are not the result of other organic com¬ 
binations. But if the latter be the case—if ethe¬ 
real oils are formed by the mutual combination of 
organic substances—if they exhibit certain deter¬ 
mined stages of vegetable metamorphosis, then 
the decomposition of water and carbonic acid can 
not be taken into account, because these do not 
exist as such in organic combinations ; “ and then” 
(concludes Dr. M.) “ the process to which the 
above substances owe their origin is afar different 
one, and the explanation of Leibig is anything but 
a formula explaining their origin;” but is quite as 
erroneous as the assertion would be that sugar 
consists of carbonic acid and spirits of wine. 
In the fourth chapter, “ On the Origin and the 
Assimilation of Nitrogen ,” Leibig starts from the 
correct assertion, that even in a soil richest in hu¬ 
mus, no vegetation can take place without the co¬ 
operation of some nitrogenous substances; and 
that (as it has been proved by Boussingault) their 
nitrogen is derived from the atmosphere. But 
Leibig subsequently rejects the opinion (p. 65) 
that plants assimilate the nitrogen of the air in a 
direct manner, and derive their nitrogen from the 
ammonia contained in the rain-water, (a discovery 
made by himself,) adducing in proof that nitrogen 
is conveyed to plants in the form of ammonia, the 
analysis of the sap of acorns, birches, &c., in which 
ammoniacal salts have been found. This idea is 
certainly the most valuable in the whole of Lei¬ 
big’s work. But here, as elsewhere, he has been 
satisfied with generalities, without looking to de¬ 
tails of great importance in vegetable physiology. 
Considering the ammonia of rain-water sufficient 
for explaining the amount of nitrogen contained in 
plants, he has entirely neglected the nitric salts, 
and asserts (without adducing any proof) that 
