FOREIGN AGRICULTURAL NEWS. 
253 
salt, as a manure, have hitherto been as different as 
the soils on which they were made. I am aware that 
it has been frequently asserted by learned and scientific 
men, that salt is only a stimulant, and possesses no 
nourishment, but may excite the vegetable absorbent 
Vessels into greater action than usual. I am not pre¬ 
pared to controvert this assertion; but the result of 
.several experiments would go far to establish a different 
view of the matter. 
Liebig’s Theory of the Nutrition of Plants.-— 
Though immediately ordered, we have not yet received 
the pamphlet of Dr. Mohl alluded to in our July No., 
attacking Liebig’s assertions in regard to the nutrition 
of plants. Instead of this, therefore, we commence a 
review of it from the London Gardeners’ Chronicle by 
Prof. Lindley. It is entitled Dr. Justus Liebig in his 
relation to Vegetable Physiology. By Di\ Hugo Mohl. 
(Dr. Justus Liebig’s Verhaltniss zur Pflanzen-Physiol- 
ogie.) Tubingen-. Frue's. 1843. 
This is the pamphlet of Dr. Mohl to which we re¬ 
ferred in a leading article of May the 20th, and which 
consists of a critique on the work of Dr. Liebig, so well 
known in this country under the title of “ Chemistry, 
in its applications to Agriculture and Physiology.” Dr. 
Mohl says that work was anxiously looked for by bot¬ 
anists both on account of the reputation of Liebig as a 
chemist, and from a knowledge of the fact that they had 
much to look for from the aid of chemistry in their in¬ 
vestigation of the phenomena of the nutrition of plants. 
But Dr. Mohl observes that throughout the whole work 
there is a want of original experiment, which is the 
more wonderful, since it is written by the greatest ex¬ 
perimenter of his day, and the possessor of one of the 
largest laboratories in Europe. Nevertheless Liebig 
everywhere insists on the importance of experiments, 
and is continually appealing to those of Theodore De 
Saussure. Under these circumstances, the work can 
only be looked upon as an attempt to construct a theory 
from data already known to the world. 
The next general remark by Dr. Mohl refers to the style 
in which the book is written. If not always correct, it 
is energetic and clear; the thoughts are propounded in 
short determinate propositions, and there is not the 
slightest indication of doubt or uncertainty about any¬ 
thing ; the author seems to know everything for certain, 
and says it boldly out. This sort of style is apt to mislead 
the uninitiated, and frequently leads the author himself 
into positive contradictions ; in fact, a thing is stated to 
be black or white according as it suits the author’s 
purpose. For instance, in one place he says that leaves 
do not decompose carbonic acid in the shade (in which 
he is wrong), and in another place he says leaves do 
decompose carbonic acid in the shade (in which he is 
right). Such contradictions are frequent, and prove 
that the author is neither well grounded in the subjects 
on which he has undertaken to write, nor has fully 
considered them. The manner in which Liebig at¬ 
tributes erroneous views, entertained perhaps by indi¬ 
vidual botanists, to “ vegetable physiologists” and 
“ botanists” in general, is objectionable and liable to 
mislead. Thus he says that “ vegetable physiologists” 
consider humus as the principal food of plants. Now 
this is not true : vegetable physiologists have no sacred 
books in which their code of laws is contained, and if 
any individuals have maintained such a view, the great 
body has not. In fact, Ingenhousz, Senebier, Curt Spren- 
gel, Link, and De Candolle, have all either denied it or 
taken other views. The doctrine of humus is altogether 
a chemical one, and has only been supported by chemists. 
Again, Liebig says that « all botanists and vegetable 
physiologists have doubted the assimilation of the 
carbon of the atmosphere by plants.” Yet all books on 
vegetable physiology Contradict such a statement; and 
the absorption of carbonic acid from the atmosphere is 
so generally admitted, that Adolphe Brongniart, in the 
13th volume of the “ Annales des Sciences,” has even 
proposed to account for the excessive vegetation of the 
primitive world upon the supposition, that the atmo¬ 
sphere, at the period those plants were growing, con¬ 
tained a larger amount of carbonic acid in its composi¬ 
tion than it now does. This might have been considered 
misrepresentation, had not Liebig in many other in¬ 
stances displayed an equal amount of ignorance of 
botanical literature and facts. As, for example, when 
he says that the woody fibre of lichens may be replaced 
by oxalate of lime, and that in equisetum and the bam¬ 
boo silica assumes the form and functions of the woody 
bundles, and that a leaf secreting oil of lemons or oil 
of turpentine, has a different structure from one secre¬ 
ting oxalic acid. 
An instance of Liebig’s misrepresentation of facts 
occurs in his rejecting the theory of the respiration of 
plants. It is well known that plants absorb oxygen in 
the dark, and give out carbonic acid ; and this has been 
attributed by botanists to a true process of respiration. 
Tliis, Liebig thinks, betrays great ignorance on the 
part of botanists. He believes the giving out of the 
carbonic acid to be merely a mechanical process, and 
the absorption of oxygen to be a chemical one. He 
says all leaves, dead or living, absorb oxygen, and the 
more oil or tannic acid they possess, the more oxygen 
they absorb. He endeavors to prove this position by 
comparing, from tables made by De Saussure, the quan¬ 
tity of oxygen absorbed by the leaves of pinus abies, 
quercus robur, and populus alba, as compared with the 
quantity absorbed by the agave Americana. Mohl re¬ 
marks on this statement, that, in the first place, the 
quantity of oxygen absorbed by the agave is put down 
at 0.3, when it ought to have been at 0.8, so as to 
affect the calculations very considerably ; and that, in 
the second place, those plants in De Saussure’s table 
which contain neither oil nor tannic acid in any quan¬ 
tity, as the triticum sestivum and robinia pseudacacia, 
are altogether omitted, although they absorbed more 
oxygen than those mentioned by Liebig; while the 
| oily juniper and rue, which are also omitted, absorbed 
less. 
Again, Liebig states on this point, that the absorp¬ 
tion of oxygen has nothing at all to do with the pro¬ 
cesses of life. How is it, then, asks Mohl, that plants 
begin to be blighted when oxygen is withdrawn ; that 
seeds will not germinate; that leaves lose their irri¬ 
tability ; that the motions of leaves and flowers cease; 
that leaf-buds and flower-buds will not open when 
brought into an atmosphere without oxygen ? But the 
way to settle the question of respiration would appear 
to be to determine whether the asserted relation be¬ 
tween the quantity of oxygen absorbed, and the quan¬ 
tity of carbonic acid given out was wrong. In De 
Saussure’s experiments, there was found to be an exact 
relation in all cases. This would not be the case, 
were Liebig’s theory of the origin of the carbonic acid 
correct. 
These few general remarks, observes Dr. Mohl, will 
serve to indicate the claim of Liebig to become a re¬ 
former of botanical science. 
The next part of Dr. Mohl’s observations relates to 
the chapters of Professor Liebig’s book, inscribed “ The 
Assimilation of Carbon.” The question at issue is 
whether the plants owe tfreir carbon to the absorption 
of organic or inorganic substances, Professor Liebig 
having given his sanction to the latter opinion. The 
reasons which seem to have weighed most with him 
are, 1st, Humic acid loses its soluble character by ex- 
