carbon dioxide of the atmosphere without the co-operation of
the humus, and that consequently the favourable effect of a
soil rich in humus on vegetation must be due to other causes
than those which were assumed by the humus-theory. We
cannot describe the further services rendered by Boussingault
to the theory of nutrition, for this would take us too much into
technical details, and the best and most important of his
results were first given to the world after 1860, and do not fall
therefore within the limits of this history. But it should
be mentioned that Boussingault must be considered the
founder of modern methods of conducting experiments in
vegetation. Liebig had before spoken in terms of sufficient
severity of the miserable way in which experiments on the subject of the nutrition of plants were managed after de Saussure's
time till later than 1830, but he did not himself introduce better
methods; this was reserved for Boussingault. One instance
may be given; those who desired to decide the question of
the humus by experiment, such as Hartig in conjunction with
Liebig and others, generally adopted the plan of supplying
plants with compounds of humus-acid, and seeing what
would be the result. Boussingault did as Columbus with the
egg; he simply made plants supply themselves with food in a soil artificially deprived of all trace of humus and containing a mixture of food-material, in order to prove beyond question
that they do not need humus.
In Germany also Prince Salm-Horstmar made similar experiments to those of Boussingault; he occupied himself chiefly in determining the relative importance of the acids and bases of the ash in the nutrition of plants, whether any and which of them are indispensable; these are questions which approached their solution only after 1860, and some are not yet decided.
The establishment of the facts, that plants containing chlorophyll derive the whole of their carbon from the carbon dioxide of the atmosphere, and that the latter is also the original source of the carbon in plants and animals which do not contain
