examination of this subject,
and drew from his experiments the conclusion, that plants absorb the
nitrogen of the air. Saussure shortly afterwards examined the same
subject, and having found, that when grown in a confined space of air,
and watered with pure water, the nitrogen of the plants underwent no
increase, he inferred that they derived their entire supplies of that
element from ammonia, or the soluble nitrogenous constituents of the
soil or manure. Boussingault has since re-examined this question, and by
a most elaborate series of experiments, in which the utmost care was
taken to avoid every source of fallacy, he was led to the conclusion,
that when haricots, oats, lupins, and cresses were grown in calcined
pumice-stone, mixed with the ash of plants, and supplied with air
deprived of ammonia and nitric acid, their nitrogen underwent no
increase. It has been objected to these experiments, that the plants
being confined in a limited bulk of air, were placed in an unnatural
condition, and Ville has recently repeated them with a current of air
passing through the apparatus, and found a slight increase in the
nitrogen, due, as he thinks, to direct absorption. It is much more
probable, however, that it depends on small quantities of ammonia or
nitric acid which had not been completely removed from the air by the
means employed for that purpose, for nothing is more difficult than the
complete abstraction of these substances, and as the gain of nitrogen
was only 0.8 grains, while 60,000 gallons of air, and 13 of water, were
employed in the experiment, which lasted for a considerable time, it is
reasonable to suppose that a sufficient quantity may have remained to
produce this trifling increase.

While these experiments show that plants maintain only a languid
existence when grown in air deprived of ammonia and nitric acid, and
hence, that the direct absorption of nitrogen, if it occur at all, must
do so to a very small extent, the addition of a very minute quantity of
the former substance immediately produces an active vegetation and rapid
increase in size of the plants. Among the most striking proofs of this
are the experiments of Wolff, made by growing barley and vetches in a
soil calcined so as to destroy organic matters, and then mixed with
small quantities of different compounds of ammonia. He found that when
the produce from the calcined soil was represented by 100, that from the
different ammoniacal salts was--