rel="nofollow" href="#ulink_b31b077b-7e1a-589c-8cde-86dea7c330d4">[21] The rate at which the fixation of carbon takes place depends on the strength of the sun's rays. It seems to take place very rapidly under a strong tropical sun.[22] The action of sunlight on the absorption of carbon has been studied by a number of observers, among others by Sachs, Draper, Cloez, Gratiolet, Caillet, Prillieux, Lommel, &c.
Action of Light on Plant-growth.
Experiments made by several observers, more especially Pfeffer, have shown that the yellow rays of the solar spectrum are the most potent in inducing this decomposition.
Some interesting experiments have been carried out by different observers on the possibility of growing plants under the influence of artificial light. While it would seem that the light from oil-lamps or gaslight is unable to promote growth, except in very exceptional cases, the electric light, or other strong artificial light, seems to be capable of taking the place of sunlight. Heinrich was the first to show that sunlight could be replaced by the magnesium light.
Experiments with the electric light have been carried out by Hervé-Mangon in France and Dr. Siemens in England. The plants grown under the influence of the electric light were observed to be of a lighter green colour than those grown under normal conditions, thus indicating a feebler growth; in fact, Siemens was of the opinion that the electric light was about half as effective as daylight.[23]
These experiments are interesting from an industrial point of view; for it is conceivable that at some distant time electricity might be called to the aid of the agriculturist.
Source of Plants' Oxygen.
With regard to the source of the oxygen, which, next to carbon, is the element most largely present in the plant's substance—amounting to, roughly speaking, about 40 per cent—all evidence seems to indicate that it is chiefly derived from water, which is also the source of the plant's hydrogen. In addition to water, carbonic acid and nitric acid may also furnish small quantities. It has been pretty conclusively proved that the atmospheric oxygen, while necessary to plant-growth, and promoting the various chemical vital processes, is not a direct source of the plant's oxygen. The important function played by atmospheric oxygen in certain stages of the plant's growth has been long recognised. Malpighi, nearly two hundred years ago, observed that for the process of germination atmospheric air was necessary; and shortly after the discovery of the composition of the air was made, oxygen was identified as the important gas in promoting this process. Oxygen is also especially necessary during the period of ripening.
Source of Plants' Hydrogen.
Hydrogen, which amounts to about 6 per cent, is, as has already been pointed out, chiefly derived from water. It is possible that ammonia also may form a source.
Source of Plants' Nitrogen.
When we come to treat of the source of the nitrogen, which is found in the plant's substance to an extent varying from a fraction of a per cent to about 4 per cent, we enter on a much more debated question.
What is the source, or, what are the sources, of plant-nitrogen? is a question to the solution of which more time and more research have been devoted than to the solution of any other question connected with agricultural chemistry.
The most obvious source is the free nitrogen, which forms four-fifths of the atmospheric air. Reference has already been made to this question.[24] Priestley was the first of the long list of experimenters on this interesting question.
As far back as 1771 he affirmed that certain plants had the power of absorbing free nitrogen; and this opinion he supported by the results of certain experiments he had made on the subject. Eight years later—viz., in 1779—Ingenhousz further supported this conclusion, and stated that all plants could absorb, within the space of a few hours, noticeable quantities of nitrogen gas. The first to oppose this theory was de Saussure, who, in 1804, carried out experiments which showed that plants were unable to utilise free nitrogen.
Subsequent experiments, carried out by Woodhouse and Sénébier, supported de Saussure's conclusions. Mention has already been made of Boussingault's elaborate researches on the subject.[25] His first experiments were carried out in 1838. He concluded that plants did not absorb free nitrogen. Georges Ville was the first to reassert the older theory, put forward by Priestley and Ingenhousz. His opinion was founded on experiments he had carried out during the years 1849–52. The subject created so much interest at the time, that a committee of the French Academy—consisting of Dumas, Regnault, Péligot, Chevreul, and Decaisne—were appointed to investigate Ville's experiments. The result of the investigation of the Commission was to confirm Ville's experiments. It is a significant fact, however, that the plant experimented with by the Commission was cress—a non-leguminous plant. It has been commonly assumed that the results of recent experiments have confirmed Ville's experiments. It is only proper to point out that this is not a necessary inference. The assimilation of free nitrogen by the leguminosæ, so far as modern research has revealed, only takes place under the influence of micro-organic life. Ville's experiments, however, were supposed to be conducted under sterilised conditions.
In the meantime the results of Boussingault's second series of experiments, carried out between the years 1851 and 1855, were published, and confirmed his earlier experiments.
The results of a large number of experiments subsequently carried out were in support of Boussingault's conclusions. Among them may be mentioned Mène, Harting, Gunning, Lawes, Gilbert and Pugh, Roy, Petzholdt, and Bretschneider.
Such an amount of overwhelming evidence might naturally have been regarded as conclusively proving that the free nitrogen of the air is not an available source of nitrogen to the plant. The question, however, was not decided. In 1876 Berthelot reopened it. From experiments he had carried out, he concluded that free nitrogen was fixed by various organic compounds, under the influence of silent electric discharges. In 1885 he carried out further experiments, from which he concluded that argillaceous soils had the power of fixing the free nitrogen of the atmosphere. This they effected, he was of opinion, through the agency of micro-organisms. Schloesing has recently shown that this fixation of free nitrogen by soils is extremely doubtful.[26] The gain of nitrogen observed under such conditions can be explained by the absorption by the soil of combined nitrogen—viz., ammonia—from the air.
Berthelot's early experiments in 1876 had the effect of stimulating a number of other experiments, with the result that we now possess the solution of this long-debated and most important problem.
The names of the better known investigators on this subject, in addition to Berthelot's, are those of Hellriegel, Wilfarth, Dehérain, Joulie, Dietzell, Frank, Emil von Wolff, Atwater, Woods, Nobbe, Ward, Breal, Boussingault, Wagner, Schultz-Lupitz, Fleischer, Pagnoul, Schloesing, Laurent, Petermann, Pradmowsky, Beyrenick, Lawes, and Gilbert.
It is impossible to enter into the details of these most important experiments. An attempt may be made, instead, briefly to epitomise them.
Recent Experiments on Nitrogen question.
In the first place, it may be asked, How is it possible that the previous elaborate experiments, published prior to 1876, should now prove unreliable? A satisfactory explanation may be found in the fact, as Lawes and Gilbert have recently pointed out, that the fixation of the free nitrogen by the plant, or within the
