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, Deherain, 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 soil, takes place, if at all, through the agency of electricity or of micro-organisms, or of both. The earlier experiments, however, were so arranged as to exclude the influence of either of those agencies. The question has further been limited in its scope. It is now supposed that only plants of the _leguminous_ order have the power of drawing upon the free atmospheric nitrogen. Of the experiments above referred to, those of Hellriegel and Wilfarth are the most striking and important. They found in their experiments, that while the legumes have the power of obtaining their nitrogen from the air, cereals have not. Similar experiments by Atwater in America, and others, support this conclusion. Their conclusions may be briefly epitomised as follows:-- (_a_) That the leguminous plants--such as peas, &c.--have the power of drawing their nitrogen supplies from the free nitrogen of the air in a way not possessed by other plants; and that they thus possess two sources of nitrogen--the soil and the air.