If we leave out of the calculation this last cause of loss, which it is impossible to estimate and which is doubtless of little importance under ordinary circumstances, we shall find that a piece of arable land of average quality loses, by exhaustion from the crops, the infiltration of rain water and the ammonia which it disengages, an amount of nitrogen equal to a minimum of 120 kilograms per hectare annually. Therefore, as its soil contains scarcely 10,000 kilograms, its exhaustion would be complete in less than a century if these losses were not compensated by gains of about the same extent. Let us now examine into these causes of gain.

The soil receives nitrogen principally by the fertilizers given to it. Their proportion and richness are very variable, but experience shows that in general they do not suffice to supply the loss occasioned by cultivation alone. The difference which is found between the quantity of the nitrogen contained in the crop and that contained in the fertilizers is sometimes very great. Boussingault, to whom we are indebted for very precise researches on this subject, mentions a field where lucerne grass and wheat were cultivated, which having originally received 225 kilograms of nitrogen in the form of manure, furnished in a space of six years 44,000 kilograms of dry hay and 5,550 kilograms of wheat, straw, and grain, containing altogether 1,078 kilograms of nitrogen. The total excess, 854 kilograms, amounts in this case to a little more than 140 kilograms per hectare annually. In general, this difference is less, but, I repeat, it is always in the same direction and may be estimated on an average at 30 or 40 kilograms annually; it remains then for us to provide for this excess, increased as it is by the losses caused by drainage of nearly 100 kilograms per hectare annually.

The most diverse and sometimes the most improbable reasons have been brought forward to account for this fact. It has even been suggested that the atmospheric dust acted as a natural fertilizing agent; but let us go on to more serious hypotheses. It has been thought that the rain water in taking from the air its soluble compounds might furnish a certain proportion of ammonia or nitric acid to the soil. Analysis has shown that this proportion is extremely small; water caught in a rain gauge contains, indeed, only a mere trace of nitric substances, scarcely 2 grams of ammonia and less than 1 gram of nitric acid per cubic meter, which corresponds to a maximum of 5 to 8 kilograms of nitrogen a year per hectare. This quantity would, then, be barely sufficient to compensate for the losses due to the gaseous ammoniacal emanations from the earth.

On the other hand, Schloesing admits that the earth, and the plants by means of their foliage, directly attract the ammonia existing in the air. This ammonia, according to the learned agronomist, is constantly emitted by the sea water, which thus restores to us under another form the nitrogen which is constantly being brought to it by the drainage water.

It is certain that humid soil can attract the ammoniacal vapors, but it is also certain, as proved by the experiments of Boussingault, that such soil can also emit them; there is, therefore, a tendency to establish, in this respect, an equilibrium between the soil and the atmosphere, the result of which is probably not far from a perfect compensation.

If, then, it is true that the leaves of plants can assimilate gaseous

ammonia, we know that the average air contains extremely few nitric compounds. According to the analyses made first by G. Ville and later by Schloesing, the atmosphere contains at most from 25 to 30 grams of ammonia per cubic kilometer. It would, therefore, be necessary, in order to provide for the loss which we have just spoken of, that the soil and its plants should absorb in the space of a year all the ammonia contained in a column of air having the surface of the field for its base and a height of 400 kilometers under a constant pressure equal to the barometric height at sea level. This is about 50 times the quantity required for the carbonaceous nutrition of a crop weighing when dry 5,000 kilograms.

Such an hypothesis is inadmissible; besides, if it were correct we should not be able to understand why a crop of gramineæ cultivated in a sterile soil, aided only by a small quantity of fertilizer, never contains more nitrogen than was contained in the seed and in the manure given to it.

The above-mentioned deficiency, then, always remains, whichever way we look at it. Let us see if it is real or if the soil receives any compensation.

Since the application of chemical analysis to agricultural researches no decrease in the average fertility of our arable lands has been discovered; on the contrary, many have become richer in consequence in the improvements in the methods of cultivation and, above all, in the regular use of fertilizers. They have therefore become more productive, and the average yield of wheat in France, which, at the beginning of this century, was only at the rate of 11 hectoliters to the hectare, has gradually risen to 15 and 16 hectoliters. This fact alone is in direct opposition to the hypothesis of a gradual impoverishment of the soil. Here are other objections more striking still:

The forests, the meadows high up on the mountains, which are never manured, have from the remotest ages furnished, in the form of wood, milk, cheese, wool, or viands, quantities of nitrogen inferior, no doubt, to what it would be under a more intense cultivation, but constant and without the soil which produces them showing the least sign of exhaustion.

This virgin soil is even more fertile than our best arable lands. In Auvergne Truchot saw meadow lands containing 9 grams of combined nitrogen per kilogram; Joulie mentions some which contain 1.5 grams, and 1.8 grams per 100 of nitrogen, while land of good quality on which cereals were cultivated yielded ordinarily ten times less. Finally, and it is with this that we terminate this part of our subject, certain plants, among which we must place in the first rank grasses of natural or artificial meadows, cause a progressive enriching of the soil even in the absence of every species of fertilizer, and notwithstanding that they contain more nitrogen than other crops, said to be exhausting, such as the root plants and cereals.

Practical agriculture has long since demonstrated this fact in regard to leguminous plants; all farmers know that wheat planted after a crop of clover or of lucerne grass yields a much better harvest than it would have done under the most copious fertilizing, and it is for this reason they speak of the leguminous plants as ameliorators or natural fertilizers of the soil.

The action of natural meadows in enriching arable soils is of the same nature; here follow some curious results on this subject which I have borrowed from the works of Messrs. Lawes and Gilbert and those of Déhérain.

In 1856 Messrs. Lawes and Gilbert transformed into meadow lands a portion of the domain of Rothamsted, which for a long series of years had been used only for raising grains. The soil contained then 1.52 grams per 1,000 of nitrogen; it was manured regularly and in what would be called excessive doses in such a way that the nitrogen of the fertilizers always exceeded that of the crop by about 15 kilograms every year.

It is evident that they could not pretend with this small surplus to compensate entirely for the losses caused by the drainage; nevertheless the soil, instead of becoming impoverished, was constantly enriched, and at the end of the year 1888 its proportion of nitrogen was 2.35 grams per 1,000-that is to say, 0.83 gram more than at the beginning. This difference corresponds to a total of 1,813 kilograms to the hectare for the entire time that the experiment lasted—that is to say, an annual gain of 50 kilograms per hectare.

The phenomenon is moreover progressive, and nothing in its rate gives any reason for supposing that it is approaching its limit.

At the experiment field of Grignon, my learned instructor, Déhérain, observed similar facts. From 1875 to 1879 he raised beets and maize for fodder upon a piece of land freshly cleared of lucerne grass and containing a proportion of 2.05 per 1,000 of nitrogen. In spite of the fertilizers given to it during that time, the land became rapidly impoverished, no doubt from excessive nitrification, and in 1879 its fertility had declined to 1.50 grams-that is to say, to about three-quarters of its former value.

The maize was then replaced by French grass [sainfoin] from 1879 to 1883, then with a meadow of Gramineæ from 1884 to 1888, inclusive, this time, however, without giving it any kind of fertilizer. The soil then began gradually to increase in fertility and has now returned to its former state of richness.

Another experiment very similar to the preceding, but in which they had not manured the soil since 1875, gave nearly identical results.

If we admit that at Grignon the soil of a hectare weighs on an average 4,000 tons, we see that in ten years, from 1879 to 1888, the soil gained under the influence of the prairie grass alone 1,920 kilograms of nitrogen, to which we must add 1,210 kilograms taken away by the crops, or a total of 3,130 kilograms, or more than 300 kilograms a year per hectare.

Here again the limit is far from being attained, and it can be easily understood that soils subjected to this treatment would in time. come to contain 10 grams per 1,000, or a hundredth or more of nitrogen, like the meadows mentioned by Messrs. Truchot and Joulie.

It is clear that this natural phenomenon can not be owing to the contributions of nitric compounds brought by the rain water or by the atmosphere, for, even by attributing to these sources a power much beyond that which we have recognized as belonging to them, all plants should then behave in the same manner; whereas we have seen that we must distinguish between the cereals which impoverish the soil continually and the leguminous plants which always enrich it.

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Lawes and Gilbert have thought to find an explanation of the ameliorating influence of leguminous plants on the soil in the fact that plants of that kind generally have very long roots and are therefore able to go much deeper in search of their nourishment than the depth at which the roots of the Gramineæ are developed; the enriching of the earth would therefore be due to the organic débris that cultivation leaves there after the harvest, the nitrogen in which had been taken from the subsoil. The defect of this view is that the fertility of the soil decreases rapidly as the depth increases, and in the majority of cases the subsoil contains only such very insignificant quantities of nitrogen that it is impossible to conceive that any plant could be nourished by it, particularly a leguminous plant which contains in its tissues five or six times more nitrogen than does a Gramineæ.

In a word, the most simple observations of practical agriculture show us that the amount of nitrogenous substances furnished by nature would not suffice for the requirements of vegetation; it is therefore indispensable that gaseous nitrogen should interpose directly, and that, too, to an important extent, at least for the cultivation of leguminous plants.

Mr. G. Ville proved this experimentally as early as 1849, and he has not ceased repeating it since then, in spite of the systematic opposition of most physiologists and agronomists.

The primitive experiment of Mr. G. Ville has now become, by recent labors in connection with it, an established fact. Allow me, then, to describe it briefly, dwelling principally upon its results.

In a sterile soil, containing at least 1 kilogram of calcined sand, various leguminous plants, such as peas, beans, lupins, and others, were sown; then were added some nutritive substances, either mineral substances alone or a mixture of mineral fertilizers with a small quantity of nitrate of soda, the object of which was to aid the young plant to pass safely over the critical period of its growth, or, in other words, the time when, having exhausted the alimentary reserves provided for it by its cotyledons, it must henceforth nourish itself with substances entirely inorganic.

The plants were watered with pure water free from ammonia; every precaution was taken to assure the aeration of the soil; finally the plants were kept in as pure an atmosphere as possible, either in a glass cage, where from time to time carbonic-acid gas was introduced, or, what is preferable in the open air, far from the laboratory, and, in general, far from everything which could contribute to the disengagement of ammonia.

Under these conditions, and particularly when the soil received no nitrogenous fertilizer, the plant remained puny at first, suffering from what the German physiologists have called "nitrogen famine." Some plants even do not survive this painful stage of their existence, but die without having sensibly increased their dry weight; others, more vigorous, yield a mediocre crop; finally some, by the side of other dying stalks, become suddenly very flourishing. Upon the first stalk, which up to that time has been lank and without strength, a new stalk seems in some way to graft itself-stronger, stiff, turgescent-which soon becomes covered with broad, well-developed leaves of a green that are entirely different from the yellowish tint of the

first leaves, and this plant is soon as full of flowers and fruit as if its entire growth had taken place in a soil of excellent quality. The crop is then very good. It contains a large quantity of nitrogen, which evidently could only come to it from the atmosphere.

This recrudescence of vegetation shows itself at a time when the weight of the plant is eight or ten times that of the seed, and similar contrasts are often observed in two stalks grown in the same pot, which are, therefore, consequently in the same soil, under the same conditions, the seeds being as similar as possible.

In a word, the experiments of Ville teach us two unforeseen and equally remarkable facts. The first and most important is that a leguminous plant can live and prosper in a soil entirely destitute of all nitrogenous compounds, thus necessitating the direct assistance of the atmosphere; the second is that all seeds of the same kind are far from behaving in the same manner, whence it results that the course of the experiment is eminently uncertain.

With plants of the family of the Gramineæ nothing similar takes place. The results are absolutely invariable; the crop is zero if the soil does not contain nitrogenous substances. It increases regularly with the quantity of fertilizer, and each seed produces about the same weight of dry material.

The irregularity of the results given by the leguminosa under the same conditions shows that there could be in this case no question as to the accidental gains of nitrogen, attributable to ammonia or to atmospheric dusts, or to the water used in watering; the fact had been discovered, but its true cause had escaped the discoverer.

G. Ville, convinced of the correctness of the positive results obtained by him, was certainly right in concluding from them that certain kinds of plants attract carbonic-acid gas, but he was not master of his experiment. Other observers also tried to repeat it after him, but did not succeed. Boussingault, in particular, having placed his plants in spaces that were too restricted to allow of the free development of their roots, only obtained stunted plants weighing scarcely four or five times as much as the seed and containing no more nitrogen than the latter, because they had never attained the second stage of their growth.

In consequence Boussingault, who, however, had several years before obtained results similar to those of Ville, thought himself justified in laying down as a principle that vegetables, no matter to what variety they belong, are always incapable of taking even the smallest quantity of nitrogen from the air.

I shall not dwell upon this discussion, which has remained celebrated and which is very much to be regretted, inasmuch as the result of it was that by deterring those students who would have liked to pursue the study of the question further its definitive solution was retarded for thirty years. I only wish here to confine myself to a single point in it, which is that the fixing of free nitrogen by plants was observed already in 1850, with all the characteristics of irregularity belonging to it and as they have been again described in recent physiological researches of German physiologists.

I now come to the recent works, and I shall commence by those of Berthelot, in which we shall be confronted by an entirely new ideathat of the interrelation of microscopic life and the phenomena of vegetable nutrition.