can scarcely be described as having a distinct existence as a branch of chemistry. Much valuable work, it is true, had already been done, especially by his two great predecessors, de Saussure and Boussingault; but it was, down to the year 1840, a science made up of isolated facts. Liebig's genius formed it into an important branch of chemistry, supplied the necessary connection between the facts, and by a series of brilliant generalisations formed the principles upon which all subsequent advance has been built.
As has already been indicated, Liebig's chief claim to rank as the greatest agricultural chemist of the century does not rest upon the number or value of his actual researches, but on the formative power he exercised in the evolution of the science. His master-mind surveyed the whole field of agricultural chemistry, and saw laws and principles where others saw simply a confusion of isolated, and, in many cases, seemingly contradictory facts.
But great as the direct value of Liebig's work was, it may be questioned whether its indirect value was not even greater. The publication of his famous work had the effect of giving a general interest to questions which up till then had possessed a special interest, and that for comparatively few. Both on the Continent and in England a very large amount of discussion took place regarding his various theories.
Development of Agricultural Research in Germany.
It was especially in Germany, however, that Liebig's work bore its greatest and most immediate fruit. Thanks to the great chemist, the German Government recognised the importance of forwarding scientific research by State aid. Agricultural Departments were added to some of the universities, largely at State expense, while agricultural research stations were, one after another, instituted in different parts of the country.
The first of the agricultural research stations to be founded was the now famous one of Möckern, near Leipzig. It was instituted in the year 1851. Others followed, until at the present day there are some seventy to eighty of these Versuchs-Stationen scattered throughout Germany, all well equipped and doing excellent work. Some idea of the activity of the German stations may be inferred when it is stated that up to the year 1877 the total number of papers embodying the results of their experiments published by them amount to over 2000.[16]
To trace the development of agricultural chemistry, subsequent to Liebig's time, in the way it has been done prior to the year 1840, is no longer possible. This is due to the enormous increase in the number of workers in the field, as also to the overlapping nature of their work, which renders a strict chronological record wellnigh an impossibility. It will be better, therefore, to attempt to give a brief statement of our present knowledge on the subject, naming the chief workers in the various departments of the subject.
The Rothamsted Experiments.
Before doing so, it is fitting that reference should be made to the work and experiments of two living English chemists, who have done much to contribute to our knowledge in every branch of the science—viz., Sir John Lawes, Bart., and Sir J. H. Gilbert, F.R.S.
The fame of the Rothamsted experiments is now world-wide; and no single experiment station has ever produced such an amount of important work as the magnificently equipped research station at Rothamsted. The Rothamsted station may be said to date from 1843, although Sir John Lawes was engaged in carrying out field experiments for ten years previous to that date.[17] In 1843 Sir John Lawes associated with himself the distinguished chemist Sir J. H. Gilbert, and the numerous papers since published have almost invariably borne the two names. The expense of working the station has been borne entirely by Sir John Lawes himself; who has further set aside a sum of £100,000, the Laboratory, and certain areas of land, for the continuance of the investigations after his death. The fields under experimentation amount to about fifty acres. By a Trust-deed, which was signed on February 14, 1889, Sir John Lawes has made over the Rothamsted Experimental Station to the English nation, to be managed by trustees.
It is impossible to enter, in any detail, into the nature and scope of the Rothamsted experiments.[18] It may be stated that, since the year 1847, some eighty papers have been published on field experiments, and experiments on vegetation; while thirty papers have been published recording experiments on the feeding of animals.[19]
What has all along characterised these valuable experiments has been their practical nature. While their aim has been entirely scientific, the scale of the experiments and the conditions under which they have been carried out, have been such as to render them essentially technical experiments. For this reason their results possess, and will always possess, a peculiar interest for every practical farmer.
The greatest services the Rothamsted experiments have rendered agricultural chemistry have been the valuable contributions they have made to our knowledge of the function of nitrogen in agriculture; its relation in its different chemical forms to plant-life; and the sources of the nitrogen found in plants. Researches of a most elaborate nature have been carried out on what is still one of the most keenly debated questions of the present hour—viz., the relation of the "free" nitrogen in the atmosphere to the plant. Of the very highest value also have been the elaborate researches of Mr. R. Warington, F.R.S., on the important question of Nitrification, which have been in course in the Rothamsted Laboratory for the last fifteen years, and to which full reference will be made in the chapter on Nitrification.
To the Rothamsted experiments also we owe the refutation of Liebig's mineral theory. In fact it may safely be said that no experimenters in the field of agricultural chemistry have made more numerous or valuable contributions to the science than these illustrious investigators.
Review of our present Knowledge of Agricultural Chemistry.
Some attempt may now be made to indicate briefly our present knowledge of the more important facts regarding plant physiology, agronomy, and manuring.
Proximate Composition of the Plant.
The great advance made in the direction of the improvement of the accuracy of old analytical processes and the discovery of numerous new ones have furnished us with elaborate analyses of the composition of plants. We now know that the plant-substance is made up of a large number of complex organic substances, formed out of carbon, hydrogen, oxygen, and nitrogen,[20] and that these substances form, on an average, about 95 per cent of the dry vegetable matter; the other 5 per cent being made up of mineral substances. As to the source of these different substances, our knowledge is, on the whole, pretty complete. With regard to the carbon of green-leaved plants, which amounts to from 40 to 50 per cent, subsequent research has confirmed Sénébier and de Saussure's conclusions, that its source is the carbonic acid gas of the air. The decomposition of the carbonic acid gas is effected by the leaves under the influence of sunlight. That a certain quantity of carbon may be obtained from the carbonic acid absorbed by plant-roots, is indeed probable. Especially during the early stages of plant-growth this source of carbon may be of considerable importance. Generally speaking, however, it may be said of all green-leaved plants, that the chief source of their carbon is the carbonic acid gas in the atmosphere.
Carbon Fixation by Plants.
The exact way in which this decomposition of carbonic acid gas is effected by the leaves is not yet clear. It seems to be directly dependent, in some way or other, on the chlorophyll, or green colouring matter. This decomposition of carbonic acid, and the fixation of the carbon by the plant with the formation of starch, takes place only under the influence of sunlight. During the night a reflex action takes place, which is commonly known as respiration, and which is exactly analogous to animal
