researches formed the main groundwork of Liebig's treatise, as they had before done for Davy's; but the progress of science had supplied many new facts which confirmed the opinions of the older chemists in most respects, and enabled Liebig to generalise with greater confidence, and illustrate more fully the principles upon which chemistry ought to be applied to agriculture. Few works have ever produced a more profound impression. Written in a clear and forcible style, dealing with scientific truths in a bold and original manner, and producing a strong impression, as well by its earnestness as by the importance of its conclusions, it was received by the agricultural public with the full conviction that the application of its principles was to be immediately followed by the production of immensely increased crops, and by a rapid advance in every branch of practical agriculture. The disappointment of these extravagant expectations, which chemists themselves foresaw, and for which they vainly attempted to prepare the agriculturist, was followed by an equally rapid reaction; and those who had embraced Liebig's views, and lauded them as the commencement of a new era, but who had absurdly expected an instantaneous effect, changed their opinion, and contemned, as strongly as they had before supported, the application of chemistry to agriculture.

That this effect should have been produced is not unnatural; for practical men, having at that time little or no knowledge of chemistry, were necessarily unable to estimate its true position in relation to agriculture, and forgetting that this department of science was still in its early youth, and burthened with all the faults and errors of youth, they treated it as if it were already perfect in all its parts. Neither could they distinguish between the fully demonstrated scientific truths, and the uncertain, though probable conclusions deduced from them; and when the latter, as occasionally happened, proved to be at variance with practice, it is not surprising: that this should have produced a feeling of distrust on the part of persons incapable, from an imperfect, and still oftener from no knowledge of science, of drawing the line of demarcation, which Liebig frequently omitted to do, between the positive fact and the hypothetical inference, which, however probable, is, after all, merely a suggestion requiring to be substantiated by experiment. This omission, which the scientific reader can supply for himself, becomes a source of serious misapprehension in a work addressed to persons unacquainted with science, who adopt indiscriminately both the facts and the hypotheses of the author. And this is no doubt the cause of the vary different estimation in which the work of the Giessen Professor was held by scientific and practical men.

Liebig's treatise was followed, in the year 1844, by the publication of Boussingault's Economic Rurale, a work winch excited at the time infinitely less interest than Liebig's, although it is really quite as important a contribution to scientific agriculture. It is distinguished by entering more fully into the special details of the application of chemistry to agriculture, and contains the results of the author's numerous searches both in the laboratory and the field. Boussingault possesses the qualification, at present somewhat rare, of combining a thorough knowledge of practical agriculture with extended scientific attainments; and his investigations, which have been made with direct reference to practice, and their results tested in the field, are the largest and most valuable contribution to the exact data of scientific agriculture which has yet been made public.

The year 1844 was also distinguished by the foundation of the Agricultural Chemistry Association of Scotland, an event of no small importance in the history of scientific agriculture. That association was instituted through the exertions of a small number of practical farmers, for the purpose of pursuing investigations in agricultural chemistry, and affording to its members assistance in all matters connected with the cultivation of the soil, and has formed the model of similar establishments in London, Dublin, and Belfast, as well as in Germany; and it is peculiarly creditable to the intelligence and energy of the practical farmers of Scotland, that with them commenced a movement, which has already found imitators in so many quarters, and conferred such great benefits on agriculture. Within the last ten or twelve years, and mainly owing to the establishment of agricultural laboratories, great progress has been made in accumulating facts on which to found an accurate knowledge of the principles of agricultural chemistry, and the number of chemists who have devoted themselves to this subject has considerably increased, though still greatly less than its exigencies require.

Notwithstanding all that has recently been done, it must not be forgotten that we have scarcely advanced beyond the threshold, and that it is only by numerous and frequently repeated experiments that it is possible to arrive at satisfactory results. Agricultural inquiries are liable to peculiar fallacies due to the perturbing influence of climate, season, and many other causes, the individual effects of which can only be eliminated with difficulty, and much error has been introduced, by hastily generalising from single experiments, in place of awaiting the results of repeated trials. Hence it is that the progress of scientific agriculture must necessarily be slow and gradual, and is not likely to be marked by any great or startling discoveries. Now that the relations of science to practice are better understood, the extravagant expectations at one time entertained have been abandoned, and, as a necessary consequence, the interest in agricultural chemistry has again increased, and the conviction daily gains ground that no one who wishes to farm with success, can afford to be without some knowledge of the scientific principles of his art.

CHAPTER I.

THE ORGANIC CONSTITUENTS OF PLANTS.

When the water naturally existing in plants is expelled by exposure to the air or a gentle heat, the residual dry matter is found to be composed of a considerable number of different substances, which have been divided into two great classes, called the organic and the inorganic, or mineral constituents of plants. The former are readily combustible, and on the application of heat, catch fire, and are entirely consumed, leaving the inorganic matters in the form of a white residuum or ash. All plants contain both classes of substances; and though their relative proportions vary within very wide limits, the former always greatly exceed the latter, which in many cases form only a very minute proportion of the whole weight of the plant. Owing to the great preponderance of the organic or combustible matters, it was at one time believed that the inorganic substances formed no part of the true structure of plants, and consisted only of a small portion of the mineral matters of the soil, which had been absorbed along with their organic food; but this opinion, which probably was never universally entertained, is now entirely abandoned, and it is no longer doubted that both classes of substances are equally essential to their existence.

Although they form so large a proportion of the plant, its organic constituents are composed of no more than four elements, viz.:—

Carbon.
Hydrogen.
Nitrogen.
Oxygen.

The inorganic constituents are much more numerous, not less than thirteen substances, which appear to be essential, having