permit. Now indeed many new friends will certainly contribute with pleasure to increase these means, and thereby to render the science itself capable of a more rapid growth and a more fructifying influence on practice. This is effected by making rational comparative experiments in the departments of the feeding of animals and plants. Such experiments however are only rational and instructive to science when not only the effect, but also all those circumstances which have operated and cooperated, and are necessary to the discovery of the causes of the observed effect, have been ascertained as accurately as possible.

In experiments on cultivation and manuring, therefore, the situation and mode of tillage, the character and composition of the soil and subsoil, must be taken note of as well as the character and composition of the manure applied, the weather of the year, the quality of the product obtained, and other note-worthy occurrences; likewise in feeding experiments, at least the principal constituents of the kind of food used. If the experiments give no information on these points, it is almost a pity that care and time should be employed on them, especially in regard to materials such as manures or food, which vary so much in their composition. What can the farmer or the chemist gain, when he learns, even from the most accurate figures, that guano and bone-dust as manure, or potatoes and hay or straw as food, have displayed very good effects in one case and very bad in others, unless he learns at the same time whether the guano contains 1 or 13 per cent. of nitrogen and 20 or 80 per cent. of phosphate of lime, whether the bone-dust was coarse or fine, pure or adulterated, whether the potatoes contained 12 or 24 per cent. of starch, the straw or 1 per cent. of nitrogen, and the hay 1 or 2 per cent.? From such experiments practice gathers nothing and science nothing, while both perhaps might have drawn the most valuable information from them, had the key been given by which the causes of the contradictory phænomena and experiences could be unlocked.

To those friends of Agricultural Chemistry, or what is the same, Rational Practice, who have no opportunity of getting the chemical investigations required to complete these practical experiments executed in their own neighbourhood, I freely offer my assistance. It will be a true

delight to me if German agriculture right often places me in the position, to set in motion the abundant chemical means and operative power, now placed at my disposal by an enlightened government, for the simultaneous enlargement of the fields of practice and of science.

The high value attached even by Thaer, in his day, to comparative experiments, is witnessed by the following words from him:-"Such accurate experiments are of course not easy, but they are in the power of every thinking farmer. Whosoever carries out correctly and carefully, and reports clearly and truly a single one, be it ever so small, promotes the science, and consequently at the same time the practice of agriculture, and earns thereby a claim to the thanks of his contemporaries and successors. To make many experiments of this kind will be rarely permitted indeed to one individual; but it should be regarded as the first and most important object of societies collected for the promotion of agriculture, to lay down plans for such experiments and distribute them for execution among their members." The Royal Agricultural Society of England has placed this sentence of our great master as the motto at the head of the latest volume of their excellent journal; may it also secure in Germany true and productive attention!

350

APPENDIX.

LIEBIG'S MINERAL THEORY.

In a little work which has just appeared in England, ‘Principles of Agricultural Chemistry, with special reference to the late researches made in England,' Baron Liebig undertakes the explanation and defence of his doctrines against the adverse conclusions deduced by Mr. Lawes from his experiments at Rothampstead. The principles are laid down in fifty axioms, which however are by no means so accurately or definitely constructed as they might be, and the author undertakes to show that these are not only the same views originally propounded in his 'Agricultural Chemistry,' but that they are true and unassailable.

In reading over carefully his answers to Mr. Lawes's objections, and comparing the papers of that gentleman with this new work of Baron Liebig, it has appeared to us that the question is scarcely put upon its true footing by the latter, and that while Mr. Lawes devotes his attention principally to the practical deductions, Liebig dwells too exclusively upon certain abstract results, which, although scientifically correct, have not in most cases any direct practical application; and that in urging these, he does not quite fairly meet the practical question, indeed, so expresses his views as to render himself very liable to the kind of misconstruction of which he complains on the part of Mr. Lawes.

For example, in the 'Agricultural Chemistry,' 4th ed. p. 210, occurs the following passage:-" Hence it is certain, that in our fields the amount of nitrogen in the crops is not at all in proportion to the quantity supplied in the manure, that we cannot augment the fertility of our fields by supplying them with manures rich in nitrogen, or with am

moniacal salts alone. The crops on a field diminish or increase in exact proportion to the diminution or increase in the mineral substances conveyed to it in the manure.”

Liebig complains that Mr. Lawes has here taken notice only of the last sentence; and that, taking it without the context, he has totally misunderstood it. He says, “in the sentences just quoted from my book, the produce of the land is compared with the proportion of nitrogenous matter, inclusive of mineral substances, supplied in the manure, and with amount of mineral constituents, inclusive of nitroge nous substances, supplied in the manure.

"The words by ammoniacal salts alone' and 'in the manure,' show that I never thought of excluding carbonic acid and ammonia in the manure. According to Mr. Lawes's mistaken notion of my meaning, I ought to have said, omitting the word manure, that on the contrary the fertility of the land rises and falls with the amount of the mineral substances supplied to it.' But this I have not said.

"The meaning of these sentences in my work is this:that ammoniacal salts alone have no effect; that, in order to be efficacious, they must be accompanied by the mineral constituents, and that the effect is then proportional to the supply -not of ammonia, but of the mineral substances.'

"The following diagram will explain my meaning:

We have extracted this passage because it appears to us hat Baron Liebig here sets forth a true and clear principle, ut one which his works, both the present and former, have n indirect tendency to place in the background. In fact he mass of his arguments and the experiments quoted are

calculated to lead any one to take his experience in the sense Mr. Lawes did, and which Liebig says is not that which he intended.

For who indeed would question that, if the soil were absolutely devoid of the essential mineral constituents, no amount of ammonia and carbonic acid would produce a crop? It would be nearly the same as to suppose that soil is unnecessary, and that the plants would thrive equally well in water or damp air as in the earth. But the question is not of absolute deprivation at all: in the ordinary course of agricultural operations, and of real cases, in this country at least, the greater part would fall under the second rather than the first of the above diagrams.

The weight of Liebig's argument, however, is to impress the importance of considering the cases belonging to the first diagram, and in an abstract point of view there is some show of reason for this. The atmosphere does supply ammonia and carbonic acid, and thus the crops are not entirely dependent on the soil for those constituents; for the mineral constituents properly so-called, the soil has exclusively to provide, so that while the supply (theoretically) of ammonia and carbonic acid is unlimited, that of the minerals is limited by the composition of the particular soil.

We do not suppose any one will question this; but when it is urged that therefore the mineral substances are most important in manure, we pause and look at the practical question, and finding that the result of manuring experi ments is that the addition of ammoniacal manures (with wheat) ordinarily causes increased product, year after year, we reason that the mineral constituents are ordinarily in excess in the soil, and that consequently we get the full value out of the latter by applying the ammoniacal manure stimulus*; whereas the application of additional mineral manure, to that existing excess, is useless.

At the same time certain of the mineral constituents of the soil may be limited in quantity while others are in enormous excesst. Hence it may happen that the addition of certain mineral substances will become necessary at particular periods. But in such a case it would be a clumsy proceeding to give to the soil a mineral manure containing * See page 338 of this Volume.. + See Chap. iii.