ammonia when it is not present already in sufficient quantity in the soil. On reading his chapter on this subject, we could not help granting-what we were indeed prepared to concede, and in which all recent practical and theoretical writers concur-that ammonia is of great value in promoting vegetation, and that it is one source of the nitrogen of plants; but when we found ourselves in the midst of a paragraph, which assumed it to be proved that ammonia is the only source from which their nitrogen is derived, we fancied that we must have inadvertently missed an important step in the argument, and we turned back again and carefully retraced our steps in search of the missing link in the chain of rea. soning-but in vain. It is proved that ammonia is very useful to vegetation, and ought never to be wasted in good husbandry; but it is thence assumed by Liebig to be the only source of nitrogen to living vegetables. We are the more particular in insisting upon this, because the writer, borne away by his own belief, expresses himself as if he had really deduced his opinion from legitimate premises, and because we have seen many notices of his book, in which, what is merely an opinion of the writer, is really supposed to be proved. But we should be sorry to think that such an opinion was capable of proof, for we are quite sure that it does not truly represent the ordinary procedure of nature. The author of the works now before us, appears to be so far of the same opinion with ourselves, that he mentions another important source of nitrogen to the general vegetation of the globe, as he expresses it, besides the ammonia-namely, nitric acid, a compound consisting, as we have already said, of nitrogen and oxygen only. The arguments upon which he rests this opinion are stated in his eighth lecture, and are perfectly satisfactory. To us one authority, not quoted by Mr Johnston, is conclusive. In a work of the celebrated Berzelius of Stockholm, now before us, and which was written in 1829, long before the opinions now in question were agitat ed, it is stated, "that every flash of lightning that passes through the air must produce a not inconsiderable quantity of nitric acid;" and we have seen enough of the incessant lightnings of South America, and of the thunders that precede the monsoons in India, to satisfy us that the quantity of nitric acid produced in the air must be really enormous. When, in addition to this, we recollect that the nitrates of potash and soda have been observed in all climates to promote vegetation when artificially applied to plants, we are inclined to think that in the following summary of his views, which for the sake of brevity we extract from his Elements, the agency of nitric acid is rather under than over stated by our author. "Nitric acid is also naturally formed, and in some countries probably in large quantities, by the passage of electricity through the atmosphere. The air, as has been already stated, contains much oxygen and nitrogen mixed together, but when an electric spark is passed through a quantity of air, a certain quantity of the two unite together chemically, so that every spark that passes forms a small portion of nitric acid. A flash of lightning is only a large electric spark; and hence every flash that crosses the air produces along` its path a quantity of this acid. Where thunderstorms are frequent, much nitric acid must be produced in this way in the air. It is washed down by the rains, in which it has frequently been detected, and thus reaches the soil, where it produces one or other of the nitrates above mentioned. "It has been long observed that those parts of India are the most fertile in which saltpetre exists in the soil in the greatest abundance. Nitrate of soda, also, in this country, has been found wonderfully to promote vegetation in many localities; and it is a matter of frequent remark, that vegetation seems to be refreshed and invigorated by the fall of a thunder-shower. There is, therefore, no reason to doubt that nitric acid is really beneficial to the general vegetation of the globe. since vegetation is most luxuriant in those parts of the globe where thunder or lightning are most abundant, it would appear as if the natural production of this compound And body in the air, to be afterwards brought to the earth by the rains, were a wise and beneficent contrivance by which the health and vigour of universal vegetation is intended to be promoted. "It is from this nitric acid, thus universally produced and existing, that plants appear to derive a large, probably, taking vegetation in general, the largest portion of their nitrogen. In all climates they also derive a portion of this element from ammonia; but less from this squrce in tropical than in temperate climates." Besides nitric acid and ammonia, manufacture by art those necessaries and luxuries for which he is now wholly dependent on the vegetable kingdom. there are other substances to which we believe plants owe a part of their nitrogen; but these, we allow, are to be considered as the most important, and we shall therefore hasten forward to the consideration of other topics. "And having conquered the winds and the waves by the power of steam, is man really destined to gain a victory over the uncertain seasons too? Shall he come at last to tread the soil beneath his feet as a When plants have obtained carbon, in the form of carbonic acid, hydro- really useless thing to disregard the ge gen and oxygen in the form of water, and nitrogen in that of nitric acid and ammonia-how does the living plant transform these substances, so as to convert them into portions of its own substance? By what power is the alteration effected-by what succession of chemical changes is the final result brought about? This is a most interesting part of the subject, which our limits prevent us from attempting to enter upon. We may merely state, that the author shows, in a rigorously exact manner, how, in the interior of the plant, the starch of the seed is first converted into the sugar of the sap, how this is then converted into starch, or into woody fibre, and the latter again, when it is required, brought back into the state of starch or sugar. He shows also, how nearly all these changes can be imitated by the art of the chemist in his laboratory; and hence infers that what takes place in the interior of the plant, is no way mysterious in kind-it is mainly a series of purely chemical changes. But the mystery in living vegetables, as in living animals, is the process by which all these changes are regulated which says, here such a change must A take place, and to such an extent. living plant is a cunningly contrived machine, of which chemistry can describe the wheels and the movements, but of which she may never be able to discover, certainly she can never hope to supply, the moving power. What says our author? "Looking back at the vast strides which organic chemistry has made within the last twenty years, and is still continuing to make, and trusting to the continued progress of human discovery,—some sanguine chemists venture to anticipate the time when the art of man shall not only acquire a dominion over that principle of life, by the agency of which plants now grow and alone produce food for man and beast, but shall be able also, in many cases, to imitate or dispense with the operations of that principle; and to predict that the time will come when man shall nial shower, to despise the influence of the balmy dew-to be indifferent alike to rain and drought, to cloud and to sunshine to laugh at the thousand cares of the husbandman-to pity the useless toil and the sleepless anxieties of the ancient tillers of the soil? Is the order of nature through all past time to be reversed? Are the entire constitution of society, and the habits and pursuits of the whole human race, to be completely altered by the progress of scientific knowledge? "By placing before man so many incitements to the pursuit of knowledge, the will of the Deity is that out of this increase of wisdom he should extract the means of increased happiness and enjoyment also. But set a man free from the necessity of tilling the earth by the sweat of his brow, and you take from him at the same time the calm and tranquil pleasures of a country life-the innocent enjoyments of the returning seasons-the cheerful health and happiness that wait upon labour in the free air and beneath the bright sun of heaven. And for what? only to imprison him in manufactories, to condemn him to the fretful and feverish life of crowded cities. "To such ends, I trust, science is not destined to lead; and he is not only unreasonably, but thoughtlessly sanguine, who would hope to derive from organic chemistry such power over dead matter as to be able to fashion it into food for living animals. With such consequences before us, it seems almost sinful to wish for it." The second part of the Lectures is devoted to the inorganic constituents of plants, and to the geological and chemical relations of soils. We shall first complete our survey of the newest views in regard to the food of plants. We have already adverted to the fact that the various parts of vegetables, when burned in the air, leave behind a comparatively inconsiderable proportion of earthy or incombustible matter. Such is the common woodash, and such the ash left when heaps of weeds or the parings of our grass fields are burned. What is the use of this incombustible matter-what purpose does it serve in the plant 14 H USUS IS that the = les to mined to TRUIM NT NO THE ་་ 4 runt in those བའི་ ོགས ས འ པར འོན་ * the area wise and c is in manufacture by art those necessaries and luxuries for which he is now wholly dependent on the vegetable kingdom. there are other substances to which we believe plants owe a part of their nitrogen; but these, we allow, are to be considered as the most important, and we shall therefore hasten forward to the consideration of other topics. When plants have obtained carbon, in the form of carbonic acid, hydrogen and oxygen in the form of water, and nitrogen in that of nitric acid and ammonia-how does the living plant transform these substances, so as to convert them into portions of its own substance? By what power is the alteration effected-by what succession of chemical changes is the final result brought about? This is a most interesting part of the subject, which our limits prevent us from attempting to enter upon. We may merely state, that the author shows, in a rigorously exact manner, how, in the interior of the plant, the starch of the seed is first converted into the sugar of the sap, how this is then converted into starch, or into woody fibre, and the latter again, when it is required, brought back into the state of starch or sugar. He shows also, how nearly all these changes can be imitated by the art of the chemist in his laboratory; and hence infers that what takes place in the interior of the plant, is no way mysterious in kind-it is mainly a series of purely chemical changes. But the mystery in living vegetables, as in living animals, is the process by which all these changes are regulatedwhich says, here such a change must take place, and to such an extent. living plant is a cunningly contrived machine, of which chemistry can describe the wheels and the movements, but of which she may never be able to discover, certainly she can never hope to supply, the moving power. What says our author? A "Looking back at the vast strides which organic chemistry has made within the last twenty years, and is still continuing to make, and trusting to the continued progress of human discovery,- some sanguine chemists venture to anticipate the time when the art of man shall not only acquire a dominion over that principle of life, by the agency of which plants now grow and alone produce food for man and beast, but shall be able also, in many cases, to imitate or dispense with the operations of that principle; and to predict that the time will come when man shall "And having conquered the winds and the waves by the power of steam, is man really destined to gain a victory over the uncertain seasons too? Shall he come at last to tread the soil beneath his feet as a really useless thing to disregard the genial shower, to despise the influence of the balmy dew-to be indifferent alike to rain and drought, to cloud and to sunshine -to laugh at the thousand cares of the husbandman-to pity the useless toil and the sleepless anxieties of the ancient tillers of the soil? Is the order of nature through all past time to be reversed? Are the entire constitution of society, and the habits and pursuits of the whole human race, to be completely altered by the progress of scientific knowledge? "By placing before man so many incitements to the pursuit of knowledge, the will of the Deity is that out of this increase of wisdom he should extract the means of increased happiness and enjoyment also. But set a man free from the necessity of tilling the earth by the sweat of his brow, and you take from him at the same time the calm and tranquil pleasures of a country life-the innocent enjoyments of the returning seasons-the cheerful health and happiness that wait upon labour in the free air and beneath the bright sun of heaven. And for what? -only to imprison him in manufactories, to condemn him to the fretful and feverish life of crowded cities. "To such ends, I trust, science is not destined to lead; and he is not only unreasonably, but thoughtlessly sanguine, who would hope to derive from organic chemistry such power over dead matter as to be able to fashion it into food for living animals. With such consequences before us, it seems almost sinful to wish for it." The second part of the Lectures is devoted to the inorganic constituents of plants, and to the geological and chemical relations of soils. We shall first complete our survey of the newest views in regard to the food of plants. We have already adverted to the fact that the various parts of vegetables, when burned in the air, leave behind a comparatively inconsiderable proportion of earthy or incombustible matter. Such is the common wood ash, and such the ash left when heaps of weeds or the parings of our grass fields are burned. What is the use of this incombustible matter-what purpose does it serve in the plant— whence is it derived? Is it only present by accident-or does the healthy plant always contain it? Is its quantity constant, and does the same plant always yield the same kind of ash? For an answer to all these questions we must refer the reader to the works before us; but it will be interesting to trace the progress of opinion upon this branch of vegetable chemistrynot only because the subject is exceedingly curious in itself, but because the present state of opinion on this subject is connected with all the recent improvements in agricultural practice, has suggested many of them, and is now directing the enlightened farmer, often unconsciously, in all his most promising practical investiga tions. The ash of plants, until a very recent period, was considered by the vegetable physiologists-who, indeed, esteemed themselves the judges paramount in this domain-as accidentally present, or at least as performing no important or necessary function in reference to vegetable life. No account was taken of it, therefore, in theory: and in practice-little as some may think agricultural practice in past times to have been affected by theory -in practice this opinion of the physiologists led to the adoption and jus. tification of the most pernicious system of husbandry; for they naturally reasoned thus: If the organic or combustible part of plants be the only important, vital or essential portion, then of manures such as are prepared in the farm-yard, it is also the only essential part and in soils the vegetable or animal matter-the combustible parts of them, that is, which alone supply the plant with organic food, must likewise be the only essential portion. Accordingly, vegetable manures, or animal excrements, directly derived from vegetables, were alone considered necessary to fertilize the land and to feed the crop; and the vegetable matter in the soil was esteemed to be at once the source and the index of its agricultural capabilities. So firmly rooted was the opinion, and so widely spread, and so long has it lingered among practical men, that some of our readers may, even while they peruse these sentences, be unwilling to acknowledge, or have had no opportunity of being satisfied, that it is entirely erroneous. However, among other errors in practice, it led to the total neglect of the inorganic or incombustible part of the crops carried from the land. It was supposed that if a sufficient quantity of vegetable matter was always kept in the soil, there could be no harm in carrying off what the land produced, or in neglecting that part of the crop which was of no economical value for any other purpose. this by a reference both to still existing, and to many now exploded practices among ourselves, but we rather advert to a custom prevailing among our transatlantic brethren, as described in the very interesting work of one of the best race of our own existing practical farmers. They grow wheat, sell the grain, and either burn or otherwise waste the straw. After wheat they grow clover, which they never cut, but plough in for green manure. The vegetable matter in the soil is thus kept up, all the conditions of the old theory are fulfilled, and yet the land is inevitably going to ruin. The father may not be aware that he is eating sour grapes; but the children's teeth are sure to be set on edge nevertheless. We might illustrate Again, our West India planters, on their estates in Jamaica and elsewhere, find the land impoverished, and they hardly know why. There may be special local causes by which it is aided here and there, but the main cause is the same as in the United States. The canes are deprived of their juice in the mill, and the trash, as the refuse canes are called, is then burned to boil the sugar. The ash they leave is often considered as worse than useless-it is only a troublesome refuse. Can any one doubt, after such examples as these, that all our practice is modified and has been modified by theory, either for good or for bad? Not that theory has at once said to the farmer, Do this, and he hath done it, but the opinions of the vegetable physiologists gradually became a part of the general stock of knowledge among all well-informed men, and, through them, gradually influenced the agricultural practice in every country of Europe. Then these same theoretical opinions led to curious shifts and contrivances in nomenclature. It was now and then observed, that the addi |