elements, and how, when introduced into the roots and leaves, do they be come changed into the substances of which full-grown plants consist? This is an exceedingly natural order, and each of the topics is clearly, and, so far as they admit of it, simply explained. The unlearned reader will, indeed, if he dip at once into the seventh or eighth lecture, find himself occasionally perplexed by new names and symbols of unknown signification; but if he begin at the beginning, he will find every thing easy and readily intelligible.

When vegetable substances, wheat, oats, potatoes, turnips, straw, hay, wood, &c., are burned in the air, they almost entirely disappear, leaving only from one to five, and in a few rare cases, as much as ten per cent of ash. That which burns away is called the organic part, and consists entirely of charcoal (carbon) in combination with one or more of three well-known gases, hydrogen, oxygen, and nitrogen. The properties of these elementary substances are first explained, and the attention drawn to the astonishing fact, that by the union of these four elements only, the skill of the Deity has known to build up all those varied forms of vegetable life that ornament the face of the dead earthministering at the same time to the delight of the eye, and to the support of the life of all living beings. After this comes the enquiry-in what form these four elementary substances enter into the circulation of plants? As to the hydrogen, it appears to be agreed on all hands that a sufficient supply enters into and always exists in the interior of plants in the form of water, which consists of hydrogen and oxygen only. The fact that water is composed of these two gases is also familiar to us; and yet how truly wonderful it is that hydrogen, which lifts balloons from the earth, and oxygen, which is to us the breath of life, and in which all bodies burn with an intolerable brilliancy-that these two gases, by the single snap of an electric spark, should be at once condensed, compressed, chained down as it were into a fluid which is the greatest enemy to fire, which is of weight enough to bear every thing before its resistless torrents, and on the surface of which, in the "great deeps," the mightiest works of man are tossed as

a child's plaything. No fiction which the mind of man ever conceived is half so wonderful as this one natural truth.

Water is the life of plants-" Can the rush grow without mire, can the flag grow without water?" The value of this liquid is more seen in tropical climates, but it strikes the willing observer wherever his lot may be cast. Its relations to vegetable life are explained at some length by our author, and we select, as an illustration of his mode of treating such subjects, what he says in regard to dew, and the more willingly, because he concludes with some general reflections which flow naturally from the considerations he has presented, and with which, from a purely scientific writer, we can cordially coincide.

"The dew, celebrated through all times and in every tongue for its sweet influences, presents the most beautiful and striking illustration of the agency of water in the economy of nature, and exhibits one of those wise and bountiful adaptations, by which the whole system of things, animate and inanimate, is fitted and bound together.

"All bodies on the surface of the earth radiate, or throw out rays of heat, in straight lines-every warmer body to every colder; and the entire surface is itself continually sending rays upwards through the clear air into free space. Thus on the earth's surface all bodies strive, as it were, after an equal temperature, (an equilibrium of heat,) while the surface as a whole tends gradually towards a cooler state. But while the sun shines this cooling will not take place, for the earth then receives in general more heat than it gives off; and if the clear sky be shut out by a canopy of clouds, these will arrest and again throw back a portion of the heat, and prevent it from being so speedily dissipated. At night, then, when the sun is absent, the earth will cool the most; on clear nights also more than when it is cloudy; and when clouds only partially obscure the sky, those parts will become coolest which look towards the clearest portions of the heavens.

"Now when the surface cools, the air in contact with it must cool also; and, like the warm currents on the mountain side, must forsake a portion of the watery vapour it has hitherto retained. This water, like the floating mist on the hills, descends in particles almost infinitely minute. These particles collect on every leaflet, and suspend themselves from every blade of grass, in drops of pearly dew.'

"And mark here a beautiful adapta

tion. Different substances are endowed with the property of radiating their heat, and of thus becoming cool with different degrees of rapidity; and those substances which in the air become cool first, also attract first and most abundantly the particles of falling dew. Thus in the cool of a summer's evening the grass plat is wet, while the gravel walk is dry; and the thirsty pasture and every green leaf are drinking in the descending moisture, while the naked land and the barren highway are still unconscious of its fall.

"How beautiful is the contrivance by which water is thus evaporated or distilled as it were into the atmosphere largely perhaps from some particular spots -then diffused equably through the wide and restless air-and afterwards precipitated again in refreshing showers or in long mysterious dews! But how much more beautiful the contrivance, I might almost say the instinctive tendency, by which the dew selects the objects on which it delights to fall; descending first on every living plant, copiously ministering to the wants of each, and expending its superfluity only, on the unproductive waste!

"And equally kind and bountiful, yet provident, is nature in all her operations, and through all her works. Neither skill nor materials are ever wasted; and yet she ungrudgingly dispenses her favours, apparently without measure-and has subjected dead matter to laws which compel

it to minister, and yet with a most ready willingness, to the wants and comforts of every living thing.

"And how unceasingly does she press this her example not only of unbounded goodness, but of universal charity-above all other men-on the attention of the

tiller of the soil! Does the corn spring more freshly when scattered by a Protestant hand are the harvests more abundant on a Catholic soil-and does not the sun shine alike, and the dew descend, on the domains of each political party?

"So science, from her daily converse with nature, fails not to take her hue and colour from the perception of this universal love and bounty. Party and sectarian differences dwindle away and disappear from the eyes of him who is daily occupied in the contemplation of the boundless munificence of the Great Impartial; he sees himself standing in one common relation to all his fellow-men, and feels himself to be most completely performing his part in life, when he is able in any way or in any measure to contribute to the general welfare of all.

"It is in this sense too that science, tracing the footsteps of the Deity in all his works, and from them deducing his intel

ligence and his universal goodness-it is in this sense that science is of no sect, or of no party, but is equally the province, and the property, and the friend of all,"

The atmosphere consists chiefly of hydrogen and nitrogen, but contains also a very small proportion of carbonic acid-one gallon in two thousand five hundred. This carbonic acid consists of oxygen and carbon only-it is the gas that escapes from soda water and sparkling champagne. The leaves and other green parts of plants, in the sunshine, absorb it from the air, decompose it, as chemists call the process, returning the oxygen to the atmosphere, and retaining the carbon. Plants thus clearly obtain carbon from the air, and in larger quantity in proportion to the extent of leaf they hang out, and to the duration of the sunshine. But the earth also contains vegetable matter, from which the roots draw part of their sustenance-of their carbon, that is-in the form of certain soluble organic compounds, which are naturally produced during the decay of vegetable and animal substances. But here physiologists and chemists are at real or apparent variance with each other, in regard to the amount of carbon which plants derive from the soil. The old physiologists, and some of the less instructed of the existing race, seeing that soils rich in vegetable matter gave generally luxuriant crops, that by adding vegetable and animal manures to these soils they were rendered still more productive, and that the quantity of carbonic acid in the atmosphere was so very small-have generally advanced and maintained the opinion that the sustenance of plants

their carbon-is chiefly derived from the soil-that what they draw from the air forms but a small portion of their actual substance. This opinion has been subjected by its defenders to strange twistings and stretchings, to account for such facts as these: A field almost destitute of vegetable matter is laid down to grass, when year by year, though depastured all the while, the vegetable matter increases, till at length four or six inches of rich, dark, vegetable mould are formed upon its surface. Or a waste is planted with trees, which every s shed their leaves though much is car and the entire fore

when the trees are of sufficient size, yet the soil contains more vegetable matter at the end of all this than it did at the beginning. Or a tree falls across a stream, dams up the water, and produces a marshy spot, rushes and weeds spring up, mosses take root and grow, year after year new shoots are sent forth and die, vegetable matter accumulates, a bog, and finally a thick bed of peat is formed. Physiologists of the old school may doubt, but common sense tells us that the increase of vegetable matter in all these cases-of its carbon, that is-must have been derived from the air.

This conclusion does not imply that a given plant or crop, that the individual grasses, or trees, or mosses, in our three cases, have not each derived a portion of their sustenance from the soil. The roots of our trees, for ex. ample, are continually drawing soluble organic matter from the soil, which they send up to the branches and leaves. But the quantity they return to it in the leaves they shed, and in the roots themselves, which remain buried, is something greater than what they thus send up; and thus the organic matter slowly increases. In our arable lands the same is shown by the slow decrease of vegetable matter through prolonged culture, and the consequent necessity of either adding a fresh supply of organic matter to maintain their fertility, or of leaving them for a time to a process of natural recovery. Still the question remains undecided between the two partieswhat portion of their carbon do plants thus derive from the soil, and what from the air? It appears that borage has been long grown in Germany, for the purpose of ploughing in as a green manure. Nearly twenty years ago Lampadius, who has done much good service to scientific agriculture, made an experiment with the view of determining the amount of vegetable matter with which this plant was capable of enriching the soil. This experiment led him to the conclusion that borage draws no less than nine tenths of its carbon from the air. Much, however, must depend upon the climate and soil; and later experiments have shown that the crops we usually cultivate for food, derive, on an average, about two-thirds of their carbon from the air, and, consequently that, if we add to the soil, in the form of manure,

one-third of what we take off in the form of a crop, we should maintain it in its existing state of richness, in so far as this depends upon vegetable matter, were there not other causes in operation which tend to lessen the amount of organic matter in soils that are continually turned up by the plough. Our author thus concludes his review of this question :

"Being thus fitted by nature to draw their sustenance-now from the earth, now from the air, and now from both, according as they can most readily obtain it-plants are capable of living,-though rarely a robust life-at the expense of either. The proportion of their food which they actually derive from each source, will depend upon many circumstances on the nature of the plant itself -on the period of its growth-on the soil in which it is planted-on the abundance of food presented to either extremity— on the warmth and moisture of the climate --on the duration and intensity of the sunshine, and upon other circumstances of a similar kind--so that the only general law seems to be, that, like animals, plants have also the power of adapting themselves, to a certain extent, to the conditions in which they are placed; and of supporting life by the aid of such sustenance as may be within their reach.

"Such a view of the course of nature in the vegetable kingdom, is consistent, I believe, with all known facts. And that the Deity has bountifully fitted the various orders of plants with which the surface of the earth is at once beautified and rendered capable of supporting animal lifeto draw their nourishment, in some spots more from the air, in others more from the soil, is only in accordance with the numerous provisions we every where perceive, for the preservation and continuance of the present condition of things."

Another point in connexion with the organic food of plants, and especially the source of their carbon, has recently been brought into perhaps unnatural prominence by Liebig. The physiologists, believing that plants derived from the soil by far the greatest proportion of their carbon, naturally enquired what vegetable substances in the soil entered into their roots and ministered in the greatest degree to their growth. To vegetable matter, generally, they gave the name of humus, and to a dark brown substance which dissolves out of the soil, when it is boiled with a solution of common soda, the name of humic acid was ap

plied. This humic acid being often met with in considerable quantities in fertile soils, has been generally mentioned by foreign agricultural writers as the principal source of that portion of the carbon which plants derive from the soil. It will be observed that this opinion may be entertained, without denying, at the same time, that plants derive the largest portion of their sustenance from the carbonic acid of the atmosphere. Liebig has shown that the humic acid and its earthy compounds are so sparingly soluble, that, were all the water which enters the roots of plants to carry with it as much of them as it could hold in solution, it would still convey to the stem and branches only a small fraction of the carbon they contain. So far Liebig's argument is unassailable; but there is a great gulf, as it appears to us, betwixt the obvious conclusion from this-that humic acid, namely, and its earthy salts, can contribute but little to the general nourishment of plants, and that which he affects to draw from it-that humic acid in no other way can directly contribute to the nourishment of plants, and that the only use of the entire vegetable matter in the soil is, by its decay, to yield carbonic acid to the roots. The compounds of humic acid, with potash, soda, and ammonia, are very soluble in water, and it has yet to be proved that they may not, especially in manured land, be present in the soil, and be thence taken up by the roots, while it is certain, also, that other soluble organic substances do exist about the roots, which, therefore, may enter into the circulation of the plant and aid its growth. In philosophical questions, a distinct line should be drawn between clearly established facts, or conclusions legitimately formed from them, and the mere opinions even of the most eminent men. On the whole, we are inclined to hold a middle way, and with our author to consider it to be satisfactorily established that, while a plant sucks in by its leaves and roots much carbon, in the form of carbonic acid, it derives a variable portion of its immediate sustenance (of its carbon) from the soluble organic substances that are within reach of its roots.

This fact is never doubted by the practical husbandman. It forms the basis of many of his daily and most

important operations, while the results of these operations are further proofs of the fact.

We pass on to another important question connected with the food of plants, in regard to which the views of our author differ to some extent from another of those opinions propounded in so broad and general a manner in the recent work of Liebig. We allude to the source of the nitrogen of plants. There are two compounds from which, according to our author, there is reason to suppose that plants, in the general vegetation of the globe, derive the greatest proportion of their nitrogen. These compounds are ammonia, which consists of nitrogen and hydrogen; and nitric acid, which is composed of nitrogen and oxygen only. The beneficial action of ammonia upon vegetation has been long recognized in practice, experimented upon by philosophical agriculturists, and brought prominently forward by writers upon theoretical and practical agriculture. It is given off in the gaseous state during the decay of the bodies and excretions of animals; and therefore wherever such are added, intentionally or otherwise, to the soil, the ammonia they yield must be considered as the source of a portion of the nitrogen which the plants that grow there are found to contain. In one of those numerous experimental papers with which Boussingault has enriched theoretical agriculture, he adverted to the opinion which had been long entertained by some that ammoniacal vapours probably float in the atmosphere in minute quantity, and suggested that those vapours might probably be a natural source of a portion of the nitrogen which is known to be present in plants. This opinion in regard to the presence of ammonia in the atmosphere, was founded upon the known fact that ammonia does escape into the air during the decay of animal substances, and that experimenters, Brandes among others, as early as 1825, had found ammoniacal salts in rain water. Liebig, who also found a sensible quantity of ammonia in rain water, has recently been led to propound the opinion that all the nitrogen contained in plants enters them in the form of ammonia, and that the minute portion which floats in the atmosphere, is the source from which they derive this