But the greatest of all these circular changes is that which subsists between the animal and vegetable kingdoms - the principles of the two systems of life requiring the refuse, the one of the other. It would be altogether out of place here to mention any statistics as to the large amount of carbonic acid derived from artificial combustion, from thermal springs, from extinct volcanoes, or from the respiration of animals; but we know that if it were not removed the atmosphere could not for any length of time support life. But vegetation does this great work, and cultivation in most cases produces a purer atmosphere; so these plants serve the animal kingdom in two ways - by supplying it with food, and by affording it a pure atmosphere.

I can hardly conclude this sketch without making some reference to the origin of vegetable life, a subject which it is well to be cautious in approaching, because of the present imperfect state of our knowledge. Among the many theories of life, (so, I suppose, they must be termed,) although we cannot even attempt to reach ultimate causation, I may perhaps be allowed to express what may be termed a fancy of vegetable life, (for theory is far too grave a term to be applied here) - a fancy of vegetable life founded upon a review of Dr. Faraday's magnificent discoveries.

The vitality of a plant forms an episode in the history of discovery, because at the first animal life was regarded as totally distinct from vegetable life; but as first great moving power. Vegetable physiology advances, and seems to incline to the idea that the vitality of a plant is a purely chemical process.

Upon what is dependent, then, this chemical action that enables a plant to decompose both carbonic acid and water, that is always going on in it as a part of its life, and the source of its development? The magnetic discoveries of Dr. Faraday of the connection between magnetism, light, and crystallisation, have no doubt brought us to the eve of a great generalisation. We are in a state so happily described by Humboldt as "a state of prcsagement" - in one of those intervals that renders the history of philosophy so indistinct between the presentiment of an epoch and its actual realisation. We have the strongest grounds for considering galvanism, electricity, magnetism, as one and the same force; which force is a certain quality of matter termed by Professor Graham polarity, which is due to the presence of inherent magnetism. So then, we may almost go so far as to say, that the same force that deflects a ray of polarised light - that causes the crystal to elect a certain determinate position in the magnetic field - that gives polarity to the needle of the compass - that this same force causes the formation and growth of an inorganic cell.

Such may be considered as an unfinished and imperfect glance at some of the most striking points in the chemical history of vegetation; but our survey is not yet finished. The vegetable flame requires the introduction of certain peculiar elements which are not organic, but which, nevertheless, makes it subservient for the support of life, and for the growth of structure.

We do not now wish to discuss the exact degree of alkalies or alkaline earths required in certain cases, nor to trouble ourselves about the presence of organic acids in plants. We know that oaks require certain conditions for their existence; in some soils they flourish, in others they will not live. A barren and granitic soil amply supports the Pine and the Fir, but not the Oak; and this because such soils do not contain sufficient alkalies for the purpose. The Equisotacete, like the Oak, require much potash; a soil formed from the grauwacke and porphyry, nourishes these plants.

The Tobacco-plant and the Vine require lime; the Beet and Potatoes magnesia; Cheno-podium likes nitrates; and the Fuchsia iodine; Cereals phosphates of ammonia and magnesia. For the growth of these plants, one or other of these constituents is required; but there are some soils composed of pure sand and limestone - they support no species of vegetation; they are absolutely barren. Argillaceous earths always exist in a fertile soil; their fertility being doubtless derived from the alkali they contain, because rich soils were formed by the disintegration of potash and soda felspars. Alumina is never found in the ashes of plants; its action is merely indirect. It is useful in condensing the water and the ammonia of the atmosphere. A cubic foot of felspar contains enough potash to supply an Oak-copse of twenty-six square feet with potash for five years.

We find, even in short excursions in our own country, great changes in the scenery of different places; this is often brought about by the predominance of a certain class of shrubs. We may pass a woody country - a country occupied as arable land - an entirely pastoral country. Whence this differenee? From the soil, which influences the producIn speaking of the imperishability of matter, I alluded to the changes which it is constantly undergoing, because the earth, from the first moment of creation, has been subject to the never-ending play of natural forces; and by looking at the present state of the globe, and then by reading its past history, the mutation of all material substance is thoroughly illustrated.

The progress of disintegration has not been much investigated; but Liebig has collected into his "Agricultural Chemistry" the results of most experiments on this subject. He divides the process of disintegration into two groups - waste, the result of mechanical force, and disintegration, properly so called, a product of chemical action. It by no means follows that both disintegration and waste may not be simultaneously taking place in the same rock, or in other words, both chemical and mechanical causes operating in unison, and by their union producing great effects.

The action of carbonic acid and water is generally to liberate in a soluble form the alkaline bases, producing frequently as an ultimate product, hydrate of silica, before which is often formed a soluble silicate. I presume that a descriptive detail of the properties of silicic acid would be superfluous; but an experiment of that talented chemist, Lavoisier, deserves notice here.