Silicates are more or less decomposed by the action of hot water; the opacity of the windows in hotbeds is an example of this. Lavoisier, on distilling some water from a clean glass vessel, found it left a residue; on weighing it, he also found that the glass retort had lost in weight what the water had gained: from this experiment it was obvious that a portion of the silica of the glass had been dissolved during the distillation.

It is needless to enumerate all the substances upon which chemists have operated; suffice it to say that their experiments have had a very extended range, and that they confirm all the statements made by those who preceded them in this investigation. In this memoir there are two points of especial interest. One is, that the alkalies are not quite so essential to the disintegration and decomposition of mere rocks, as it was at first supposed: for hornblende, epotite, chlorite, and rocks composed mainly of these substances, underwent rapid decomposition by pure as well as by carbonated water, and this without calling in the agency of an alkali; this experiment accounts for the fact that rocks of this kind are often more readily decomposed by meteoric agencies than are felspars: it enables us to trace the simple process by which plants are furnished with the lime and the magnesia they require, without our having recourse to any mysterious decomposing power of the roots of the growing vegetable. The second and most important result is, that potash, soda, and their carbonates, but especially carbonate of potash, is volatile at a red heat. - that many plants contain much alkali, whereby a very little is found in the ashes after incineration.

So, by this incineration of the ashes of a plant, according to the ordinary rules for the analysis of vegetables, the Professors Rogers' statements show that a very large amount of error must not only have been by such analyses introduced, but by them perpetuated. The ashes of anthracite, of bituminous coal, of lignite, contain not a trace of alkali, but digestion with water previously to incineration, reveals to us their presence - thus adding another proof to the vegetable origin of coal.

I have not gone into the minutiae of any actual decompositions which take place during the disintegration of certain rocks, because my object is more to point out a train of thought than to dwell upon the facts by which these reflections may be produced. Our facts may be likened to the landmarks of the journey, but their attainment is not its ultimate aim.

constituent, and that for the most part soils are formed by the disintegration of the parent rock of the district, it is obvious that these facts, when applied on a great scale to nature, most divide and influence vegetation; for according to the geological conformations of a country is its soil, and so is its flora.

The subsoil is generally in connection with the original rock, by whose wearing away it was formed, and the soil is in intermediate relation to it, not always having even the same color; for it may be a transplanted soil, or separated from the parent rock by a larger amount of gravel, in which case the white subsoil from the chalk, or the yellow from the clays, would not represent the color of the land's exterior surface. The depth, texture, and fertility of a soil is dependent both upon the mineral constituents and the easily disintegrating properties of the rock whence it is formed; and it is the physical and mineral, more than the geological age of a soil, that conduces to its fertility: old rocks may be bar ren in one place, but fertile in another.

In merely a geological survey of a soil we are apt to underestimate, if not to overloook, the important fact, which is played by water in vegetation. Yet, if we cast our eyes over a hydrographic map of the world, we find certain rainless districts, destitute of water courses, and where the air is ratified to allow of the condensation of aqueous vapor. What do travellers tell us of the vegetation of a tract of country so circumstanced? Why, in this rainlesss desert let but* little spring of water rise, it generates fertility in the limited sphere of its operations; an oasis is produced; and that arid ocean of burning sand rejoices in one small island of vegetable life.

Water is one local cause of influencing the fertility of a soil, but there are various others, many physical actions in one place shifting the superficial detritus which covers the more stubborn rocks; and if we do not take circumstances like these into account, we shall be apt to consider that geology gives us more information than it really does; to form the idea, that with a geological map before us, itwould be no trouble by its inspection accurately to ascertain the soil of any particular country.

The mill-stone grit plains around Paris in a geological, map of the district, would be similarly colored; yet each one of these plains has its own particular form of vegetation. Montmorency is covered with corn-fields; Sannois supports only a short sterile rod; Meu-don is furnished with Spanish chestnut trees; the Airaflexuosa, the Melampyrum sylvati- cum, the Pteris aquilina, all grow there. Only a minute's inspection clears up this seeming incongruity. We find, though no difference is pointed out by the map - for geologically speaking, there can be no distinction made: they are all mill-stone grits - we find in the one case the millstone-grit is mixed with sand; in another case it is mixed with clay; and, in the third, it is alone and uncovered.

Thus far we have pursued our journey, and now it is time that we pause. I would hope that my sketch has been clearly followed, and that my design, imperfect as it is, has been strictly adhered to.

In following the chemical history of a plant, we saw how interwoven were a variety of subjects with each other; we saw, too, that the plant itself is capable of reading to us many a lesson from the great book of nature, of bringing before us many of her beautiful operations; of exhibiting to us clearly and distincly some fuller proofs of that design, and order, and harmony, so palpably manifested in this our universe.

So then man and nature can be viewed as two great forces here, the one progressive,and the other stationary, albeit, though not still, yet both working out their proper ends in the universal scheme of the Great Designer, which it is past the feeble ken of man to penetrate.

Such subjects as these must make us feel with Emerson that "the destiny of organised nature is amelioration, and who can tell its limits? It is for man to tame the chaos on every side, while he lives to scatter the seeds of science and of song, that climate, corn, animals, men, may be milder, and that the germs of love and benefit may be multiplied,"