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there are, in abundance, rocks of volcanic origin, though they do not rise in the form of hills with cones and craters like those of Auvergne. There can be no doubt that the remarkable rocks of Staffa and the Giant's Causeway are the productions of an extinct volcano, because their mineral compositions and columnar structure perfectly agree with basaltic lava, which have been known to flow from the craters of volcanoes.

Mr. T. Jenkyn tells us, as it is well known that water exists in the different forms of a solid, a liquid, and a vapour it is probable that the first or premordial appearance of water upon the cooling surface of the earth's crust was in the form of vapour, which, with the decrease of temperature, condensed at last into rain. It will now require a stretch of imagination to comprehend that from some cause, which geologists do not profess to understand, the heat of an ignited and burning globe began to decrease, and that a portion of this heat so radiated into space as to allow a crust to form upon its surface, just as you now see crust forming over molten lead or glass, as the fused materials cool.

It may be imagined again that this cooling process continued, probably for thousands of years, until at last the surface of the earth grew so cool that the watery elements, which during its ignited and fused state must have floated along with many other volatile constituents in this atmosphere around it, condensed into vapour and then into drops. This water would be deposited on the surface of the earth in the forms of lakes, seas, and oceans. After a time, portions of this water would be taken up by evaporation, descend again in the form of rain, which in its turn would produce streams, torrents, and rivers.

At the time of the earth's first cooling, the crust would be diversified with hollows and elevations, depressions, swellings, and peaks. As the rain would wear away, or geologically speaking, would disintegrate the upper parts or edge of the hollows, the water would run downwards to the bottom and there settle, and when in a state of rest it would deposit the loosened materials, or detritus, which it had dislodged from the higher portions of the rock. This detritus would necessarily consist of pebbles, gravel, sand, and mud, or silt. I

Every fresh shower would effect a fresh disintegration of the rock, and would also produce fresh currents and streams that would carry away more pebbles, sand, and silt, and deposit them in a fresh layer over the surface of the first. Upon every repetition of shower and flood, two results would take place. First, the lake or inland sea would by the ac

cession and rise of its unexhaled waters become deeper and deeper; and secondly, with every fresh deposit of detritus the bottom would come up higher and higher, and the water shallower and shallower, until at last the upper layer of detritus would become visible as dry land. You at once see that as these layers or beds rise in thickness, they would elevate the waters of the lake or inland sea in the depression, and eventually drive the water off to seek its level in other hollows, until finally sea would join sea and form the ocean.

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Mr. B. Having in our last lesson on vegetable physiology given a brief account of the elementary organs, the next step will be to consider the nature of plants, and the means they have of performing their vital actions. Light, heat, and water are the external agents which, acting upon the vital principle, set all the machinery of vegetation in motion. No one of these causes will by itself produce any effect, although their combined action is of the most powerful kind. Light, as we shall hereafter see, causes the decomposition of carbonic acid, fixing the carbon in the interior of tissue, and thus solidifying the most delicate parts, or altering the chemical nature of others. It is the grand cause of the varied colours of vegetation, and may be considered as being in part what produces a motion of the fluids. In its absence, plants are weak and sickly and soon perish.at

Heat, by drying the atmosphere, produces evaporation, which is one of the great means by which the crude fluids become inspissated (thickened) and altered in their nature; it causes the expansion of the gases which plants contain, distends their tissue, and renders the latter more capable of performing its contractile and hygrometrical (pertaining to an instrument for measuring the moisture of the atmosphere) functions. 1༈

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Water relaxes all the parts, dissolves the soluble matters which are laid up in a plant in a state of torpidity, and softens the tissue until it is capable of receiving the influence of temperature. It is, moreover, the medium by which the nutritious principles that are deposited in the earth are absorbed by the roots, and conveyed from one part of the system to the other.

To sum up, in few words, all that has thus far been stated, it

is light, heat, and water, acting in concert upon an irritable membrane, which enable plants, by virtue of their extensibility, elasticity, and hygrometrical powers, to perform the phenomena of contraction and endosmose (the transmission of fluids inwardly through membraneous substances), by means of which they absorb and digest their food, circulate their fluids, develope their organs, increase in size, and reproduce themselves. The general opinion of vegetable physiologists seems to be, that what may be called more particularly the food of plants, because it is that without which they cannot exist, is atmospheric air and water, in which carbonic acid is dissolved; that this alone is sufficient for their support; but that, under natural circumstances, they receive into their system a great variety of other substances dissolved in water.

It is well known that pure water, as it is furnished by distillation, does not exist in nature; but that it is constantly mixed with earthy, saline, and gaseous matters, which vary in proportion according to the circumstances under which the water is impregnated with them. Such water as this is necessarily what is offered to plants as their food. From the decomposition of the various organic substances which are buried in the soil results a considerable formation of carbonic acid-a principle readily soluble in water, and consequently forming a considerable part of all the fluid introduced into the roots of a plant. The saline and earthy matters which find their way into the system are gradually deposited, almost unchanged, in those parts where the water in which they are dissolved evaporates; namely, in the leaves or the bark, or in some other part of the surface. But the atmospheric air, water, and carbonic acid become decomposed, and their elements, recombining in new proportions, form the materials for new parts, and the gum, fecula, sugar, azotic products, and other immediate principles of the vegetable kingdom.my duty

Son. Theodore de Saussure tells us, that if we calculate with the utmost care all the weight which a plant ean gain," either by fixing carbon or by depositing earthy, saline, al alka line, and metallic matter, which they borrow from the soil," or by respiring oxygen, or from the soluble matter of soil, we shall not be able to account for more than twentieth part of the real weight of such a plant. The other nineteen twentieths must, therefore, be fixed water.

Mr. B. Whatever errors there may be in calculations of this nature, there cannot be a doubt that they are correct to so considerable an extent as to oblige us to admit that water forms a considerable part of the solid tissure of plants; so

that it would appear that, like minerals, plants have a water of crystallisation independently of their water of vegetation. By what means plants are enabled to procure the various kinds of matter which we have seen to enter into their system, is the next subject of consideration. The roots are no doubt the organs which are specially destined for this office. When young they consist of a sheath of thin and exceedingly hygrometrical cellular tissue surrounding a bundle of woody and vascular matter. At this period they contain mucilagenous matter (one of the proximate elements of vegetables) which enables them to attract fluids from the soil that surround them.

Son. I understand that if you take a parsnip and place the the point of its roots in water, it will suck up the moisture as well as if the whole root were immersed; and on the other hand if you place the whole root in water except the point or newly-formed part, it will not suck up the fluid in any considerable quantity.

Mr B. Such would be the case, and for the same reason the young tips of the roots are of the greatest importance to a plant, and its well-being depends in a greater degree upon their preservation than upon any other circumstance. In consequence of the great importance of the tips of the roots they have received the special name of Spongioles (from spongia a sponge) and are sometimes spoken of as if they were really distinct organs; they are, however, nothing more than the young and tender extremities of the roots.

Journeyman. From what I have now heard I can well understand why Mr. H. Mr. K's foreman, was so particular in his orders to us, that every root and fibre should be taken up with the greatest care. That, no doubt, was the principal reason why Mr. Khad such a demand for his trees and shrubs.

Mr. B. When the food of a plant enters the roots it passes upwards, undergoing some kind of chemical change, and dissolving whatever soluble matters it meets with in its course, so that, without having been exposed to any of those conditions by which it is ultimately and principally affected, it is considerably altered from its original nature before it reaches

the leaves.

There are three great axioms to which the experiments of careful observers and skilful reasoners have led:- 1. The quantity of water lost to a plant by evaporation, and its power of absorption from the soil, is in proportion to the quantity of light. 2. Light causes a decomposition of the carbonic acid of vegetation, and consequently, by solidifying the tissue, renders the parts most exposed to it the hardest.

3. The digestion of plants chiefly consists in a loss of water by evaporation, and in an acquisition of carbon, by the decomposition of carbonic acid.

It is only when under the direct influence of light that plants acquire their peculiar properties as to colour, perfume and flavour. It is on the leaves and succulent stems of plants that light chiefly acts, and it is in those parts where the proper juices that give colour, flavour, and the other charac ters peculiar to plants, more especially reside, they are the oils, volatile and fixed, gums, balsams and turpentines, the alkalies and acids, the earthy and saline compounds, and the tannin and extractive principles found in the sap or proper juices of trees, plants, and vegetables. The deposit of these substances which pass into the system of a plant, along with its fluid nutriment, gradually consolidates the tissue, cases over its sides, chokes up its intercellular passages, and prevents the organic membrane from performing its functions. When this happens, the part in which it occurs ceases to be capable of further action; and thus wood and bark become incapable of conveying fluid, and the fall of the leaf is the

consequence.

Son. When deprived of light, says Dr. Irvine, all plants nearly agree in the qualities of their juices. The most pungent vegetables then grow insipid; the highest flavoured, inodorous, and those of the most variegated colours are of a uniform whiteness. Vegetables which grow in a natural situation, he adds, readily burn when dry, but a vegetable bred in a dark place contains nothing inflammable.

Mr. B. From such facts it appears that when deprived of light plants continue to grow, that the juices which support their growth are then nearly alike in all, and that they acquire their peculiar properties as to colour, odour, taste, and inflammability, only when vegetation proceeds under the direct influence of light.

Journeyman. It frequently happens in America that clouds and rain obscure the atmosphere for several days together, and during that time the buds of extensive forests expand themselves into leaves. These leaves assume a whitish hue till the sun appears, when, within the short period of six hours of a clear sky and bright sunshine, their colour is changed to a beautiful green.

Mr. B. Under the name of respiration is included all that is connected with the inhaling and giving off of gaseous matter by plants.

This function is chiefly connected with the absorption of oxygen and carbonic acid, and their expiration. By a vast number of experiments, chemists have determined that the

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