be produced by "currents of electricity circulating about from east to west," then the conclusion seems inevitable, that either the electrical and magnetic fluids are identically one in essence, though, perhaps, liable to certain modifications; or, that the electric fluid is derived primarily from the sun, and induces the several magnetic phenomena, by an energy exerted upon certain substances within the earth's surface: in either case, the sun must be referred to as the prime actuating

cause.

Mr. Ritchie's farther remarks are pertinent to the idea above suggested; for he says, in reference to the questions, "How are these (electro-magnetic) currents generated? Are they voltaic or thermo-electric ? from the constitution of our globe, we can scarcely doubt that they belong to the latter class. The earth abounds with metalliferous veins, and these veins are undoubtedly of different temperatures: consequently, thermo-electric effects must take place. The rapid change in the magnetic equator, when approaching South America, renders this supposition highly probable." (Journal of the Royal Institution, Aug. 1831, p. 130.)

5. The sun's rays, for the most part, strike the surface of the earth, and produce the developement of heat, varying in intensity according to the then existing state of the atmospheric medium through which they pass; and, perhaps, according to the nature and extent of the electrising processes induced within that surface upon which the rays impinge. The light thus radiated must be either reflected, absorbed, or extinguished.

That a portion of it is reflected, no one acquainted with astronomical facts can entertain a doubt. That another and considerable portion is absorbed is proved, almost to a demonstration, by the increase of temperature at a considerable depth below the surface of the ground; and by numerous electrical and magnetic phenomena. That any portion is extinguished or destroyed, is contradicted by the direct analogy of facts; for all nature affords irrefragable evidence that not an iota of her productions ever was or can be lost.

From the foregoing and many other considerations, I have arrived at the conclusion that, as the surface of the earth is the grand absorbing medium of the rays of light, the portion absorbed, not being extinguished, must inevitably be productive of many stupendous phenomena, some of which I shall now endeavour cursorily to point out.

Without entertaining, to its full extent, the theory of Professor Leslie, that the globe is cavernous, its central cavern being replete with light, "shining with intense refulgence,

and overpowering splendour," I cannot but suggest, first, that much of the ethereal solar light is distributed interstitially among the molecular particles of the substances that compose our terraqueous globe; that this light is masked or hidden till it becomes deranged by some disturbance of the molecular particles; but, when so deranged, it gives rise to the several phenomena termed electrical, which result from friction, percussion, or disruption. Hence it may be concluded that the attraction of cohesion has its origin in the ethereal light that is dispersed throughout all material substances.

Secondly, that whatever portion of light is not so distributed may exert an energy more strictly chemical, in the decomposition and reproduction of water, the oxidation of metals, the developement of gaseous bodies, and the like. It is, as I conceive, during the induction of these phenomena that the electrising process is begun and perfected; and this, perhaps, not only by a decomposition or modification of the matter of solar light, but by its direct action upon the decomposable matters within the earth's surface; and from this electrising process result the separation of the magnetic fluid, and the developement of free electricity and heat. May not earthquakes, the eruption of volcanoes, and other stupendous convulsions, be referred to chemical or voltaic decomposition effected by the agency of absorbed light?

My ideas accord (partially, at least) with the suggestion of Dr. Hutton, at the close of his article on electricity, in his Mathematical Dictionary. "Perhaps," he observes, "we may be allowed to extend our views, and consider the sun as the fountain of the electric fluid, of the zodiacal light, the tails of comets, the aurora borealis, lightning, and artificial electricity, and of its various and not dissimilar modifications."

If I have succeeded in affording evidence of the actual existence of one great principle of union, and have shown that this principle is the sun, and therefore that it would be futile to seek for any other, it will only be required to point out the way in which that principle may farther act so as to connect all the other great natural phenomena with that of gravitation. The "attraction of gravitation" is a term that is employed by every one who attempts to explain the cause of the descent or fall of any body possessing solidity and weight. Attraction infers the operation of an agent by which bodies are induced to approach to, and come in contact with, it. Electrified bodies induce an opposite electrical condition in other bodies within the range of their influence; and the two are then attracted, the one to the other, in obedience to

the law of electrical induction: this is familiarly known to all electricians. Now, it appears certain that the agency of induction is in constant operation, and that no limits can be set to the extent of its agency. It moreover appears that the spherical form is the one most favourable to the retention of the electric fluid; while, on the contrary, points tend to produce its rapid dispersion. The earth and moon are of a form almost spherical; and there is little doubt that electricity is distributed over their surfaces, and is maintained and renewed by the electrising principle of the sun's rays. If, then, the surfaces of the planetary bodies be in an active electrical condition, will they not tend mutually to attract each other? With these considerations before us, it must be reasonable to pause, and enquire whether an agent like electricity, so subtle, so powerful, so susceptible of infinite modifications (the reality of whose existence is undeniable, and the nature of whose phenomena admits of absolute demonstration by experiment), whether such an agent may not be capable of producing all the phenomena which are usually ascribed to gravitation; whether it may not govern the movements of, and regulate those attractions between, a planet and its satellites, on which depend the vicissitudes of the seasons, the alternations of day and night, and the ebbing and flowing of the tides. Electricity is known to exist; it is known to be more or less in a state of constant operation: in a word, it is a cause producing effects. Gravitation, on the contrary, is a conventional term for an effect; certain, indeed, but resulting from no ascertained cause. We have, then, to decide, whether it be not more philosophical to admit that so tremendously powerful an agent as electricity is capable of inducing all the effects ascribed to gravitation, than to remain satisfied with the assumption that bodies gravitate; solely because, by the law of their creation, they tend to fall towards a centre; or to believe, according to some writers, that every particle of matter in nature gravitates or tends towards every other particle.'

If it be permitted to us to enlarge our views, and to suppose that the sun's electrising power operates upon the whole planetary system, and establishes all their primary and secondary relations, which act and react one on the other; if, moreover, we conceive the probability of a mutual interchange of influences existing between the sun and its planets, and that he attracts and receives from them that matter which supports and renews the luminous fluid that composes his own rays, how vastly comprehensive will appear the mighty plan of operations, a plan by which not even a particle

of light ever is or shall be ever lost! Comets have, by some, been supposed to regulate the electrical relations between the sun and his planets; and, by others, to supply the loss which the sun sustains by emitting rays of light and heat. I am inclined to believe that the agency of induction and attraction is universal and complete; that a reciprocal and harmonious interchange is for ever going on between the sun and the planets; and that the sun itself is the one sole "great principle' which operates and connects together all the phenomena of the material world.'"

I have not hesitated to introduce, in the foregoing paper, several passages from the early sections of the Domestic Gardener's Manual, with trifling modifications: this the reader who is in possession of that work will scarcely fail to discover. By thus borrowing, as it were, from myself, I have been enabled, without incurring the charge of plagiarism, to embody most of the facts which constitute the basis and substance of the electrical theory that is impressed forcibly upon my mind, as being alone capable of elucidating all the great natural phenomena. There is much of the hypothetical, I admit, in the view I have taken of the predominating agency of absorbed solar light; but, wherever doubt exists at all, and mysterious effects are observed, the actuating machinery of which cannot be brought within the power of human perception, theory must be allowed; unless, indeed, it be required that the powers of the mind remain dormant. The case admits of no other alternative.

He, however, who, while he endeavours modestly to exhibit the ideas which are forcibly impressed upon his mind, in the hopes of stimulating to scientific research, neither asserts dogmatically, nor claims authority to dictate to the faith of others, cannot justly be accused of presumption. "The germs of discovery are often found in rational speculations; and that hypothesis which is presented to the test of philosophic investigation may, without arrogant assumption, be deemed the pioneer of science.

Jan. 21. 1833.

G. J. T.

ART. VI. A Reply to Mr. Main's Question to the Author of the "Domestic Gardener's Manual." By the Author of the "Domestic Gardener's Manual."

Sir,

I OBSERVE, in Vol. VIII. p. 652., that there is a paper addressing, not one but two distinct questions to me, on

points of great interest in the science of vegetable physiology, to which I had particularly adverted in an article on the sap-vessels (Vol. VIII. p. 142.).

As Mr. Main claims my earliest notice, and I very much wish to meet his views, I regret that I did not see his communication until the 18th inst. (Dec. 1832). However, I hope I may still be in time for your next publication. If I do not greatly mistake, Mr. Main is the gentleman who penned the review of the Domestic Gardener's Manual, in Vol. VII. p. 57. (as I perceive by the index; for I have not the number by me). The article now before me is written in a style that identifies it, to the best of my recollection, with that of the review; and I beg to return my best and cordial thanks for the kindly spirit which breathes through both one and the other.

The first question proposed by Mr. Main is grounded upon an extract from a letter that I had received from Mr. Knight. The hypothesis therein noticed belongs exclusively to that gentleman. It is a deduction from his own positive observations and experiments: the very expressions quoted are, verbatim, those of Mr. Knight. "The nutriment absorbed becomes the true sap or living blood of the plant, by exposure to light in the leaf; it descends by the bark, by which the matter that forms the layer of alburnum is deposited."

I cordially, it is true, acquiesce in this opinion of the origin of the alburnous deposit; still, however, although Mr. Main disclaims as inadmissible the answers of any other person but myself to his questions, he surely cannot, upon due consideration, refuse to receive Mr. Knight's own testimony in support of a hypothesis originally his own. I, therefore, must take the liberty to state the following question, and respectfully to request Mr. Main to afford it his most serious reflection; and not to stop there, but to make the fact of which it treats the subject of close investigation and critical experiment. This will not be difficult during the ensuing spring; and the results may be fully ascertained in the course of a few months.

I ask, then, has not Mr. Knight asserted, in the Philosophical Transactions, that, having partially detached strips of bark of the walnut tree, of several inches' length, from the alburnum, in the spring, he introduced beneath such bark two folds of paper, each of which was coated on both sides with bees' wax; so that such strips of bark were placed wholly out of contact with the alburnum, or other bark of the tree, except at their upper ends. Air and light were excluded by a covering of clay till autumn, when as much alburnum was deposited upon the paper, along the whole