to him belongs exclusively the merit of applying a knowledge of these phenomena to pathology." *

Even had Prochaska never lived, and as far as Hall is concerned we are bound to consider him as non-existent, the sole claim which can be made to a discovery is founded on the true spinal system, with its separate sets of nerves. That is Marshall Hall's own; and that is an error. All the rest belongs to others. That it does belong to others is seen in the fact that the Reflex Theory, divested of this hypothesis, was simultaneously promulgated, as a novelty, by Johannes Müller, who, on a comparison of dates, at once awarded to Hall the priority of publication. But Müller had no new facts to adduce; he simply generalized the known facts, respecting the action of the spinal chord; and connecting them with the assumption of the brain being the sole seat of sensation, he declared the independence of the spinal chord, and the absence of sensation in its special acts.

But while history thus irrefragably shows that Marshall Hall made no discovery at all, even if Prochaska's claims be set aside, and that it is therefore preposterous to compare him with Harvey, who revolutionized physiology, and demonstrated that which no one before him had suspected,-whereas Hall demonstrated nothing but what was already well known,— still I think that science was greatly benefited by the zeal, the energy, and the ability with which Marshall Hall worked out the Reflex Theory, and applied it to the explanation of many physiological and pathological facts. It is only necessary to compare the state of our knowledge now, with its condition when he first published, to see what an immense advance has been made, much of which is certainly due to the discussion of points raised by him. Even the hypothesis of a separate spinal system, erroneous as it was, proved of great service, like many other erroneous hypotheses: it gave a definite direction to research. And if his successors have bit by bit destroyed all that was peculiar in his statement of the Reflex Theory, extending and modifying the theory to suit the advance of knowledge, and leaving it much more like the theory advanced by Prochaska; we must nevertheless remember that it is owing to Hall's striking hypothesis, and more systematic arrangement of known facts, that successors, in a great measure, have been enabled to improve upon his theory.

To sum up, we may say, that if Marshall Hall was not a great man, he was a distinguished man, and has "deserved well of his country."

*SHARPEY, as quoted in Hall's Memoirs, p. 109. And compare, also, what MARSHALL HALL himself admitted in his first paper, presented to the Royal Society, p. 659, and in his New Memoir, p. 87.

205

SOLAR CHEMISTRY.

BY ROBERT HUNT, F.R.S.

"THE HE fable of Prometheus is but the outshadowing of a philosophic truth," was the language used by Lavoisier to express his belief in the direct dependence of all organization on influences derived from the Sun. Every advance which has been made in the examination of this most interesting subject, proves that energies originating in that vast globe, determine the phenomena of life on this earth, and regulate nearly all the conditions of the inorganic world.

Situated at the enormous distance of upwards of ninetyfive millions of miles from us is this great orb, having a diameter of 882,000 miles, which forms the centre of the solar system. Not only is the Earth and each of the other planets. chained to the Sun by the attractive power of its mass, but their motions are determined by its motion, and the physical forces which regulate all cosmical phenomena have their source within its body. The Sun, figuratively termed "the fountain of Light," is equally so, of every other Power with which science. has made us acquainted. An infinite store of creative energies is amassed in the solar orb. These are diffused in obedience to the Creator's word, and flow for ever throughout the universe, to be absorbed by the planetary spheres, producing alike, the crystalline arrangement of rock masses, and the development of vital forms.

"Man," says Bacon, "is but the minister and interpreter of nature, and can neither extend his power nor his knowledge. a hair's breadth beyond his experience and observation of the present order of things." Entirely acquiescing in this, we feel, nevertheless, that we have scarcely yet recognised the vast power of the human mind. Guided aright, man can penetrate the arcana of space, and dive into the tomb of time. With telescopic eye he can reach the worlds remote in space, and study the chemistry at work within them, while his microscopic vision enables him to detect the presence of the minutest monad, and watch the kindling of life's faintest spark. Although, as we have said, there are more than ninety-five millions of miles of space between this Earth and the Sun, the human mind has bridged the gulf, analysed the solar matter, and shown how, in all probability, the physical forces are developed. To explain this as clearly as possible is the purpose of the present paper. Since the time when Newton analyzed the solar beam, the

advance of our knowledge has been rapid. We know of luminous rays which were never seen by that philosopher. Of the chemical action of light he was ignorant. The beautiful phenomena of the Polarization of Light were unknown to him, and of the existence of numerous dark lines crossing even the most brilliant divisions of the Newtonian spectrum-and which promise to advance our knowledge by the discovery of many sublime truths, he had not the most remote idea. Yet, notwithstanding this want of the knowledge with which we are familiar, Newton possessing, in a remarkable degree, that power of thinking out a truth, which showed itself so strongly in the philosophers of Athens, proposed an hypothesis, which has been set aside and almost forgotten, but to which we are now returning, to adopt it as the most truthful theory of the physical condition of the Sun.

With the following quotation from Newton's "Optics," let us fairly introduce the inquiry, which, with every advance, appears to confirm his views :

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May not great, dense, and fixed bodies, when heated beyond a certain degree, emit light so copiously,-as, by the emission and reaction of their light, and the reflections and refractions of their rays within their pores-to grow still hotter, till it comes to a certain period of heat, such as is that of the Sun?

"And, are not the Sun and fixed stars great earths, vehemently hot, whose heat is conserved by the greatness of the bodies, and the mutual action and reaction between them and the light which they emit?"-(Newton's Optics.)

Since Newton's days, this hypothesis for a considerable period was discarded, and more especially so, since Sir William Herschel gave to the world his investigations on the solar spots. The prevailing idea has been that the Sun is a dark mass; that floating above it there exists a stratum of opaque clouds; and that surrounding-enveloping-those is the Photosphere, or sphere of light, whence we derive the luminous powers on which vision and colour are dependent.

Arago and Biot determined, by a series of beautifully devised experiments, dependent on the phenomena of the polarization of light, and by careful observations, that the luminous principle, originating in the photosphere, is produced by a gaseous or vaporiform medium in a state of intense combustion. This discovery did not, however, interfere with the idea that the mass of the Sun was dark and cold. It is only within a very recent period that inquiries in a new direction have taught us to doubt the correctness of the views originating with the elder Herschel, and have led us back to the speculations of Newton.

To explain these modern investigations, it will be necessary to examine with some care the known conditions of the solar agencies, and some of the peculiar phenomena of Light.

If through a pin-hole made in a window-shutter a sunbeam is allowed to pass into a dark room, three phenomena are rendered evident to our senses. LIGHT, producing vision and developing colour, is shown upon every particle of dust floating in the luminous pencil. HEAT is sensibly felt, if the hand is placed in the path of the ray; and ACTINISM, or chemical power, is rendered evident by the change produced, if we allow the beam to fall on any photographic preparation. That the sunbeam should have the power of breaking up the strongest combinations depending on chemical affinity, is one of the wonderful discoveries of science. Now, if we place a triangular prism of glass in the path of the sunbeam, the rays are bent out of their course, or refracted, and by this means decomposed into a beautiful flame-like chromatic image. If this solar spectrum this section of a rainbow-is received upon a screen, it will be found to consist of several coloured bands; crimson, red, and orange passing into yellow from the least refracted end, while from the most refrangible one, we have lavender, violet, indigo, blue, and green, also passing into yellow, as they advance to the true centre of the spectral image.

These rays constitute the Newtonian spectrum ; so-called from Newton's having first examined, with precision, the relative conditions of these coloured bands, and established, with any approach to correctness, the laws regulating the relations of colour and refraction.

Beyond the most refrangible end of this spectrum there exists another class of rays, which are not visible under ordinary circumstances. If, however, the rays of Light are intercepted by solutions of sulphate of quinine, or of horse-chestnut bark,-by a block of canary-yellow glass, coloured with the oxide of uranium, or by a crystal of fluor spar, those extra spectral rays are rendered apparent. Those rays, which were unknown to Newton, have been investigated by Professor Stokes, who has named them the Fluorescent rays. They are luminous, probably under all circumstances, to those animals whose eyes are adjusted as the eyes of most of the night-roaming creatures are to admit the rays of highest refrangibility, and to vibrate in unison with their vibrations; but, unless peculiar conditions are established, the fluorescent rays are not sensible to the human eye.

Such, then, is the amount of our knowledge respecting the luminous principle of the sunbeam. It must not be forgotten that the rays of which we have been speaking vary considerably in the intensity of their illuminating power. The maximum

exists in the Yellow ray, and it diminishes as we recede from it towards either end of the spectrum. The least refrangible, or the Red rays, give a modified amount of Light; but the maximum of Heat exists in them. The most refrangible, or Blue end of the spectrum, is less luminous; but the maximum of chemical action is fixed at this extremity, the fluorescent rays beyond the spectrum of Newton being visible only under the peculiar circumstances mentioned.

When these beautifully coloured bands of Light are well defined upon a screen, let the inquiring reader examine the image with a small telescope. A new set of phenomena will become apparent. The spectrum is then seen to be crossed by a vast number of black lines. Every ray, even the most brilliant, will be seen to have spaces in which there is an entire absence of light. To these we would now direct attention.

It is instructive to trace the steps by which we slowly advance to the discovery of a great truth. As in ascending a tall column, the way may, for a season, prove dark and possibly -being without promise-wearying; but, eventually, the gleams of light are seen, and presently a wide horizon is opened to our view. Thus has it been with this inquiry.

Dr. Wollaston was the first who observed the existence of non-luminous spaces in the prismatic spectrum. Dr. Ritchie proved that these lines were dependent on absorption, and showed how they could be increased in visible numbers by artificial means. Fraunhöfer, however, was the first to make a full investigation of these lines, and to publish a map of them; hence they have been generally called Fraunhöfer's lines.

These lines are of so fixed a character in relation to the coloured bands of the spectrum, that if it is desired to indicate with great precision any special ray of the spectrum, we refer to them by their letters or numbers. In the accompanying plate, the more remarkable lines only are given. The positions they occupy have been determined, by a careful examination of the map of Fraunhöfer, and the very complete delineation of those lines published in the "Philosophical Transactions for 1859," by Sir David Brewster and Dr. Gladstone. Fraunhöfer laid down on his map 354 lines, but Sir David Brewster says-"In the delineations which I have executed, the spectrum is divided into more than 2,000 visible and easily recognized portions, separated from each other by lines more or less marked."

The lines marked with capitals from A to I in the plate may be always easily detected in any Solar spectrum. Those which are indicated by small letters, and those which are numbered, are the more marked lines observable in Sir D. Brewster's map; the letters and numbers agreeing with those which he employs. The origin of the dark lines-spaces in which there is no Light can scarcely be said to be yet resolved. Fraunhöfer, and