than Kirchhoff's and Bunsen's method; thus it may detect the presence of litmus and other organic colours. But it has none of the extreme delicacy, facility, and certainty which constitute the whole value of spectrum-analysis. Dr. Gladstone had not the good luck to choose the right branch of this inquiry; but his concluding remarks prove that he was fully aware of the value of the prism. They are as follow:

"These observations will be sufficient to prove that the varying chromatic phenomena, exhibited by different substances, may be taken advantage of in qualitative analysis, to an extent which has been hitherto unappreciated. My remarks have been almost confined to transmitted light; but the phenomena of reflected light offer a similar, and as yet almost unoccupied, field of investigation. What I have here marked down must be considered rather as a tentative inquiry than as a really valuable contribution to our knowledge of the effect of different chemical substances on the rays of light; but should any one be induced to take up the matter systematically, he might easily make such a series of observations as would furnish data for regular tables of comparison; and the prism would then take its place, as the blowpipe now does, among the recognised and almost indispensable instruments of the analytical laboratory."

We should notice that Mr. Crookes and Mr. Pearsall have also, in particular cases, used the prism on the analytical principle.

From the preceding historical sketch it would appear probable that Talbot discovered the very method of analysis now performing such great things; that Wheatstone recognised the principle, and that Gladstone developed it into a complete method in one branch, which did not prove to be luckily chosen. To all the other inquirers on the subject, the utility of the spectrum in analysis seems not to have occurred. It would be superfluous here again to eulogise the achievements of the German professors, whose published results we are reviewing.

Some persons, we believe, consider it a trifling, if not an illnatured, occupation to trace back the occurrence of thoughts and works which only, after a time, are recognised at their true worth. But the same principle might be applied to other parts of history. It amounts to saying, "Let bygones be bygones. We have a great deal of science and learning, and many pleasant things in the present day, and we shall not increase them by rooting up old records, and inquiring who secured these advantages to us." It must not be so; the very history of science is a science of itself, showing the gradual development of modes of thought, and the errors and difficulties into which genius falls are some of the most interesting facts in the range of knowledge.

But independently of this interest, we have a strong opinion

that formal honour and, if possible, general reputation should be meted out to scientific men by laws as strict and impartial as those which secure the execution of a dead man's will. In the first place, it is obvious that the human race owes to scientific discoverers a debt which it is not merely impossible to discharge, but even to conceive. Suppose Providence had decreed that Watt, instead of being plagued by patent laws, should have enjoyed a small percentage of the advantages which he has caused and will cause to others; how long the life, how great the benefits that would have fallen to his lot! Watt, indeed, was partly a practical man; but the same could be said of most abstruse philosophers. Again, it is, we believe, generally true that an earnest discoverer is never so well rewarded as when the worth of his results is proved and acknowledged, and the expectation of the proof and acknowledgment is nearly equal to the fact. What is wealth, or a title, or a splendid funeral to a man who has laid the foundations of a new science, knowing that it will grow through all time more perfect and useful, and bear his name upon its front? But the beauty and grandeur of the edifice will seldom be apparent in the few first stones, except to him who has the whole design mentally before him. So he who publishes a single new and original thought may well know what it will grow to be, and yet feel that future times only will witness the growth. That a single thought may turn the affairs of men, no reflective person can deny. Every science, and consequently every material achievement of modern times, is the development of a few simple grand thoughts, apparently incidental in their occurrence, but generally the result of enormous labour. May there be none, then, who shall go without their due award of honour for any labour or ability which they apply to the advancement of the general good! Thus we shall best insure future energy. But in any case, let us remember the soliloquy of Falstaff: "Honour pricks me on. What is that honour? Air: a trim reckoning! Who hath it? He that died o' Wednesday. Doth he feel it? No. Doth he hear it? No. Is it insensible, then? Yea, to the dead. But will it not live with the living? No. Why? Detraction will not suffer it; therefore I'll none of it."

We have but little space left to consider one obvious but exceedingly interesting application of spectrum-analysis, viz. to the determination of the chemical composition of astronomical self-luminous bodies. If we receive from a distant body rays of light, which, when examined by the prism, are found to correspond, to the utmost degree of accuracy that our observing powers can attain, with the light given off by sodium and calcium, here the induction is quite irresistible, that sodium and

calcium exist in that distant body. It is possible that this direct mode of observation may be applied to the stars; but when we come to consider the sun's spectrum in this relation, a most remarkable phenomenon meets us. The bright bands of coloured light observed in the spectra of sodium and the various elements correspond, not to bright bands in the solar spectrum, but to the dark lines long since observed by Fraunhofer. This coincidence is so striking in the case of the conspicuous line D corresponding to the line of the sodium spectrum, that Fraunhofer himself noticed the fact, as we have seen, and all subsequent observers have confirmed it. From this and other coincidences many natural philosophers probably had become convinced of a physical connection of the bright terrestrial and the dark solar lines. It remained, however, for Kirchhoff to show experimentally, as well as theoretically, the nature of this relation; and his results are published in the second paper at the head of this article.

Kirchhoff finds that any element in the gaseous state has the power both of radiating and absorbing light rays of those definite kinds which are shown in its spectrum. If, from its condition of heat, the light radiated is more intense than that absorbed, the body appears luminous; but if the rays falling upon it are the most intense, the body gives out less light than it absorbs. In the last case, the gaseous body will intercept light, or be opaque to rays of the definite kinds belonging to the gaseous substance. If, then, a compound ray of light containing rays of all kinds or colour pass through sodium in a gaseous state, the yellow rays which belong to sodium are intercepted, and the spectrum of the compound light would be marked with a dark band, which corresponds in position to the line Na. a of the sodium spectrum, and the dark line D of the solar spectrum. It must be understood, however, that this line is only dark relatively to the rest of the spectrum, for the sodium vapour which is absorbing yellow light may be giving out yellow light of less intensity. What is thus shown in the case of sodium is obviously applicable to potassium, lithium, and all other elements, however complicated their spectra.

The fact of the absorption of definite rays by certain elements was proved in the most convincing manner by the following experiments:

"The bright line produced in the spectrum of a gas-flame by the presence of a bead of chloride of lithium, is changed into a dark one when direct sunlight is allowed to pass through the flame. When the bead of lithium is replaced by one of chloride of sodium, the dark double line D (coincident with the yellow sodium line) appears with uncommon distinctness."

The same line D was also produced when light from a white

hot platinum-wire, which would otherwise give a continuous or complete spectrum, was passed through a flame of weak aqueous alcohol in which common salt was dissolved.

A still more striking proof of the same fact was shown in an ingenious experiment by Professor Roscoe in his lecture at the Royal Institution (partially frustrated, however, by an unfortunate breakage). A piece of sodium was placed in a glass tube exhausted of air, and then heated until the tube was filled with pure sodium vapour. When viewed by pure yellow sodium light this tube appeared nearly opaque, while by all ordinary light it was perfectly transparent!

But how are the conditions of these experiments fulfilled by the sun? Kirchhoff makes the very reasonable assumption, that the central solid and fluid mass of the sun gives off light rays of every character, such as would produce a continuous and complete spectrum, like that of incandescent platinum, but that the sun's atmosphere contains the gaseous elements which absorb certain definite rays, and produce Fraunhofer's lines. By these processes of experiment and inductive reasoning, Kirchhoff has already proved that the sun's atmosphere contains, amongst other substances, sodium, iron, magnesium, chromium, and nickel, but that lithium is not present in any appreciable quantity.

Indeed it wants little cool reasoning to be convinced of these statements when we see the spectra of the sun and of one of these elements side by side. Thus Professor Roscoe, in his lecture, eloquently described the indelible impression made on his mind by the splendid spectacle of the coincidence of the bright lines of the iron spectrum with a part of the dark solar lines:

"In the lower half of the field of the telescope were at least seventy brilliant iron lines, of various colours, and of all degrees of intensity and of breadth; whilst in the upper half of the field the solar spectrum, cut up, as it were, by hundreds of dark lines, exhibited its steady light. Situated exactly above each of the seventy bright lines was a dark solar line. These lines did not only coincide with a degree of sharpness and precision perfectly marvellous, but the intensity and breadth of each bright line was so accurately preserved in its dark representative, that the truth of the assertion that iron was contained in the sun flashed upon the mind at once."

In the whole range of the sciences there will scarcely be found an experiment so intrinsically beautiful in itself, or so pregnant with interesting information, as this. By the aid of such experiments the chemist will not always grovel on the earth. Light has now become his servant, and will bring him telegraphic information from every part of space.

The present occasion is not quite the first on which our thoughts have been raised to the chemical composition of as

tronomical bodies. The aerolites which occasionally fall upon the earth's surface from the unknown regions of space have been found to contain in greater or less quantity many of the terrestrial elements, such as iron, nickel, manganese, chromium, cobalt, aluminium, calcium, magnesium, sodium, silicon, sulphur, carbon, hydrogen, and oxygen. No new element unknown to us here has been found in these aerolites; but at the same time no combination of elements, such as the aerolites almost invariably present, has ever been met with among the plutonic or sedimentary rocks of our planet. Light itself has previously been made to yield some slight information by observations as to its state of polarization.

It is very uncertain to what extent we may be able, by observing the spectra of the stars, to determine their chemical composition. A statement of Sir J. Herschel bears closely on this matter, and is not very encouraging. He says:*

"Nothing short of a separate and independent estimation of the total amount of the red, yellow, and the blue rays in the spectrum of each star would suffice for the resolution of the problem of astrometry in the strictness of its numerical acceptation; and this the actual state of optical science leaves us destitute of the means even of attempting with the slightest prospect of success."

But in spite of this we entertain no doubt that every principal star will, in the course of time, be analysed by spectrum observation, and that Bunsen will then be one of the chief founders of the Chemistry of the Universe, as he is already the chief contributor to the chemical geology of our own globe. In thus speaking we are not making random generalisations, but have facts to warrant what we say. Fraunhofer did not neglect the spectra of the stars, and has given us nearly the whole of the information we possess about them. In the spectrum of Sirius he observed no fixed lines within the orange and yellow spaces, but in the green there was a very strong streak, and two other very strong ones in the blue. The star Castor gave a spectrum exactly like that of Sirius, the streak in the green being in the very same place. The spectrum of Pollux contained, besides many other weak lines, a line corresponding to that of D in the solar spectrum. The line D was also seen in the spectra of the stars Capella, Betalgeus, and Procyon. But observations of this nature are surrounded with difficulties such as even Fraunhofer, with his incomparable prisms and lenses, could scarcely overcome.

The interest of this subject is immensely increased if we consider that the colours of the fixed stars appear even to the naked

* Results of Astronomical Observations at the Cape of Good Hope, p. 304. † Brewster's Edinburgh Journal of Science, vol, viii, p. 7.