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observed by Sir David Brewster, in 1842, between the bright red line of potassium and a dark line in the solar spectrum called Fraunhofer's A. The fact of the coincidence of these lines is easily rendered visible if the solar spectrum is allowed to fall into the upper half of the field of our telescope, whilst the sodium or potassium spectrum occupies the lower half. The bright lines produced by the metal, as fine as the finest spider's web, are then seen to be exact prolongations, as it were, of the corresponding dark solar lines.
Although the fact of the coincidence of several bright metallic lines with the dark solar lines was well known, yet the exact connexion between the two phenomena was not understood until Professor Kirchhoff, in the autumn of 1859, investigated the subject. Nevertheless, before he gave the exact proof of their connexion, some few bold minds had foreseen the conclusions to which these observations must lead, and had
predicted the existence of sodium in the sun.
Foremost among these stand Professors Stokes and William Thomson, and the Swedish philosopher Ångström. It is, however, to Kirchhoff that we are indebted for the full and scientific investigation of the subject, and he must be considered as the founder of the science of solar and stellar chemistry.
Wishing to test the accuracy of this frequently asserted coincidence of the bright metallic and dark solar lines with his very delicate instrument, Professor Kirchhoff made the following very remarkable experiment, which is interesting as giving the key to the solution of the problem regarding the existence of sodium and other metals in the sun:
• In order to test in the most direct manner possible the frequently asserted fact of the coincidence of the sodium lines with the lines D, I obtained a tolerably bright solar spectrum, and brought a flame coloured by sodium vapour in front of the slit. I then saw the dark lines D change into bright ones. The flame of a Bunsen's lamp threw the bright sodium lines upon the solar spectrum with unexpected brilliancy. In order to find out the extent to which the intensity of the solar spectrum could be increased without impairing the distinctness of the sodium lines, I allowed the full sunlight to shine through the sodium flame, and to my astonishment I saw that the dark lines D appeared with an extraordinary degree of clearness. I then exchanged the sunlight for the Drummond's, or oxy-hydrogen lime-light, which, like that of all incandescent solid or liquid bodies, gives a spectrum containing no dark lines. When this light was allowed to fall through a suitable flame coloured by common salt, dark lines were seen in the spectrum the position of the sodium lines. The same phenomenon was observed if instead of the incandescent lime a platinum wire was used, which being heated in a flame was brought to a temperature near its melting point by passing an electric current through it. The phenomenon in question is easily explained upon the supposition that the sodium flame absorbs rays of the same degree of refrangibility as those it emits, whilst it is perfectly transparent for all other rays.' (Kirchhoff Researches, &c., pp. 13, 14.)
Thus Kirchhoff succeeded in producing artificial sunlight, at least as far as the formation of one of Fraunhofer's lines is concerned. He proved that the yellow soda flame possesses thisat first sight anomalous-property of absorbing just that kind of light which it emits; it is opaque to the yellow D light, but transparent to all other kinds of light. Hence, if the yellow rays in the spectrum produced by the Drummond's light in the above experiment are more intense than those given off by the soda flame, we shall see in the yellow part of the spectrum shadows, or dark lines; and if the difference of intensity be very great, these shadows may by contrast appear perfectly black. This opacity of heated sodium vapour for the particular kind of light which it is capable of giving off, was strikingly exhibited by Professor Roscoe, in one of a course of lectures on Spectrum Analysis, lately delivered by him in London at the Royal Institution. A glass tube, containing a small quantity of metallic sodium, was rendered vacuous and then closed. On heating the tube, the sodium rose in vapour, filling a portion of the empty space. Viewed by ordinary white light this sodium vapour appeared perfectly colourless, but when seen by the yellow light of a soda-flame the vapour cast a deep shadow on a white screen, showing that it did not allow the yellow rays to pass through.
This remarkable property of luminous gases to absorb the same kind of light as they emit, is not without analogy in the cognate science of Acoustics. Sound is produced by the vibration of the particles of gravitating matter, whilst light is supposed to be produced by a similar vibration of the particles of a non-gravitating matter, called the luminiferous ether. In the case of sound, a similar phenomenon to the one under consideration is well known. We are all acquainted with the principle of resonance; if we sound a given note in the neighbourhood of a pianoforte, the string capable of giving out the vibrations producing that note takes up the vibrations of the voice, and we hear it answering the sound. The intenser vibrations proceeding in one direction are absorbed by the string, and emitted as waves of slighter intensity in every direction.
Not only did Professor Kirchhoff show experimentally that luminous gases absorb the kind of light which they emit, by reversing the spectra of several of the metals, but by help of theoretical considerations he arrived at a very important general formula concerning the emission and absorption of rays of heat and light, which includes these phenomena as a particular case. The general law is called the law of exchanges, and it asserts that the relation between the amount of heat or of light which all bodies receive and emit is for a given temperature constant. Somewhat similar results were arrived at independently by Mr. Balfour Stewart in this country.
In order to determine and map the positions of the bright lines produced by the electric spectra of the various metals, Kirchhoff employed the dark lines in the solar spectrum as his guides. Much to his astonishment, he observed that dark solar lines occur in positions coincident with those of all the bright iron lines. Exactly as the sodium lines were identical in position with Fraunhofer's lines D, for each of the iron lines (and Kirchhoff examined more than sixty) a dark solar line was seen to correspond. Not only had each bright iron line its dark representative in the solar spectrum, but the breadth and degree of distinctness of the two sets of lines agreed in the most perfect manner; the brightest iron lines corresponding to the darkest solar lines. These coincidences cannot be the mere effect of chance; in other words, there must be some causal connexion between these dark solar lines and the bright iron lines. That this agreement between them cannot be simply fortuitous is proved by Kirchhoff, who calculates — from the number of the observed coincidences, the distances between the several lines, and the degree of exactitude with which each coincidence can be determined—the fraction representing the chance or probability that such a series of coincidences should occur without the two sets of lines having any common cause; this fraction he finds to be less than 1.000.000.000.000.000.000, or, in other words, it is practically certain that these lines have a common cause.
• Hence this coincidence,' says Kirchhoff, 'must be produced by some cause, and a cause can be assigned which affords a perfect explanation of the phenomenon. The observed phenomenon may be explained by the supposition that the rays of light which form the solar spectrum have passed through the vapour of iron, and have thus suffered the absorption which the vapour of iron must exert. As this is the only assignable cause of this coincidence, the supposition appears to be a necessary one.
These iron vapours might be contained eith in the atmosphere of the sun or in that the earth. But it is not easy to understand how our atmosphere can contain such a quantity of iron vapour as would produce the very distinct absorp
tion-lines which we see in the solar spectrum ; and this supposition is rendered still less probable by the fact that these lines do not appreciably alter when the sun approaches the horizon. It does not, on the other hand, seem at all unlikely, owing to the high temperature which we must suppose the sun's atmosphere to possess, that such vapours should be present in it. Hence the observations of the solar spectrum appear to me to prove the presence of iron vapour in the solar atmosphere with as great a degree of certainty as we can attain in any question of natural science.' (Kirchhoff Researches, 8c., p. 20.)
This statement is not one jot more positive than the facts warrant. For to what does any evidence in natural science amount to, beyond the expression of a probability ? A mineral sent to us from New Zealand is examined by our chemical tests, of which we apply a certain number, and we say these show us that the mineral contains iron, and no one doubts that our conclusion is correct. Have we, however, in this case proof positive that the body really is iron? May it not turn out to be a substance which in these respects resembles, but in other respects differs from, the body which we designate as iron? Surely. All we can say is, that in each of the many comparisons which we have made the properties of the two bodies prove identical; and it is solely this identity of the properties which we express when we call both of them iron. Exactly the same reasoning applies to the case of the existence of these metals in the sun. Of course the metals present there, causing these dark lines, may not be identical with those which we have on earth ; but the evidence of their being the same is as strong and cogent as that which is brought to bear upon any other question of natural science, the truth of which is generally admitted.
We do not think we can give our readers a more clear and succinct account of the development of this great discovery than by quoting from Kirchhoff's admirable memoir the following passage:
* As soon as the presence of one terrestrial element in the solar atmosphere was thus determined, and thereby the existence of a large number of Fraunhofer's lines explained, it seemed reasonable to suppose that other terrestrial bodies occur there, and that, by exerting their absorptive power, they may cause the production of other Fraunhofer's lines. For it is very probable that elementary bodies which occur in large quantities on the earth, and are likewise distinguished by speciał bright lines in their spectra, will, like iron, be visible in the solar atmosphere. This is found to be the case with calcium, magnesium, and sodium. The number of bright lines in the spectrum of each of these metals is indeed small, but those lines, as well as the dark lines in the solar spectrum with which they coincide,
VOL, CXVI. NO. CCXXXVI.
are so uncommonly distinct that the coincidence can be observed with great accuracy. In addition to this, the circumstance that these lines occur in groups renders the observation of the coincidence of these spectra more exact than is the case with those composed of single lines. The lines produced by chromium, also, form a very characteristic group, which likewise coincides with a remarkable group of Fraunhofer's lines; hence, I believe that I am justified in affirming the presence of chromium in the solar atmosphere. It appeared of great interest to determine whether the solar atmosphere contains nickel and cobalt, elements which invariably accompany iron in meteoric masses. The spectra of these metals, like that of iron, are distinguished by the large number of their lines. But the lines of nickel, and still more those of cobalt, are much less bright than the iron lines, and I was therefore unable to observe their position with the same degree of accuracy with which I determined the position of the iron lines. All the brighter lines of nickel appear to coincide with dark solar lines ; the same was observed with respect to some of the cobalt lines, but was not seen to be the case with other equally bright lines of this metal. From my observations I consider that I am entitled to conclude that nickel is visible in the solar atmosphere; I do not, however, yet express an opinion as to the presence of cobalt. Barium, copper, and zinc appear to be present in the solar atmosphere, but only in small quantities; the brightest of the lines of these metals correspond to distinct lines in the solar spectrum, but the weaker lines are not noticeable. The remaining metals which I have examined — viz., gold, silver, mercury, aluminium, cadmium, tin, lead, antimony, arsenic, strontium, and lithium-are, according to my observations, not visible in the solar atmosphere.' (Kirchhoff: Re: searches, gc., p. 21.)
We are now in a position to understand why the discovery of the existence of these metals in the sun is no myth, no vague supposition, or possible contingency. We now see that this conclusion is derived, by a severely correct process of inductive reasoning, from a series of exact and laborious experiments and observations, and that the presence of these metals in the solar atmosphere has been determined with as great a degree of certainty as is attainable in any question of physical science. But it is only to those who have witnessed the spectacle of the coincidence of the bright iron with the dark solar lines, shown in such an apparatus as that of Kirchhoff's, that it is given adequately to feel the force of this conclusion; and the impression made by such a sight is not one likely to be easily effaced from the mind.
The mode in which new and perhaps startling facts in science, such as those we are now considering, are unwittingly misinterpreted and inisapplied by certain minds to suit their own preconceived notions, must be an interesting branch of study to