coloured rays together in the eye, and again obtaining the white image of the hole in the shutter, he proved that the kind of light which produces on the eye the sensation we term whiteness, is in reality made up of an infinite number of differently coloured rays.

The coloured band thus obtained by Newton did not, however, reveal to him all the characteristic beauties of solar light, because in his spectrum the tints were created by the partial superposition of an infinite number of differently coloured images of the round hole through which the light came. It was not until the year 1802 that Dr. Wollaston, by preventing the different coloured lights from overlapping, and thus interfering with each other, discovered that great peculiarity in solar light which has led to such startling discoveries in the composition of the sun itself. Dr. Wollaston noticed, when he allowed the sunlight to fall through a narrow slit upon the prism, that a number of dark lines cutting up the coloured portions of the spectrum, made their appearance. These dark lines, or spaces, of which Wollaston counted only seven, indicate the absence of certain distinct kinds of rays in the sunlight; they are, as it were, shadows on the bright background.

It is, however, to the celebrated German optician Fraunhofer, that we owe the first accurate examination of these singular lines. By a great improvement in the optical arrangements employed, Fraunhofer, re-discovering these lines, was able to detect a far larger number of them in the solar spectrum than had been observed by Wollaston. He counted no less than 590 of these dark lines, stretching throughout the length of the spectrum from red to violet, and in the year 1815 drew a very beautiful map of them, some of the most important of which he designated by the letters of the alphabet. Fraunhofer carefully measured the relative distances between these lines, and found that they did not vary in sunlight examined at different times. He also saw these same dark fixed lines in reflected as well as direct solar light; for on looking at the spectrum of moonlight and of Venus-light, the same lines appeared quite unaltered in position. But he found that the light of the fixed stars was not of the same kind as direct or reflected sunlight, as the spectra of the starlight contained dark lines entirely different from those which are invariably seen in the solar spectrum. From these observations Fraunhofer, so early as 1815, drew the important conclusion that these lines, let them be caused by what they may, must in some way or other have their origin in the sun. The explanation of the production of these lines was reserved for a subsequent time; but

Fraunhofer opened the inquiry, and all his conclusions have been borne out by recent and more elaborate investigations.

Since the time of Fraunhofer our knowledge of the constitution of the solar spectrum has largely increased. Professor Stokes, in his beautiful researches on Fluorescence, has shown that similar dark lines exist in that part of the spectrum extending beyond the violet, which require special arrangements to become visible to our eyes; and Sir David Brewster and Dr. Gladstone have mapped with great care about 2000 lines in the portion of the spectrum from red to violet.

But it is to Kirchhoff, the Professor of Physics in the University of Heidelberg, that we are indebted for by far the best and most accurate observations of these phenomena. In place of using one prism, as Fraunhofer did, Kirchhoff employed four prisms of most perfect workmanship, and thus enjoyed the advantage of a far greater dispersion, or spreading out, of the different rays than the Munich optician had obtained. The lines were observed through a telescope having a magnifying power of 40, and when the whole apparatus was adjusted with all the accuracy and delicacy which the perfection of optical instruments now renders possible, Kirchhoff saw the solar spectrun with a degree of minute distinctness such as had never before been attained; and of the beauty and magnificence of the sight thus presented those only who have been eye-witnesses can form any idea.

Kirchhoff's purpose was not merely to observe the fine vertical dark lines which in untold numbers crossed the coloured spectrum, stretching from right to left. He wished to measure their relative distances, and thus to map them, exactly as the astronomer determines the position of the stars in the heavens, and the surveyor triangulates and marks out the main features of a country; so that future wanderers in this new field may find fixed and well-recognised points from which to commence their own excursions. Professor Kirchhoff is far from thinking that his measurements, delicate and numerous though they be, have exhausted the subject. The further we penetrate into the secrets of nature, the more we find there remains to be learnt. He saw whole series of nebulous bands and dark lines which the power of his instrument did not enable him to resolve; and he thinks that a larger number of prisms must be employed to effect this end. He adds-The resolution of these 'nebulous bands appears to me to possess an interest similar to that of the resolution of the celestial nebule; and the investigation of the spectrum to be of no less importance than the 'examination of the heavens themselves.' True, indeed, does

this appear, when we learn that it is by the examination of these lines that we can alone obtain the clue to the chemical composition of sun and stars!

The exact measurement of the distances between the lines was made by moving the cross wires of the telescope from line to line by means of a micrometer screw with a finely divided head, and reading off the number of divisions through which the screw had to be turned. The breadth and degree of darkness were also noticed, and thus the lines were mapped. In order to give a representation in the drawing of the great variety of the shade and thickness of the lines, they were arranged according to their degree of blackness, and drawn of six different thicknesses. First, the darkest lines were drawn with thick black Indian ink; the ink was then diluted to a certain extent, and the lines of the next shade drawn, and so on to the lightest series. As soon as a portion of the spectrum had been drawn in this manner, it was compared with the actual spectrum, and the mistakes in the breadth and darkness of the lines, as well as in their position, corrected by fresh estimations, and the drawing made anew. A second comparison and another drawing were then made, and this process repeated until all the groups of lines appeared to be truthfully represented. Copies from the same lithographic stones accompany the English edition of the memoir as are appended to the original, and these are masterpieces of German artistic skill. They are printed on six different stones, with ink of six different tints, and reproduce with marvellous fidelity the appearance which the solar spectrum presents when viewed through the magnificent Heidelberg instrument.

These maps extend, however, over only one-third part of the visible portion of the solar spectrum, and it will, we fear, be long before the other two-thirds are completely surveyed, as the following note, telling of the failing eyesight of the ingenious observer, touchingly explains:- My drawing,' he says, 'is 'intended to include that portion of the spectrum contained between the lines A and G. I must, however, confine myself at present to the publication of a part only of this, as the re'mainder requires a revision, which I am unfortunately unable to undertake, owing to my eyes being weakened by the con'tinual observations which the subject rendered necessary.'

Before it can be understood how these dark lines reveal the chemical composition of the solar atmosphere, it must be shown how the constitution of terrestrial matter can be ascertained by the examination of the nature of the light which such heated matter emits. That certain substances, when heated or burnt,

give off peculiar kinds of light, has long been known; and this fact has been made use of by the chemist to distinguish and detect such substances. Thus compounds of the earth strontia, when burnt with gunpowder, produce the peculiar mixture well known as the red fire' of the pyrotechnist; the salts of baryta give colour to the green fires of the stage; and we all see in the Christmas game of snap-dragon that a handful of salt (chloride of sodium) thrown into the dish imparts to the flame a yellow colour.

This property of substances to give off certain kinds of light was formerly only known to hold good for a few bodies; but the progress of science has taught us that it is not confined to one substance, but is applicable to all. We only require to examine a body under the proper conditions, in order to see that when heated it emits a peculiar and characteristic kind of light; so that each elementary substance—that is, a substance which has not been split up, or decomposed, or out of which no two or more bodies differing in their properties have been obtained-whether it be a gas, a solid, or a liquid, may by heating be made to emit a kind of light peculiar to itself, and different from that given off by any other substance. Here, then, is the basis of this new method of spectrum analysis—a science which demonstrates the chemical composition of a body by the colour or kind of light emitted from it when heated. We now only need to know, in order to understand the subject, the proper conditions under which bodies can be made to develope this beautiful property, by help of which their chemical natures can be thus easily investigated, and analysis rendered not only independent of test-tubes, but likewise of distance; for it is clear that so long as light can be seen, it matters not how far removed its source may be. The sole condition which must be fulfilled in order to attain the object, is that the body to be analysed must be in a condition of luminous gas or vapour; for it is only in the gaseous state that each kind of matter emits the light peculiar to itself. It is somewhat difficult at first to understand how a gas or air can be heated until it emits light, and yet familiar instances are not wanting of such a condition of things. Flame, indeed, is nothing else than heated and luminous gas; and in the blue part of the flame of a candle, and in the lambent blue flame which plays on the top of a large fire, we have examples of a truly gaseous body heated until it becomes luminous.

The modes in which the various elements can be best obtained in the condition of luminous gases are very different. For the compounds of the metals of the alkalies and alkaline earths, it

suffices to bring a small quantity of one of their salts into a flame of a spirit lamp, or into a gas flame. The salt then volatilises, or becomes gaseous; and this vapour, heated to the temperature at which it is luminous, tinges the flame with a peculiar colour. For the compounds of the other metals, such as iron, platinum, or silver, a much higher temperature is needed; whilst for bodies such as air and hydrogen, which are gases at the ordinary temperature, a different mode of manipulation is necessary.

In order to become acquainted with the exact nature of the light which bodies in the condition of luminous gases emit, their light must be examined otherwise than by the naked eye. The same kind of apparatus is used in this investigation which Fraunhofer and Kirchhoff applied to the investigation of solar light; in short, the distinctive qualities of these luminous gases are ascertained by their spectra. Then only is it that the full beauty of this property of matter becomes apparent, and the character of each elementary body is written down in truly glowing language-language different for every element, but fixed and unalterable for each one, as to the interpretation of which no variety of opinion can possibly exist.

To Professors Bunsen and Kirchhoff science is mainly indebted for the examination of this hitherto hidden language of nature. These philosophers undertook an investigation of theSpectra of the Chemical Elements,' and nobly have they carried out their intention; unfolding a vast store of nature's secrets to the knowledge of mankind, and revealing the existence of much more yet to be learnt in unlimited fields which promise a rich harvest of discovery to the patient and exact inquirer. Seldom indeed has it been the privilege of men in a single discovery to found a science, or to open a subject so pregnant with important results as that of spectrum analysis.

Those alone who are acquainted with the practical details of the science of Chemistry will be able fully to appreciate the grand change which the introduction of this new method effects in the branch of their science devoted to analysis. Qualitative analysis thereby undergoes a complete revolution; the tedious operations of precipitation and filtration must now be superseded by the rapid observation of the spectra of the coloured flames by which the presence of the most minute trace of the substance far too small to be found by the older and coarser methods can be surely and clearly detected. Let us endeavour to form an idea of the appearance of the peculiar spectra thus obtained; the most complete or eloquent description must, however, feil to give more than a bare idea of the reality.