measure the various parts of the spectrum. Long ago Frauenhofer observed that in the solar spectrum the band of colored light was not absolutely continuous, but was crossed by a system of lines, which he marked B, C, D, E, F, G, H; which have since been called Frauenhofer's lines, but of which he knew not the significance. Bunsen and Kirchoff, on further investigating this matter, found that the Drummond light, when surrounded by burning gas, gave out another system of lines; and that different gases gave different systems, according to their constitution, or to the different metals burned in their flames; one metal showing one set of lines in one part of the spectrum, another a different set in another part; and that each mineral gives lines of its own peculiar color. Thus, for instance, the sodium in common salt gives a bright, distinct, yellow line; lithium a pale yellow, and a bright red line; while strontium gives a band of six red, one orange, and one blue. The very smallest quantities of these metals present in the atmosphere are easily discovered by the spectroscope, and several new metals, never known before, have been discovered by it. The six-hundred-thousandth part of a millogramme of strontium, in the dust of a room where spectrum experiments were in progress was distinctly displayed upon the spectrum of the gaseous flame.* But while a gas

eous flame gives bright lines in the spectrum, the sun gives dark lines. We can not, of course, distinguish these by their colors, but we may by their number and position. Thus, we find in the spectrum of the sun's rays the lines peculiar to iron, about seventy well-marked lines between the Frauenhofer lines D and F, and conclude that iron exists in the sun; since all the bright lines of the iron spectrum in gas correspond to the dark lines in the sun's spectrum. The two bright yellow

*Annual of Scientific Discovery, 1862, p. 137.

sodium lines are coincident with two dark lines in the solar spectrum: and the bright red line of potassium coincides exactly with a dark line of the same breadth. Thus, we discover in the sun, iron, calcium, magnesium, sodium, aluminum, nickel, barium and copper. The sunlight reflected from the planets gives the same system of lines; but the fixed stars give each a different system, showing the presence of different metals in each. Thus we are enabled to discover, not only whether a ray of light comes from a solid coal or from a flame of gas, but we can also tell of what gas the flame consists. Mr. Huggins thus describes

THE VARIOUS ORDERS OF SPECTRA:

"When light which has emanated from various sources is decomposed by a prism, the spectra which are obtained may differ in several important respects from each other. All the spectra which may present themselves may be conveniently arranged in three groups.

"1. The special character which distinguishes spectra of the first order consists, in that the continuity of the colored band is unbroken by dark or bright lines. We learn from such a spectrum that the light has been emitted by an opaque body, and, almost certainly, in the solid or liquid state. A spectrum of this order gives us no knowledge of the chemical nature of the incandescent body from which the light comes.

"2. Spectra of the second order are very different. These consist of colored lines of light separated from each other. From such a spectrum we may learn much. It informs us that the luminous matter from which the light comes, is in the state of gas. It is only when a luminous body is free from the molecular trammels of liquidity and solidity that it can exhibit its own peculiar power of radiating some colored rays alone. Hence,

substances when in a state of gas may be distinguished from each other by their spectra. Each element, and every compound body that can become luminous in the gaseous state without suffering decomposition, is distinguished by a group of lines peculiar to itself.

"3. The third order consists of the spectra of incandescent solid or liquid bodies, in which the continuity of light is broken by dark lines. These dark spaces are not produced by the source of the light. They tell us of vapors through which the light has passed on its way, and which have robbed the light by absorption of certain definite colors or rates of vibration. Such spectra are formed by the light of the sun and stars.

"Kirchoff has shown that if vapors of terrestrial substances come between the eye and an incandescent body, they cause groups of dark lines; and further, that the group of dark lines produced by each vapor is identical in the number of the lines, and their position in the spectrum, with the groups of bright lines of which its light consists when the vapor is luminous.

"It is evident that Kirchoff, by means of this discovery, has furnished us with the means of interpreting the dark lines of the solar spectrum. When a group of bright lines coincides with a similar group of dark lines, then we know that the terrestrial substance producing the bright lines is present in the atmosphere of the sun; for it is this substance, and this substance alone, which, by its own peculiar power of absorption, can produce that peculiar group of lines. In this way Kirchoff discovered the presence of several terrestrial elements in the solar atmosphere.'

The result of these investigations of the sun, stars, and nebula is, that they are all composed of several

*Lecture before the British Association, Nottingham, August 23, 1866, by W. Huggins, F. R. S.

elements, some of which are earthly, others unearthly, and that they are widely different in their chemical compositions. Not only do the nebulæ differ from the stars in being in a different mechanical condition through heat, as if they might be stars burned up into gas retaining all the elements of stars; they present radical differences of chemical composition, such as could by no means originate in any change of temperature. The stars also differ from each other in the same remarkable way; they are all composed of several elements, and differ in the elements of which they are composed. There is

NOT A TRACE OF HOMOGENEOUS MATTER,

containing only one simple uncompounded element, in the heavens, neither in the original nebulæ, nor in the suns and stars which were said to be composed of it.

The remarkable difference in the material of the stars overthrows the notion of their homogeneous origin. It is thus stated, and its undeniable consequence drawn with emphasis, by Mr. Rutherford of New York, one of the highest authorities on Spectrum Analysis; "The star spectra present such varieties that it is difficult to point out any mode of classification. For the present, I divide them into three groups: first, those having many lines and bands, and most nearly resembling the sun; viz., Capella, Geminorum, a Orionis, Aldebaran, 7 Leonis, Arcturus, and ẞ Pegasi. These are all reddish or golden stars. The second group, of which Sirius is the type, present spectra wholly unlike that of the sun, and are white stars. The third group, comprising a Virginis, Rigel, etc., are also white stars, but show no lines—perhaps they contain no mineral substance, or are incandescent without flame.

"It is not my intention to hazard any conjectures based upon the foregoing observation; this is more properly

the province of the chemist; and a great accumulation of accurate data should be obtained before making the daring attempt to proclaim any of the constituent ele

ments of the stars.

"One thought I can not forbear suggesting. We have long known that 'one star differeth from another star in glory; we have now the strongest evidence that they also differ in constituent materials, some of them perhaps having no elements to be found in some others. What, then, becomes of that homogeneity of original diffuse matter which is almost a logical necessity of the Nebular Hypothesis?" +

THE CONSTITUTION OF THE NEBULE.

Of the constitution of the nebula, William Huggins of London, the leading spectroscopist, says:

"The telescope has failed to give any certain information of the nature of the nebulæ. It is true that each successive increase of aperture has resolved more of these objects into bright points; but, at the same time, other fainter nebulæ have been brought into view, and fantastic wisps, and diffused patches of light have been seen which the mind almost refuses to believe can be due to the united glare of innumerable suns still more remote.

"Spectrum analysis, if it could be successfully applied to objects so excessively faint, was obviously a method of investigation specially suitable for determining whether any essential physical distinction separates the nebulæ from the stars.

"I selected, for the first attempt, in August, 1864, one of the class of small but comparatively bright nebulæ. My surprise was very great, on looking into the small

*1 Cor. xv. 41.

† Annual of Scientific Discovery, 1865, p. 331.