the electric lamp, L a converging lens, B the birefracting spar, and P the thermo-electric pile.) If time permitted we might finish the series of demonstrations by magnetizing a ray of heat as we magnetized a ray of light. We have finally to determine the position and magnitude of the invisible radiation which produces these results. For this purpose we employ a particular form of the thermo-electric pile. Its face is a rectangle, which by movable side-pieces can be rendered as narrow as desirable. Throwing a concentrated spectrum upon a screen, by means of an endless screw, we move this rectangular pile through the entire spectrum. Its surface is blackened so that it absorbs all the light incident upon it, converting it into a curve which exhibits the distribution of heat in our spectrum. It is represented in the adjacent figure. Beginning at the blue, the curve rises, at first very gradually; then, as it approaches the red more rapidly, the line CD representing the strength of the extreme red radiation. Beyond the red it shoots upwards in a steep and massive peak to B, whence it falls, rapidly for a time, and afterwards gradually fading from the perception of the pile. This figure is the result of more than twelve careful series of measurements, for each of which the curve was constructed. On superposing all these curves, a satisfactory agreement was found to exist between them. So that it may safely be concluded that the areas of the dark and white spaces respectively represent the rela FIG. 25. G P heat, and thus enabling it to declare its power | tive energies of the visible and invisible by the deflection of the magnetic needle. radiation. The one is 7.7 times the other. When this instrument is brought to the But in verification, as already stated, conviolet end of the spectrum, the heat is found sists the strength of science. Determining to be almost insensible. As the pile grad- in the first place the total emission from the ually moves from the violet towards the red, electric lamp; then by means of the iodine it encounters a gradually augmenting heat. filter determining the ultra-red emission; the The red itself possesses the highest heating difference between both gives the luminous power of all the colors of the spectrum. emission. In this way, it was found that the Pushing the pile into the dark space beyond the energy of the invisible emission is eight times red, the heat rises suddenly in intensity, and, at that of the visible. No two methods could some distance beyond the red, attains a max-be more opposed to each other, and hardly imum. From this point the heat falls somewhat more rapidly than it rose, and afterwards gradually fades away. Drawing an horizontal line to represent the length of the spectrum, and erecting along it, at various points, perpendiculars proportional in length to the heat existing at those points, we obtain any two results could better harmonize. I think, therefore, you may rely upon the accuracy of the distribution of heat here assigned to the prismatic spectrum of the electric light. There is nothing vague in the mode of investigation, nor doubtful in its conclusions. LECTURE VI. Priasis of Spectrum Analysis: Solar Chemistry: Summary and Conclusions. We have employed, as our source of light in these lectures, the ends of two rods of coke rendered incandescent by electricity. Coke is particularly suitable for this purpose, because it can bear intense heat without fusion or vaporization. It is also black, which helps the light; for, other circumstances being equal, as shown experimentally by Bal four Stewart, the blacker the body the brighter will be its light when incandescent. Still, refractory as carbon is, if we closely examine our voltaic arc, or stream of light between the carbon-points, we should find there incandescent carbon-vapor. We might also detach the light of this vapor from the more dazzling light of the solid points, and obtain its spectrum. This would be not only less brilliant, but of a totally different character from the spectra that we have already seen. Instead of being an unbroken succession of colors from red to violet, the carbon-vapor would yield a few bands of color with spaces | You never get other bands than these two of darkness between them. What is true of the carbon is true in a still more striking degree of the metals, the most refractory of which can be fused, boiled, and reduced to vapor by the electric current. From the incandescent vapor the light, as a general rule, flashes in groups of rays of definite degrees of refrangibility, spaces existing between group and group, which are unfilled by rays of any kind. But the contemplation of the facts will render this subject more intelligible than words can make it. Within the camera is now placed a cylinder of carbon hollowed out at the top to receive a bit of metal; in the hollow is placed a fragment of the metal thallium, and now you see the arc of incandescent thallium-vapor upon the screen. It is of a beautiful green color. What is the meaning of that green? We answer the question by subjecting the light | to prismatic analysis. Here you have its spectrum, consisting of a single refracted band. Light of one degree of refrangibility, and that corresponding to green, is emitted by the thallium-vapor. green ones from the silver, never other than the single green band from the thallium, never other than the three green bands from the mixture of both metals. Every known metal has its bands, and in no known case are the bands of two different metals alike. Hence these spectra may be made a test for the presence or absence of any particular metal. If we pass from the metals to their alloys, we find no confusion. Copper gives us green bands, zinc gives us blue and red bands; brass, an alloy of copper and zinc, gives us the bands of both metals, perfectly unaltered in position or character. But we are not confined to the metals; the salts of these metals yield the bands of the metals. Chemical union is ruptured by a sufficiently high heat, the vapor of the metal is set free and yields its characteristic bands. The chlorides of the metals are particularly suitable for experiments of this character. Common salt, for example, is a compound of chlorine and sodium; in the electric lamp, it yields the spectrum of the metal sodium. The chlorides of lithium and of strontium yield in like manner the bands of these metals. When, therefore, Bunsen and Kirchhoff, the celebrated founders of spectrum We will now remove the thallium and put a bit of silver in its place. The arc of silver is not to be distinguished from that of thallium; it is not only green, like the thallium-analysis, after having established by an exvapor, but the same shade of green. Are they, then, alike? Prismatic analysis enables us to answer the question. It is perfectly impossible to confound the spectrum of incandescent silver vapor with that of thallium. Here are two green bands instead of one. Adding to the silver in our camera a bit of thallium, we obtain the light of both metals, and you see that the green of the thallium lies midway between the two greens of the silver. Hence this similarity of color. haustive examination the spectra of all known substances, discovered a spectrum containing bands different from any known bands, they immediately inferred the existence of a new metal. They were operating at the time upon a residue obtained by evaporating one of the mineral waters of Germany. In th water they knew the new metal was c cealed, but vast quantities of it had to be evaporated before a residue could be obtained sufficiently large to enable ordinary chemistry to grapple with the metal. But they hunted it down, and it now stands among chemical subsequently discovered a second metal, which they called Casium. Thus, having first placed spectrum analysis on a safe foundation, they demonstrated its capacity as an agent of discovery. Soon afterwards Mr. Crookes, pursuing this same method, obtained the salts of the thallium which yielded that bright monocromatic green band. The metal itself was first isolated by a French chemist. But you observe another interesting fact. The thallium band is now far brighter than the silver bands; indeed, the latter have won-substances as the metal Rubidium. They derfully degenerated since the bit of thallium was put in. They are not at all so bright as they were at first, and for a reason worth knowing. It is the resistance offered to the passage of the electric current from carbon to carbon that calls forth the power of the current to produce heat. If the resistance were materially lessened, the heat would be materially lessened; and, if all resistance were abolished, there would be no heat at all. Now, thallium is a much more fusible All this relates to chemical discovery upon and vaporizable metal than silver; and its earth, where the materials are in our own vapor facilitates the passage of the current to hands. But Kirchhoff showed how spectrum such a degree as to render it almost incom-analysis might be applied to the investigation petent to vaporize the silver. But the thalhum is gradually consumed; its vapor diminishes, the resistance rises, until finally you see the two silver bands as brilliant as they were at first. The three bands of the two metals are now of the same sensible brightness. We have in these bands a perfectly unalterable characteristic of these two metals. of the sun and stars, and on his way to this result he solved a problem which had been long an enigma to natural philosophers. A spectrum is pure in which the colors do not overlap each other. We purify the spectrum by making our slit narrow and by augmenting the number of our prisms. When a pure spectrum of the sun has been obtained in this way it is found to be furrowed by in numerable dark lines. Four of them were But let me not skim so lightly over this first seen by Dr. Wollaston, but they were great subject. I have spoken of emisafterwards multiplied and measured by Fraun- sion and absorption, and of the link that hofer with such masterly skill that they are binds them. Let me endeavor to make plain now universally known as Fraunhofer's lines. to you, through the analogy of sound, their To give an explanation of physical meaning. I draw a fiddle-bow across these lines was, as I have said, this tuning-fork, and it immediately fills the a problem which long chal- room with a musical sound; this may be relenged the attention of philos-garded as the radiation or emission of sound ophers. (The principal lines from the fork. A few days ago, on sounding are lettered according to Fraun- this fork, I noticed that, when its vibrations hofer in the annexed sketch of were quenched, the sound seemed to be conthe solar spectrum A, it may tinued, though more feebly. The sound apbe stated stands near the ex-peared to come from under a distant tabl treme red, and J near the where stood a number of tuning-forks of difextreme violet.) ferent sizes and rates of vibration. One of these, and one only, had been started by the fork, and it was one whose rate of vibration was the same as that of the fork which started it. This is an instance of the absorption of sound of one fork by another. Pacing two forks near each other, sweeping the bow over one of them, and then quenching the agitated fork, the other continues to sound. Placing a cent-piece on each prong of one of the forks, we destroy its perfect synchronism with the other, and then no communication of sound from the one to the other is possible. 1 D Now, Kirchhoff had made thoroughly clear to his mind the princ ples which linked together the emission of light and the absorption of light; he had proved their inseparability for each particular kind of light and heat. He had proved, for every specific ray of the spectrum, the doctrine that the body emitting any ray absorbed with special energy a ray of the same refrangibility. Consider, then, the effect of I will now do with light what has been here knowledge, such as you now done with sound. Placing a tin spoon con possess, upon a mind prepared taining sodium in a Bunsen's flame, we ob like that of Kirchhoff. We tain this intensely yellow light, which corr have seen the incandescent va- sponds in refrangibility with the yellow ban i pors of metals emitting defi- of the spectrum. Like our tuning-fork, it nite groups of rays; accord- emits waves at a special period. I will send ing to Kirchhoff's principle, the white light from our lamp through that those vapors, if crossed by solar flame, and prove before you that the yellow light, ought to absorb ays of flame intercepts the yellow of the spectrum the same refrangibility as those SS, Fig. 28; in other words, absorbs w. ves which they emit. He proved of the same period as its own, thus producthis to be the case; he was ing, to all inter.ts and purposes, a da k able, by the interposition of a Fraunhofer's band in the place of the yellow. vapor, to cut out of the solar (A Bunsen's flume contained within the chimspectrum the band correspond-ney Cis placed in front of the lamp L. Th: ing in color to that vapor. tin spoon with its pellet of sodium is plunged Now, the sun possesses a pho- into the flame. Vivid combustion soon sets tosphere, or vaporous enve- in, and, when it does, the yellow of the spec lope-doubtless mixed with vi- trum, at D, is furrowed by a dark ban'. olently agitated clouds-and Withdrawing and introducing the sodium. Kirchhoff saw that the power- flame in rapid succession, the sudden disap ful rays coming from the solid, or the pearance and reappearance of the strip of molten nucleus of the sun, must be inter- darkness are observed). cepted by this vapor. One dark band of Mentally, as well as physically, every age Fraunhofer, for example, occurs in the of the world is the outgrowth and offspring yellow of the spectrum. Sodium vapor is of all preceding ages. Science proves itself demonstrably competent to produce that dark to be a genuine product of Nature by growband; hence Kirchhoff inferred the existing according to this law. We have no soence of sodium-vapor in the atmosphere of .the sun. In the case of metals, which emit a large number of bands, the absolute coic'dence of every bright band of the metal with a dark Fraunhofer line, raises to the highest degree of certainty the inference that the metal is present in the atmosphere of the In this way solar chemistry was founded on spectrum analysis. sun. FIG. 27. lution of continuity here. Every great discovery has been duly prepared for in two ways: first, by other discoveries which form its prelude; and, secondly, through the sharpening, by exercise, of the intellectual instrument itself. Thus Ptolemy grew out of Hipparchus, Copernicus out of both, Kepler out of all three, and Newton out of all the four. Newton did not rise suddenly Irom the sea-level of the intellect to his amazing | in plain words, my subject before you, and to elevation. At the time that he appeared, the table-land of knowledge was already high. He juts, it is true, above the table-land, as a massive peak; still he is supported by it, and a great part of his absolute height was the height of humanity in his time. It is thus FIG. 28. alta the discoveries of Kirchhoff. Much had been previously accomplished; this he mastered, and the by the force of individual genius went beyond it. He replaced uncertainty by certainty, vagueness by definiteness, confusion by order; and I do not think that Newton has a su er claim to the discoveries that have made A's name immortal than Kirchhoff has to the credit of gathering up the fragmentary knowledge of his time, of vastly extending it, and of infusing into it the life of great principles. Splendid results have since been obtained with which the names of Janssen, Huggins, Lockyer, Respighi, Young, and others, are honorably associated, but, splendid as they are, they are but the sequel and application of the principles established in his Heidelberg laboratory by the celebrated German investigator. SUMMARY AND CONCLUSIÓN My desire in these lectures has been to show you, with as little breach of continuity as possible, the past growth and present as pect of a department of science, in which have labored some of the greatest intellects the world has ever seen. My friend Professor Henry, in introducing me at Washington, spoke of me as an apostle; but the only apostolate that I intended to fulfil was to place, permit its own intrinsic attractions to act upon your minds. In the way of experiment, I have tried to give you the best which, under the circumstances, could be provided; but I have sought to confer on each experiment a distinct intellectual value, for experiments ought to be the representatives and exposi tors of thought—a language addressed to the eye as spoken words are to the ear. In association with its context, nothing is more impressive or instructive than a fit experi, ment; but, apart from its context, it rather suits the conjuror's purpose of surprise than that purpose of education which ought to be the ruling motive of the scientific man. And now a brief summary of our work will not be out of place. Our present mastery over the laws and phenomena of light has its origin in the desire of man to know. We have seen the ancients busy with this problem, but, like a child who uses his arms aimlessly for want of the necessarv muscular exercise, so these early men speculated vaguely and confusedly regarding light, not having as yet the discipline needed to give clearness to their insight, and firmness to their grasp of principles. They assured themselves of the rectilineal propogation of light, and that the angle of incidence was equal to the angle of reflection. For more than a thousand years-I might say, indeed, for more than fifteen hundred years subsequently - the scientific intellect appears as if smitten with paralysis, the fact being that, during this time, the mental force, which might have run in the direction of science, was diverted in o other directions. The course of investigation as regards light was resumed in 1100 by an Arabian philosopher named Alhazan. Then it was taken up in succession by Roger Bacon, Vitellio, and Kepler. These men, though failing to detect the principle which ruled the facts, kept the fire of investigation constantly burning. Then came the fundamental discovery of Snell, that corner-stone of optics, as I have already called it, and immediately afterward we have the application by Des cartes of Snell's discovery to the explanation of the rainbow. Then came Newton's crowning experiments on the analysis and synthesis of white light, by which it was proved to be compounded of various kinds of light of different degrees of refrangibility. In 1676 an impulse was given to optics by astronomy. In that year Olaf Roemer, a learned Dane, was engaged at the Observatory of Paris in observing the eclipses of Jupiter's moons. He converted them into so many signal-lamps, quenched when they plunged into the shadow of the planet, and relighted when they emerged from the shadow. They enabled him to prove that light requires time to pass through space, and to assign to it the astounding velocity of 190,ooo miles a second. Then came the English |