Spectroscopy

“ It is one of the problems of spectroscopy to find out the composition of bodies, not only qualitatively, but also quantitatively, and, when we shall know in what proportion different bodies are distributed in the sun, we may reduce the problem of finding out this temperature to the much simpler one of finding out the temperature of a given electric spark. ”

“ The important point is not whether a body is solid, liquid, and gaseous, but how many atoms are bound together in a molecule, and how they are bound together. This is one of the teachings of modern spectroscopy. A molecule containing a few atoms only gives a spectrum of lines. Increase the number of atoms, and you will obtain a spectrum of fluted bands; increase it once more, and you will obtain a continuous spectrum. The scientific evidence for the statements I have made is unimpeachable. In the first place, I may examine spectra of bodies which I know to be compound. ”

“ Later, using spectroscopy, which is a technique used in every natural science, they compare the colors missing with the ones that atoms emit.
The light of the stars interacts with electrons within atoms. Photons with the exact energy are absorbed and drive electrons to higher energy levels. ”

“ The difficulty that a number of spectroscopic lines seem to involve at least an equal number of electrons may be got over by imagining that the atom may present several positions of equilibrium to the electron, which it may occupy in turn. A collision may be able to throw the electrons from one of these positions to another. According to this view the different lines are given out by different molecules, and we should have to take averages over a number of molecules to obtain the complete spectrum just as we now take averages of energy to obtain the temperature. ”

“ It has been denied by some that pure thermal motion can ever give rise to line spectra, but that either chemical action or impact of electrons is necessary to excite the regular oscillations which give rise to line spectra. There is no doubt that the impact of electrons is likely to be effective in this respect, but it must be remembered that all bodies raised to a sufficient temperature are found to eject electrons, so that the presence of the free electrons is itself a consequence of temperature. ”

“ Compounds generally show spectra of resolvable bands, and
if an elementary body shows a spectrum of the same type we are probably justified in assuming it to be due to a complex molecule. But that it may be given by the ordinary diatomic molecule is exemplified by oxygen, which gives in thick layers by absorption one of the typical sets of Bands which were used by Deslandres and others to investigate the laws of distribution of frequencies. ”

“ Leaving the consideration of radical changes of a vibrating system out of account for the present, the minor differences which have been observed in the appearances of spectra under different sparking conditions are probably to a large extent due to differences in the quantities of material examined, though temperature must alter the violence of the impact and there is a possible effect due to a difference in the impact according as the vibrating system collides with an electron or with a body of atomic dimensions. ”

“ Spectroscopic Measurements and Standards of Wave-Length.—All spectroscopic measurement should be reduced to wave-lengths or wave-frequencies, by a process of interpolation between lines the wave-lengths of which are known with sufficient accuracy. ”

“ Special precautions often are necessary to accomplish this purpose, for too high a temperature would invariably break up the compound molecule into its more elementary constituents. For some bodies I may employ the low temperature of an ordinary Bunsen burner. With others, a weak electric spark taken from their liquid solutions will supply a sufficient quantity of luminous undecomposed matter to allow the light to be analyzed by a spectroscope of good power. The spectrum of a compound body is never a line-spectrum. It is either a spectrum of bands or a continuous spectrum. ”

“ The spectroscope, however simple or complex it may be, is an instrument which allows us to observe the image of the slit through which the light enters it, in the most perfect manner. If the light contains rays of every wave-length, then the images formed by each will be so close together that the spectrum will be continuous, that is, without break. If the light contains only certain wave-lengths, then we shall get certain, and not all, of the possible images of the slit, and the spectrum will be discontinuous. ”

“ With regard to the well-known statement that solids and liquids give continuous spectra, while gases give line-spectra, it must be remarked that metallic vapors show in nearly all cases a continuous spectrum before they condense. Oxygen gives a continuous spectrum at the lowest temperature at which it is luminous. Examining liquids and solids by the method of absorption, we find that many of them show discontinuous spectra, presenting fairly narrow bands. It is not denied that the nearness of molecules does not affect the spectrum. ”

“ Fundamentally, spectroscopic analysis depends upon the principle of refraction, of which we have spoken in connection with the atmosphere. Although the most powerful 147spectroscopes are now made on a different plan, the working of the instrument can best be comprehended by considering it in the form in which it was first invented, and in which it is still most often used. In its simplest form the spectroscope consists of a three-angle prism of glass, through which a ray of light is sent from the sun, star, or other luminous object to be examined. ”

“ Light from any substance which has been made incandescent may be observed with the spectroscope in the same way, and each element can be thus separated. It is found that each substance (in the same conditions of pressure, etc.) gives a constant spectrum of its own. _Each metal displays its own distinctive colour. It is obvious, therefore, that the spectrum provides the means for identifying a particular substance._ It was by this method that we discovered in the sun the presence of such well-known elements as sodium, iron, copper, zinc, and magnesium. ”

“ If I have a transparent body and allow light sent out by a body giving a continuous spectrum to fall through it, I often observe that the transparent body sifts out of the light falling through it certain kind of rays. Spectra are thus produced which are called absorption spectra, because the body which is under examination does not send out any light, but absorbs some vibrations which are made to pass through it. It is an important fact that a molecule absorbs just the rays which it is capable itself of sending out. ”

“ We now come to another very important discovery, made by means of our prism and narrow slit—the determination of the composition of the photosphere or mass of luminous vapour surrounding the body of the sun.
A simple experiment will show how this brilliant discovery was arrived at. The light of a candle or other flame containing incandescent solid matter is passed through the spectroscope, and is found to decompose into a continuous spectrum, uninterrupted by dark lines. Between the light and the slit a spirit-lamp is placed, but no difference in the appearance of the spectrum is perceived. ”

“ But as I have succeeded in demonstrating, on the one hand, that this spectrum does not belong to carbon, and on the other hand that the spectrum of the candle-flame was brightly visible in the ignited filament of the incandescent lamp when the vacuum is as perfect as it is possible to make it, I think there should now be little doubt respecting the identity of the two spectra. Carbon, being found in various combinations everywhere on the surface of the globe, should of necessity reveal its presence in most of the bodies subjected to spectrum analysis. ”

“ Ordinary incandescent vapours from highly heated substances give discontinuous spectra, i.e. spectra in which the rays of coloured light are quite limited, and they appear in the spectroscope only as lines of the breadth of the slit. These are called line-spectra, and every chemical element possesses in the incandescent gaseous state its own characteristic lines of certain colour and certain refrangibility, by means of which that element can be recognised. ”

“ The study of the way in which atoms combine into molecules belongs to chemistry, and will not much concern us. We are concerned with the structure of atoms, the way in which electrons and nuclei come together to build up the various kinds of atoms. This study belongs to physics almost entirely. There are three methods by which most of our knowledge is obtained: the spectroscope, X-rays, and radio-activity. The hydrogen atom, which has a simple nucleus and only one electron, is studied by means of the spectroscope almost alone. ”

“ But it would be altogether a mistake to suppose that some element existing in exceedingly small quantities, or, more correctly, existing in the form of an exceedingly rare vapour in the sun’s atmosphere, would be detected by means of its dark lines, or by any other method depending on the study of the solar spectrum. When we place a small portion of some substance in the space between the carbon points of an electric lamp, and volatilize that substance in the voltaic arc, we obtain a spectrum including all the bright lines of the various elements contained in the substance ”

“ It must be remembered that the light in which the object is to be photographed must be the same as that in which the luminosity of the glasses or pigments is measured.
Action of the Spectrum on Chromic Salts.—The salts most usually employed in photography are the bichromates of the alkalis. The result of spectrum action is confined to its own most refrangible end, commencing in the ultra-violet and reaching as far as in the solar spectrum. Fig. 2 shows the relative action of the various parts of the spectrum on potassium bichromate. ”

“ When a body loses energy by radiation, that energy is restored during an encounter; the way in which this energy is restored will profoundly affect the vibrations of the molecule, and hence the observed spectrum. I have endeavored by means of theoretical considerations, or speculations, as you may perhaps feel inclined to call them, to lead you on to an experimental law which I believe to be of very great importance. The spectrum of a molecule is in fact variable at any given temperature, and changes if the molecule is surrounded by others of different nature. ”

“ Stars with fluted spectra indicate the presence of vapours of the metalloids or of compound substances, while the reversed flutings indicate the presence of carbon. These conclusions have been reached by careful laboratory experiments which are now carried on at the same time as the spectral examination of the stars and other heavenly bodies, so that each peculiarity of their spectra, however puzzling and apparently unmeaning, has been usually explained, by being shown to indicate certain conditions of chemical constitution or of temperature. ”

“ Laboratory observations have established that the spectra of the elements are not the same under all circumstances. We formerly thought it remarkable that nitrogen should have two or three characteristic spectra, or that a metal should have a spark spectrum and an arc spectrum. We are now confronted with the potent fact that an element may have a variety of spectra, depending upon the nature and the intensity of the forces employed in rendering it luminous. ”

“ Let’s see now, how a real spectroscope is made, for it isn’t always convenient to have a pitch dark room, nor is it a very exact way to hold the prism to your eye when examining the light of burning sodium, or hydrogen gas, or other substances. ”

“ I should like to explain to you how I believe the spectroscope may contribute its share to the settlement of that question. In his first great paper on the molecular theory of gases, the late Professor Clerk Maxwell assumed that two molecules may actually come into contact, that they may strike each other, as two billiard-balls do, and then separate, according to the laws of elastic bodies. This theory is difficult of application when a molecule contains more than one atom, and, especially as it did not in the case of conduction of heat give results ratified by the experimental test. ”

“ Of course, if the photosphere were really composed of glowing solid and liquid matter, or of masses of gas so compressed and so intensely heated as to give a continuous spectrum, no gas existing below the photosphere could send its light through, nor could its presence, therefore, be indicated in any spectroscopic manner. ”

“ The progressive changes in spectra, from the exclusive indications of the simple elements hydrogen, helium, nitrogen, possibly carbon, and the terrestrially unknown gas nebulium in the gaseous nebulæ, to the long list of familiar substances, including several chemical compounds, in the red stars, may prove to be fundamentally significant when adequately studied from the standpoint of the investigator of atomic structure. ”

“ The eye readily recognizes difference of colour in the light given off by the different gases, and when this light is examined by the spectroscope, bright lines, characteristic of each gas, are observed. Nos. 12 and 13, in Plate XVII., are the spectra of hydrogen and of nitrogen respectively, which appear when the gases are examined in the manner just described. In this manner the spectra of chlorine, bromine, iodine, oxygen, sulphur, phosphorus, &c., may be studied. ”