Franz Arthur Friedrich Schuster

“ If you can imagine one second of time to be magnified to the length of a hundred years, it would only take about a second, on the average, from the time a molecule has encountered one other molecule until it encounters the second. ”

“ 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. ”

“ 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. ”

“ Our whole life is a chemical process, and a great part of the mysteries of Nature would be cleared up if we could gain a deeper insight into the nature of chemical forces. I believe no other line of investigation to be as hopeful in this respect as the one which examines directly the vibrations of the molecules which take place under the influence of these chemical forces. If we could find a connection between the vibrations of a compound molecule and the vibrations of the simpler elements which it contains, we should have made a very decided step in the desired direction. ”

“ They move about among each other, and within each molecule the atoms are in motion. Each atom, again, has its own internal movement. But, if the world were made up of atoms and molecules alone, we should never know of their existence; and, to explain the phenomena of the universe, we must recognize the presence of a continuous universal medium penetrating all space and all bodies. This medium, which we call the luminiferous ether, or simply the ether, serves to keep up the connection between atoms or molecules. ”

“ We have extended our methods of research; we have extended our power over the physical agents; we can work with the temperature of sun and stars almost as we can with those in our laboratories. No one can foretell the result, and perhaps in twenty years time another lecturer will speak to you of a spectroscopy still more modern in which some questions will have received their definite answer, and by which new roads will have been opened to a further extension of science. ”

“ You notice at once that waves differ in height. So the waves both of light and sound may differ in height, and to a difference in height corresponds a difference in the intensity of the sound you hear or of the light you see. The higher the wave the greater its energy, the louder is the sound or the brighter is the light. But, in addition to a difference in height, you have noticed that in different waves the distance from crest to crest may vary. The distance from crest to crest is the length of the wave, and waves not only differ in height but also in length. ”

“ The gases and vapors in the solar atmosphere are for the greater part in the molecular condition in which they give a line-spectrum, and we know of stars the spectra of which resemble our solar spectrum very nearly. We shall not be far wrong in ascribing to such stars a temperature similar to that of our sun. Other stars have absorbing envelopes showing spectra of fluted bands. We know that fluted bands belong to a more complex molecular condition, which only can exist at lower temperatures. These stars, therefore, must have a lower temperature than our sun. ”

“ Those who strove to advance the one paid little attention to the other, and did not trouble to know which of their conclusions were in harmony, which in discordance, with the results of the sister science. It is time, I think, now that the bearing of one branch of inquiry on the other should be pointed out: where they are in agreement, their conclusions will be strengthened, while new investigations will lead to more perfect truths where disagreement throws doubt on apparently well-established principles. ”

“ The importance of studying the motion of molecules is therefore clear; and it is the special domain of the modern spectroscopy to investigate one kind of these motions.
When a tuning-fork or a bell is set in vibration, its motion is taken up by the surrounding air, waves are set up, they spread and produce the sensation of sound in our ears. Similarly, when an atom vibrates, its motion is taken up by the ether, waves are set up, they spread, arid if of sufficient intensity produce the sensation of light in our eyes. Both sound and light are wave-motions. ”

“ 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. ”

“ 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. ”

“ We have passed by, and no doubt still are passing by, unnoticed differences which appear slight and unimportant, but which when properly understood will give us more information than the rough and crude distinctions which have struck us at first. ”

“ We have to enter partly into a theoretical question, and I must necessarily allude to some of the facts recognized by all who believe in the molecular theory of gases. Each molecule, which, as we have seen, sends out rays of light and heat on account of its internal motion, is surrounded by other molecules. These are, indeed, very closely packed, and continually moving about with enormous velocities. Generally they move in straight lines, but it must necessarily happen that often they come very near, and then affect and deflect each other. ”

“ As there is no other instrument which can touch the conditions of the most distant bodies of our universe, bodies so large that their size surpasses our imagination, so is there no other instrument which equals it in the information it can yield on the minute particles at the other end of the scale, particles which in their turn are so small that we can form no conception of their size or number. ”

“ 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. ”

“ 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. ”

“ The spectrum of a molecule is the language which that molecule speaks to us. This language we are endeavoring to understand. The inexperienced in a new tongue which he is trying to learn does not distinguish small differences of intonation or expression. ”

“ It is, therefore, only during a very small fraction of time that one molecule is under the influence of another, and it is one of the greatest problems of molecular physics to find out what happens during that short element of time. ”

“ When a body is sufficiently hot it becomes luminous, or, to speak in scientific language, the vibrations which are capable of producing a luminous sensation on our retina are increased in intensity as the temperature is raised, until they produce such a sensation. ”

“ The longest waves which affect our eyes produce the sensation of red, then follow orange, yellow, green, blue, and the shortest waves which we ordinarily see seem violet. If a molecule vibrates, it generally sends out a great number of waves which vary in length. These fall together on our retina, and produce a compound sensation which does not allow us to distinguish the elementary vibrations, which we want to examine. A spectroscope is an instrument which separates the waves of different lengths before they reach our retina ”

“ If two dissimilar molecules encounter, the relative phase of the vibrations, and hence the forces, will constantly change. Attraction will rapidly follow repulsion, and the whole average effect will be much smaller than in the case of two atoms of the same kind. We have no clear notion how such differences may act, and we must have recourse to experiment to decide whether any change in the effect of an encounter is observed when a molecule of a different kind is substituted for a molecule having the same periods of vibration. ”

“ We measure the combined heat radiation of different layers on the solar surfaces, all of which are at different temperatures, and we ob; serve only an average effect which is much influenced by the absorption in the outer layers of the solar atmosphere and in the corona, and does not admit of easy interpretation. The spectroscopic method, which is yet in its infancy, has the advantage that we can observe separately each layer of the sun; and we thus examine the temperature not as an average, but for every part of the solar body. ”

“ Such an association of atoms is called a molecule. When a body is in the state of a gas or vapor, each molecule for the greater part of the time is unaffected by the other molecules in its neighborhood, and therefore behaves as if these were not present. ”

“ The spectra of the oxides, chlorides, bromides, or iodides of the alkaline earths, for instance, are spectra of fluted bands. All these bodies are known to contain atoms of different kinds—the metallic atoms of calcium, barium, or strontium, and the atoms of chlorine, bromine, iodine, or oxygen. ”

“ The changes of spectra due to molecular combinations and rearrangements have in addition to their theoretical importance a great practical interest, for they will afford us some day a means of answering approximately a great many questions relating to the temperature of sun and stars. ”

“ During the greatest part of this very short time, it moves in a straight line, for the forces between molecules are so small that they do not affect each other unless their distance is exceedingly small. ”

“ It may render the bands more wide and indistinct at their edges, but its influence is more of a nature which in gas spectra is sometimes observed at high pressures when the lines widen, and does not consist of an alteration in type. Though in a solid or liquid body the molecules are much nearer together, they are less mobile ”