70 



NA TURE 



[May 28, 1874 



Increase of temperature will increase amplitude. 



The shorler the free path the more complex the vibrations. 



The greater the amplitude the moie will tlie vibration of the 

 mo'ecule be brought out, not merely Ih&Juiidaiiuiilal vihratiovs, 

 as we may term them, which we get in the free path, when it is 

 longest, but the overtones. 



Now why have I risked wearying you with these detailed 

 statements concerning the vibrations of "molecules?" Because 

 we believe that tach molecular vibration disturbs the Ether ; that 

 spectra are thus begotten ; each wave-length of light being set 

 in vibrattcn by a molecular vibration of corresponding wave- 

 length. The vibration is, in fact, the sender ; the spectrum is 

 the receiving instrument, in this new telegraphy. 



Now there are two questions w-hich I propose to discuss, and 

 they are these : — What light does the spectroscope throw upon 

 molecular questions? and is there any hope that the spectro- 

 scope, as researches with it are extended, may aid the study of 

 a subject which lies at the root of chemical and physical investi- 

 gation ? 



I have written down several statements, which I propose to 

 discuss one by one. I shall state the experimental basis, when 

 it exists, on which the statements rest and the methods by which 

 the results have been obtained 



1 shall for a time use the word "particle" to represent a 

 small mass of matter, because it does not tie me to the " atom," 

 or the " molecule " of the chemist, or to the " molecule " of the 

 physicist. " Particle " is a neutral term, which I hope none of 

 you will quarrel with. 



1. My first proposition is this : — W/icit f articles are aggregated 

 together, so as to form a solid or liquid, they give out rays of light 

 of certain refrangibilities ; and the sptctrum is continuous as far as 

 it goes. This was Kirchoff's first generalisation. 



It surely is an important fact from the point of view of the 

 molecular theory that all solids and liquids, with their particles 

 moving as already stated ,do give you a perfectly distinct spec- 

 trum from that which you get when you deal with any rare 

 gas or vajjour whatever. A poker put into the fire becomes of a 

 dull red heat, after a time a white heat is arrived at. As far as 

 the vibrations exist they are continuous, there are no breaks in 

 the series of wave-lengths. You may also get a platinum wire, 

 and drive it to incandescence in the same way by means of 

 electricity. Analyse the light by means of the spectroscope, 

 the spectrum is the same as that of the poker Further, 

 we can go to the sun, and divest it in imagination of the 

 atmosphere which absorbs much of its light, and we know 

 that, with a small exception, we shall get a peifectly continuous 

 spectrum similar to that in the case of the poker or platinum wire. 

 Connected with spectroscopic investigation there is this wonder- 

 ful fact, that as it deals with matter in the most general way, it 

 is perfectly easy to carry on a line of argument, not by referring 

 to different chemical elements, but to matter, irow on the earth, 

 now in the sun, or again in some of the stars. It is a great 

 leveller. In this continuous spectrum we have a spectroscopic 

 fact connected with that kind of molecular motion which physicists 

 attribute to particles so long as they are closely packed together 

 in the solid state, and so long as they have but a small free 

 path as in the fluid state. 



2. I now come to my second proposition : — When particles are 

 in a stale of gas or vapour, and are rendered incandescent by high 

 tension electricity, lines fecti a are produced in the case of all the 

 chemical elements. 



I have several photographs which I will throw on the screen, 

 showing such spectra as these now in question. We have thus 

 the spectra of tlie light given off by the vapour of cobalt and of 

 nickel rendered incandescent by means of high-tension electri- 

 city. I will next show you the spectra of other chemical ele- 

 ments, such as aluminium and iron compared with nickel and 

 cobalt, pure and impure iron compared with a meteorite. These 

 line-spectra are only to be obtained from gases and vapours, and 

 as a rule only when we employ high-tension electricity. 



We get a perfectly distinct spectroscopic result from the one we 

 had before, precisely in the case where according to the physicists 

 we have an enormous motion and agitation of molecules. 



3. I now proceed to the next proposition : — Jn some cases f ar- 

 ticles in a state of gas or vapour can be set swinging by heat 'raves. 

 1 have here some salts of sodium and strontium, these 1 place in 

 the heat of a Bunsen burner, they are at once dissociated and the 

 particles of the nretals are set swinging by the heat waves and 

 we get their longest hncs. Now that is not only true for stron- 

 tium and sodium, but for many other elements. But if I put 

 salts of iron, or of the other heavy metals in the flame, I shall not 



get bright lines. Or again, in some other vapours, such as sul- 

 phur, we only get a spectrum, not of hnes, but continuous over 

 a limited part of the spectrum. In fact I may s.-iy that with the 

 exception of those elements which easily reverse themselves, this 

 heat is absolutely incompetent to give me anything like a blight 

 line. 



4. Partides, the amplitudes of vibrations of which may either be 

 so slight that no visible light proceeds from them, or so great that 

 they give out light of their 07vn, absorb light of th« same wave- 

 length and of greater amplitude passing through them 



Consider how beautiful this statement is when you look at it 

 in the light of its teaching with regard to particles. We throw 

 sodium into a flame and get a yellow light ; we place it on 

 the poles of our electric lamp and render it incandescent, and its 

 light is rich yellow. 



We have similarly incandescent sodium outside the sun, through 

 which the rays of sunlight pass outwards towards the earth, and 

 we may have similar non-luminous sodium vapour in a test- 

 tube ; and the vibration which gives the yellow light, in the case 

 of the sun, and which is invisible in the vapour of the tube, 

 instead of giving a bright line gives a dark one. Let me show 

 you some photographs of the solar spectrmn, so that as you have 

 seen the bright lines due to radiation, you may see the dark lines 

 in the solar spectrum which are due to absorption. 



Our knowledge of the elements existing in the sun and stars 

 depends entirely upon the principle first suggested by Stokes, 

 that particles are set swinging when light waves pass through 

 them with the particular rate of vibration which they efTect. 



The elements to which a large number of the Fraunhofer lines 

 ate due have been determined by means of the vibrations of 

 particles on the earth. Whether a particle vibrates on the earth 

 or on the sun it does not matter to the spectroscope, the vibra- 

 tion is the same, but as the particle is set vibrating at the sun by 

 a greater amplitude of the light passing through it we get a dark 

 line instead of a bright one. To show that in the stars', re- 

 presenting to us other suirs, the spectra are very various I will 

 exhibit spectra of the three classes of stars into which most 

 may be grouped. In the middle we have a simple line spectrum, 

 in the centre a more complex one, and at the bottom a chan- 

 nelled spectrunr. 



5. Next I have to point out to you that line spectra become more 

 complicated as the particles are brought nearer together, provided 

 the slate of gas or lapotir be retained. .See the importance of this 

 observed fact in connection with the molecular theory. If in the 

 solid the particles can only oscillate round their mean position, 

 if in the gas they can go through with enornrous rajjidity a 

 tremendous number of various movements of rotation and vibra- 

 tion, and along their free path ; and if spectroscopically we can 

 follow these movements by difierences in the phenomenon 

 observed, is it too much to hope that in the coming time we 

 shall have an enormous help in our inquiries ? We get a solid or 

 liquid condition, and a continuous .'spectrum ; we get the most 

 tenuous gaseous condition and then the phenomenoir is changed, 

 and the spectrum consists of a single line. So far indeed as the 

 visible spectrum goes, it is possible by workirrg with the gas at 

 low pressure, and not too high temperature, to get a spectrum 

 from any gas or vapour of only a single line, and as you increase 

 the density, and thus force the particles closer together, and 

 make the conditions of the gas approximate in the way of aggre- 

 gation more to those of a solid, so does the spectrum get more 

 and more like that of a solid, till we see at last a bright continuous 

 spectrum. Take, for instance, hydrogen, and use, not an ordinary 

 air-pump, but a sprengel mercury pump ; use tiris for three or 

 four hours, and observe the spectrum of the gas. It is a single 

 line. Fill the tube again with gas, at ordinary atmospheric pres- 

 sure, double the pressure, or multiply it ten or more times, aird 

 what becomes of the'line? Not only does that green hne which 

 first appeared get nrore and more obvious and thick, but more 

 lines appear, and they get thicker, till at last there is such a back- 

 ground of continuous spectrum that these are all invisible as lines. 

 At twenty atmospheres the spectrum is as continuous as that of a 

 solid. 



6. Here is my sixth proposition :— In the case of metals 

 the>-e are two different ways in which the continuous spectrum 

 is approached. Mind I do not say reached, for there may be much 

 more to learn on this point. To render this clear I must show you 

 some more photographs and explain the method by which they 

 have been obtained. Here I have a coil and a jar, and here the 

 poles. We drive the nretal of whicii these poles are composed 

 into vapour, the vapour is rendered incandescent ; the spectrum 

 we should get would therefore be one of bright lines. Now, 



