374 



NATURE 



[August 20, il 



the waves set up in the ether by a molecule, say of hydrogen, is 

 the most fixed and absolutely permanent quantity in nature, 

 and is so of physical necessity, for with any alteration the mole- 

 cule would cease to be hydrogen. 



Such would be the case if the coincidence were certain ; but 

 an absolute coincidence can be only a matter of greater or less 

 probability, depending on the resolving power employed, on the 

 number of the lines which correspond, and on their characters. 

 When the coincidences are very numerous, as in the case of iron 

 and I he solar spectrum, or the lines are characteristically 

 grouped, as in the case of hydrogen and the solar spectrum, we 

 may regard the coincidence as certain ; but the progress of 

 science has been greatly retarded by resting important conclu- 

 sions upon the apparent coincidence of single lines, in spectro- 

 scopes of very small resolving power. In such cases, unless 

 other reasons supporting the coincidence are present, the prob- 

 ability of a real coincidence is almost too small to be of any 

 importance, especially in the case of a heavenly body which may 

 Lave a motion of approach or of recession of unknown amount. 



But even here we are met by the confusion introduced by 

 multiple spectra, corresponding to different molecular groupings 

 of the same substance ; and, further, to the influence of sub- 

 stances in vapour upon each other ; for when several gases are 

 jiresent together, the phenomena cf radiation and reversal by 

 absorption are by no means the same as if the gases were free 

 from each other's influence, and especially is this the case when 

 they are illuminated by an electric discharge. 



I have said as much as time will permit, and I think indeed 

 sufficient, to show that it is only by the laborious and slow pro- 

 cess of most cautious observation that the foundations of the 

 science of celestial physics can be surely laid. We are at pre- 

 sent in a time of transition, when the earlier, and, in the nature 

 of things, less precise, observations are giving place to work of 

 an order of accuracy much greater than was formerly considered 

 attainable with objects of such small brightness as the stars. 



The accuracy of the earlier determinations of the spectra of 

 the terrestrial elements are in most cases insufficient for modern 

 work on the stars as well as on the sun. They fall much below 

 tlie scale adopted in Rowland's map of the sun, as well as below 

 ihe degree of accuracy attained at Potsdam by photography in a 

 part of the spectrum for the brighter stars. Increase of resolv- 

 ing power very frequently breaks up into groups, in the spectra 

 of the sun and stars, the lines which had been regarded as 

 single, and their supposed coincidences with terrestrial lines fall 

 to the ground. For this reason many of the early conclusions, 

 based on observation as good as it was possible to make at the 

 time with the less powerful spectroscopes then in use, may not 

 be found to be maintained under the much greater resolving 

 power of modern instruments. 



The spectroscope has failed as yet to interpret for us the re- 

 markable spectrum of the Aurora Borealis. Undoubtedly in 

 this phenomenon portions of our atmosphere are lighted up by 

 electric discharges : we should expect, therefore, to recognize 

 the spectra of the gases known to be present in it. As yet we 

 have not been able to obtain similar spectra from these gases 

 artificially, and especially we do not know the origin of the 

 principal line in the green, which often appears alone, and may 

 have, therefore, an origin independent of that of the other lines. 

 Recently the suggestion has been made that the aurora is a phe- 

 nomenon produced by the dust of meteors and falling stars, and 

 that near positions of certain auroral lines or flutings of man- 

 ganese, lead, barium, thallium, iron, &c., are sufficient to justify 

 us in regarding meteoric dust in the atmosphere as the origin of 

 the auroral spectrum. Liveing and Dewar have made a con- 

 clusive research on this point, by availing themselves of the 

 dust of excessive minuteness thrown off from the surface of 

 electrodes of various metals and meteorites by a disruptive dis- 

 charge, and carried forward into the tube of observation by a 

 more or less rapid current of air or other gas. These experiments 

 prove that metallic dust, however fine, suspended in a gas will 

 not act like gaseous matter in becoming luminous with its 

 characteristic spectrum in an electric discharge similar to that of 

 the aurora. Prof. Schuster has suggested that the principal 

 line may be due to some very light gas which is present in too 

 small a proportion to be detected by chemical analysis or even 

 by the spectroscope in the presence of the other gases near the 

 earth, but which at the height of the auroral discharges is in 

 a sufficiently greater relative proportioi to give a spectrum. 

 Lemstrom, indeed, states that he saw this line in the silent dis- 



NO. l\l^, VOL. 44I 



charge of a Holtz machine on a mountain in Lapland. The lines 

 may not have been obtained in our laboratories from the atmo- 

 spheric gases on account of the difficulty of reproducing in tubes 

 with sufficient nearness the conditions under which the auroral 

 discharges take place. 



In the spectra of comets the spectroscope has shown the 

 presence of carbon presumably in combination with hydrogen, 

 and also sometimes with nitrogen ; and in the case of comets 

 approaching very near the sun, the lines of sodium, and other 

 lines which have been supposed to belong to iron. Though the 

 researches of Prof. II. A. Newton and of Prof. Schiaparelli 

 leave no doubt of the close connection of comets with corre- 

 sponding periodic meteor swarms, and therefore of the probable 

 identity of cometary matter with that of meteorites, with which 

 the spectroscopic evidence agrees, it would be perhaps unwise 

 at present to attempt to define too precisely the exact condition 

 of the matter which forms the nucleus of the comet. In any case 

 the part of the light of the comet which is not reflected solar 

 light can scarcely be attributed to a high temperature produced 

 by the clashing of separate meteoric stones set up within ihe 

 nucleus by the sun's disturbing force. We must look rather to 

 disruptive electric discharges, produced probably by processes of 

 evaporation due to increased solar heat, which would be amply 

 sufficient to set free portions of the occluded gases into the 

 vacuum of space. May it be that these discharges are assisted, 

 and indeed possibly increased, by the recently discovered action 

 of the ultra-violet part of the sun's light? Lenard and Wolfe 

 have shown that ultra-violet light can produce a discharge from 

 a negatively electrified piece of metal, while Hallwachs and 

 Righi have shown further that ultra-violet light can even charge 

 positively an unelectrified piece of metal. Similar actions on 

 cometary matter, unscreened as it is by an absorptive atmo- 

 sphere, at least of any noticeable extent, may well be powerful 

 when a comet approaches the sun, and help to explain an 

 electrified condition of the evaporated matter which would 

 possibly bring it under the sun's repulsive action. W^e shall 

 have to return to this point in speaking of the solar corona. 



A very great advance has been made in our knowledge of the 

 constitution of the sun by the recent work at the Johns Hopkins 

 University by means of photography and concave gratings, in 

 comparing the solar spectrum, under great resolving power, 

 directly with the spectra of the terrestrial elements. Prof. 

 Rowland has shown that the lines of thirty-six terrestrial 

 elements at lea-t are certainly present in the solar spectrum, 

 while eight others are doubtful. Fifteen elements, including 

 nitrogen as it shows itself under an electric discharge in a 

 vacuum tube, have not been found in the solar spectrum. 

 Some ten other elements, inclusive of oxygen, have not yet been 

 compared with the sun's spectrum. 



Rowland remarks that of the fifteen elements named as not 

 found in the sun, many are so classed because they have few 

 strong lines, or none at all, in the limit of the solar spectrum as 

 compared by him with the arc. Boron has only two strong lines. 

 The lines of bismuth are compound and too diffuse. Therefore 

 even in the case of these fifteen elements there is little evidence 

 that they are really absent from the sun. 



It follows that if the whole earth were heated to the tempera- 

 ture of the sun, its spectrum would resemble very closely the 

 solar spectrum. 



Rowland has not found any lines common to several elements, 

 and in the case of some accidental coincidences, more accurate 

 investigation reveals some slight difference of wave-length or a 

 common impurity. Further, the relative strength of the lines in 

 the solar spectrum is generally, with a few exceptions, the same 

 as that in the electric arc, so that Rowland considers that his 

 experiments show "very little evidence" of the breaking up of 

 the terrestrial elements in the sun. 



Stas in a recent paper gives the final results of eleven years of 

 research on the chemical elements in a state of purity, and on 

 the possibility of decomposing them by the physical and chemical 

 forces at our disposal. His experiments on calcium, strontium, 

 lithium, magnesium, silver, sodium, and thallium, show that these 

 substances retain their individuality under all conditions, and are 

 unalterable by any forces that we can bring to bear upon 

 them. 



Prof. Rowland looks to the solar lines which are unaccounted 

 for as a means of enabling him to discover such new terrestrial 

 elements as still lurk in rare minerals and earths, by confronting 

 their spectra directly with that of the sun. He has already 

 resolved yttrium spectroscopically into three components, and 



