164 



NA TURE 



[Ju 



NE I 



O' 



189; 



not insist that that observation is absolutely conclusive. It is 

 certainly strong evidence. But the subject is dithcuh, and one 

 that has given rise to some difference of opinion among physi- 

 cists. .\l any rale this property distinguishes argon very sharply 

 from all the ordinar)- gases. 



One question which occurred to us at the earliest st,ige of the 

 inquiry, as soon as we knew that the density was not very dif- 

 ferent from 21, was the question of whether, jiossibly, argon 

 could be a more condensed form of nitrogen, denoted chemically 

 by the symbol Nj. There seem to be several dirticulties in the 

 way of this sup|Hisilion. Would such a constitution be con- 

 sistent with the ratio of s|H.-cific heats {I-65)? That seems 

 extremely doubtful, .\nolher question is. Can the density be 

 really as high as 21, the numlxjr required on the supix>silion of 

 Nj ? As to this matter, I'rof. Kanisay has repeated his measure- 

 ments of density, and he finds that he cannot get even so high as 

 20. To suppose that the density of argon is really 21, and that 

 it appears to lie 20 in consetjuence of nitrogen still mixetl with 

 it, would Iw to sup|X)se a contamination with nitrogen out of all 

 proportion to what is probable. It would mean some 14 per 

 cent, of nitnigen, whereas it seems that from ij to 2 per cent. 

 is easily enough detectetl by. the S|»:ctroscope. Another ques- 

 tion that may lie asked is, Would N3 require so much cooling to 

 condense it as argon requires ? 



There is one matter on which I would like to say a word — 

 the question as to what Nj wouUl Ije like if we had it ? There 

 seems to lie a great discre|>ancy of opinions. Some high 

 authorities, among whom must be included, I sec, the celebrated 

 Mendtleef, consider that N3 would be an exceptionally stable 

 body : but most of the chentisls «ith whom I have consulted 

 are of opinion that N3 would be explosive, or, at any rate, abso- 

 lutely unstable. That is a tpiestiun which may be left for the 

 future to decide. We must not attempt to put these matters too 

 positively. The lalance of evidence still seems to be again.st 

 the supposition that argon is Xj, but for my part I do not wish 

 to dogmatise. 



A few weeks ago we hail an eloquent lecture from Prof, 

 kiicker on the life and work of the dlustrious Helmholt?. It 

 will \hi known to many that during the last few months of his 

 life Ilelmholt/ lay prostrate in a semi-paralyse<i condition, 

 forgetful of many things, but still retaining a keen interest in 

 science. .Some little while after his death we had a letter from ' 

 his widow, in which she described how interested he had been 1 

 in our preliminary announcement at Oxford ujion this subject, ' 

 and how he desired the account of it to \k read to him over 

 again. lie added the remark, " I always thought that there 

 must be something more in the atmosphere." 



A srECTKoscoric I'KOOF or r/fE 



ME TEL m/C Cl ).\S Tl TU T/OX ( >/•' 



SATLh'.X'S A-AVcy^.' , 



'T'llE hypithesis that the rings of Saturn are com|K>sed of an 

 immense multitude of comp.iratively small IkkIIcs, re- 

 volving around .Saturn in circular orbits, has been firmly 

 establisheil since the publication of MawvelTs classical paper in 

 1859. The grounds on which the hyiMithesis is Uaseil are too 

 well known to retpiire s|)ecial mention. .Ml the observed , 

 i>hcnomcnaofthe rings are naturally and completely explained by 

 II, and mathematical investigation shows that a solid or fluid ring 

 could n>it exist imdcr the circumstances in which the actual ring 

 is placol. I 



The s]x"ctroscr)pic proof which I'rof. Kccler has recently 

 obtained of the meteoric conslilulion of ihe ring, is of interest 

 l»ecause it is the first i/imt proof of the correctness of the 

 accepted hyi>>illiesis, and liecause it illustrates in a very lieautiful 

 manner Ihe I'ruilfulness of Doppler's principle, and the value of 

 the '.jierlroscoiK; as an inslnnnent for the measurement of 

 celestial motions. 



Since the relative velocities of different i»arls of the ring 

 would l»- I'xMnlially different uniler the Iwn hypotheses of rigid 

 rtructure and meteoric constitution, it is jHissilile lo rlistinguish 

 lictwecn these hyiKitheses by measuring the motion of different 

 parts of the ring in the line of sight. The only diOiculty is to 

 fimi a methcxi so rlelicate that the very small differences of 

 velocity in <|ueslion may n<il lie masked by instiumental errors. 

 Succcs» in vLnunl ol>servalion.s of the s|)cctruni Is hardly lo \k 

 cxpcclc<l. 



I Alrtdilcil from .1 (Kipcf. Iiy l*r.,f. Jnmc* K, Kcclcr, in die . \ttivfhyiitat 

 Jpitrnal I'^r Way . 



After a nundjer of attempts. Prof. Keeler obtained two fine 

 photographs of the lower spectnnii of .Saturn on .Xpril 9 and to 

 of the present year. The exposure in each case was two hours, 

 and the image of the planet was kept very accurately central on 

 the slit-plate, .\fter the exposure the spectrum of the Moon 

 was photographed on each side of the spectrum of Saturn, and 

 nearly in contact with it. Each |^>art of the lunar si>ectrum has 

 a width of about one millimetre, which is also nearly the 

 extreme witlth of the jilanetary s]>ectrum. On lK)th sides of the 

 spectrum of the ball of the planet are the narrow s|)ectra of the 

 ans,v of the ring. The length of the spectrum from * to D is 23 

 millimetres. 



These jihotographs not only show ver)' clearly the relative 

 displiicement of the lines in the sjiectrum of the ring, due to the 

 opixisite motions of the ans,v, but exhibit another peculiarity, 

 w hich is of special importance in connection with the subject of 

 the present (xiper. The planetarj' lines are strongly inclined, in 

 conseijuence of the rotation of the ball, but the lines in the 

 spectra of the ansie do not follow the direction of the lines in the 

 central spectrum : they are nearly parallel lo the lines of the 



One niilliinclrc. 



Fli.. 1. 



comparison s|>ectrum, and, in fact, as com|>arcd with the lines of 

 the ball, have a slight tendency to incline in the opposite direc- 

 tion. Hence the outer ends of these lines are less displaced 

 than Ihe inner ends. Now it is evident that if the ring rotated as 

 a whole, the velocity of the outer e<lge would exceed that of the 

 inner edge, and the lines of the ans^e would Ik.- inclined in the 

 same direction as those of the ball of the planet. If, on the 

 other hand, the ring is an aggregation of satellites revolving 

 aroimd .Saturn, ihe \'eloclly wtmld be greatest at the inner edge, 

 and the Inclinalion of lines In llie spectra of the ans;e would be 

 rc\erse<l. The photographs are iherefttre a direct proof of the 

 approximate corrcclness of the latter supposition. 



It is interesting lo delerniine the form of a line in Ihe spectrum 

 of .Saturn when the slit is in the major axis of Ihe ring, on the 

 a.ssumpllon that the planet rotates as a solid body, and ihat the 

 ring Is a swarm f»f particles revolving in circular orbils .accord- 

 ing lo Kepler's third law, .\l present the niollon of Ihe system 

 as a whole is neglected. The upper part of I'ig. I represents 

 the image of Saturn on the slit of the sjicctroscopc (the scale 



NO. i3;,7, vor.. 52] 



