37° 



NATURE 



[August 20, 1908 



transformed by the Fresnel rhomb in two normal and 

 linear vibrations. It depends upon the position of the 

 Nicol which of the components of the doublet is the 

 more quenched. 



The initial position of the Nicol determines the 

 angle througli which it is to be turned in order to 

 reverse the relative intensities of the two components. 

 Only widened lines and no doublets, and correspond- 

 in.tjly only a shift and no extinguishing of components 

 by turning' the Nicol, will be observed, either from 

 want of uniformity of the field or from want of homo- 

 geneity of the light. 



The phenomena observed by Prof. Hale in the 

 double lines and the widened lines of the sun-spot 

 spectrum, and exemplified in the photographs under 

 review, are identical in character with those observed 

 in the laboratory under the specified conditions with 

 somewhat broad lines or in a rather non-uniform field. 

 The behaviour of a spectral line emitted in these cir- 

 cumstances by iron vapour between the poles of an 

 electromagnet cannot be distinguished from the radia- 

 tion of iron vapour immersed in the interior of a Hale 

 (electronic or corpuscular) solar vortex at a distance of 

 149 million kilometres. 



So far as we know, only a strong magnetic field can 

 resolve a single line into a doublet, having components 

 circularly polarised in opposite directions. Are we not 

 compelled, then, to admit that where these unique and 

 characteristic phenomena are present a magnetic field 

 must be their cause? The evidence is of the same 

 nature (but still more convincing by the unique char- 

 acter of the polarisations) as that for motion in the 

 line of sight from the Doppler displacement of spec- 

 tral lines in the case of moving stars or molecules. 



The absence of any shift of the red telluric lines by 

 the rotation of the Nicol, or of measurable displace- 

 ments of the cyanogen fiutings, as reported by Prof. 

 Hale, considerably strengthens the argument and ex- 

 cludes instrumental and other errors. 



A quantitative comparison of the magnetic separ- 

 ations of the iron lines as observed in the laboratory 

 and in the sun will be necessary to complete 

 the argument and to make it, if full correspond- 

 ence exists, almost insuperable. A small prob- 

 ability, of course, must be left open that under the 

 conditions existing in the solar furnace, dissociation 

 processes or something of the kind (Lockyer) occur, so 

 as to alter the whole vibrating system of the atoms 

 and therefore also the relative separations of the 

 different lines. 



Being in the country I unfortunately cannot supply 

 now the magnetic separations of the region of Prof. 

 Hale's photograph. 



The iron spectrum in the magnetic field has been 

 examined by Becquerel and Deslandres, Reese, Kent, 

 and Hartmann (Thesis, Halle, 1907), but the lines in- 

 vestigated are not far enough in the red. 



Recently Miss van Meurs made in the Amsterdam 

 laboratory a rather extensive study of the radiation 

 of iron in the magnetic field, which will be published 

 shortly ; her observations in the red are still 

 unfinished. 



In order to obtain an idea of the order of magnitude 

 of the magnetic force within the Hale solar vortex, we 

 can only compare the largest separation of an iron 

 line observed by Prof. Hale, viz. o'22 A.U., with that 

 of one of the larger separations of iron lines measured 

 in the laboratory. The separation of x 4i44'o5 is 0^67 

 A.U. in a field of 29,740 Gauss. This gives for the 

 magnetic force nearly 10,000, which may be reduced to, 

 say, 6000 Gauss, if Dr. Hale's line presents such excep- 

 tionally hig'h separation as some zinc lines. 



If we might identify the Hale vortex with a solenoid 

 with axis parallel to that of the vortex, and having 



NO. 2025, VOL. 78] 



one layer of one winding per cm., a current of 5000 

 amp. would be necessary for generating tlie Oooo 

 Gauss field. 



This current at first sight seems rather large. The 

 actual case is approached more nearly by substituting 

 for the one winding per cm. a gaseous conducting cir- 

 cular disc, one cm. thick, and of suitable radius, 

 though all calculations here are merely tentative and 

 extremely rough. Let the radius of this disc be chosen 

 equal to that of the earth, viz. 6'4 x 10' cm. (represent- 

 ing a solar vortex of, according to Hale's photographs 

 of the solar vortices, extremely moderate dimensions), 

 then the current of 5000 amp. in the disc is to be dis- 

 tributed over an area of 6'4xio* cm-., giving 

 o'S X 10-^ amp. per cm". 



The kathode rays issuing from the spot of lime (say 

 one square millimetre area) in a Wehnelt tube carry 

 something lilie 10-^ amp. (I quote from memory). 

 Hence the solar vortex would not be too crowded with 

 electrons even if the magnetic force to be accounted 

 for were much higher. 



In the last paragraph of my very first paper (1897)' 

 concerning radiation in the magnetic field, I wrote : — 

 " Further inquiry must also decide as to how far the 

 strong magnetic forces existing, according to some, at 

 the surface of the sun may change its spectrum." 

 Since I always entertained the expectation, sometimes 

 amounting almost to conviction, that some day a cos- 

 mical application of the magnetic separation of the 

 spectral lines would be discovered by astronomers. 

 This might -suggest that I am too favourably dis- 

 posed towards any evidence in such a direction. 



I trust, however, that I have not been too sangume 

 while writing this review of Prof. Hale's splendid 

 discovery. Its importance for general and solar physics 

 must be very great, and not less for the theories of 

 meteorology and terrestrial magnetism, affording, as 

 it does, a vera causa for the perturbations of the electric 

 and magnetic equilibrium of our earth and its 

 atmosphere. P. Zeeman. 



THE LIQUEFYING OF HELIUM. 



OM July 10 Prof. H. Kamerlingh Onnes, of 

 Leyden University, and his assistants had the 

 satisfaction of seeing a considerable volume of 

 liquid helium remain for some hours. This conquest 

 over the last and most refractory gas was made 

 known within a day or two, but few details were 

 given until the appearance of the official pub- 

 lication, from which this note ' is taken. 



Prof. Onnes points out that the first step neces- 

 sary was the determination of isothermals of helium, 

 and in particular of those at temperatures obtain- 

 able only with liquid hydrogen. From these the a 

 and h of van der Waals's theory can be obtained, 

 and the Boyle point, i.e. the temperature at which 

 the minimum of pv occurs with very small densities, 

 be found. This point also occurs at one-half the 

 absolute temperature of the Joule-Thomson in- 

 version point at low densities. With these data 

 he was able to apply a theorem developed in 

 1896 from an earlier and more general theorem of 

 1881 during the endeavour to liquefy hydrogen static- 

 ally. The theorem shows that the Boyle point of 

 heiium lies somewhat above the lowest temperatures 

 obtainable with hydroger;, and hence that a regenera- 

 tive process, as applied by Linde and Hampson to air 

 and by Dewar to hydrogen, could be effective with 

 helium. 



Ever since 1883, when Prof. Onnes commenced his 



1 From the Special August Supplement to the Dutch Proceedings of the 

 Royal .\cademy of Amsterdam. The note has been approved by Prof. 

 Onnes. 



