April 20, 1899] 



NA TURE 



597 



Pf/ roll- Hill. — Russia and the United States are the two great 

 petroleum producers. In the British Empire, Canada and Burma 

 are the only oil regions deserving mention at the present time, 

 though their output is, comparatively speaking, small. 



Sa/t. — The United States and the United Kingdom produce 

 about 2 million tons of salt each, Russia I J million, Germany 

 i^ million, India about i million. 



Silver. — Here again the United States are the largest pro- 

 ducers, followed closely by Mexico. Australasia furnishes an 

 output nearly equal to one-third of that of the United States, 

 and Bolivia and Germany approximately the same amount. 



7'iiz. — The Malay Peninsula is facile princeps as regards the 

 production of tin, probably yielding nearly two-thirds of the 

 world's supply ; and when aided by other British Possessions 

 fully three-quarters. 



Zinc. — The mines of Upper Silicia alone would suffice to 

 make the German Empire par excellence the zinc- producing 

 country of the whole world. The United States, after a long 

 interval, take the second place in the list. 



It must be carefully remembered that many valuable minerals 

 are not mentioned : for instance. Cape Colony produces diamonds 

 to the value of 4J millions yearly ; Italy has no equal for its 

 sulphur. Chili for its nitrate of soda, Germany for its potassium 

 salts, Spain for its quicksilver, and the United States for their 

 phosphates. 



ON THE ORIGIN OF MAGNETO-OPTIC 

 ROTATION} 



TT is known (Phil. Mag., December 1897) that when in a 

 ■'- material molecule there exists an independently vibrating 

 group of ions or electrons, for all of which the ratio ejiii of 

 electric charge to inertia is the same, then the influence of a 

 magnetic field H on the motions of this group is precisely the 

 same as that of a rotation with angular velocity w, equal to 

 JcH/wf-, imposed on the group around the axis of the field, on 

 the hypothesis that the extraneous forces acting on the ions are 

 symmetrical with respect to this axis. This result involves the 

 main features of the Zeeman effect ; it requires that the separ- 

 ations of the doublets representing the spectral lines arising from 

 such a group must all be equal when measured in differences of 

 frequency, or be inversely as the square of the wave-length in 

 vacuum when measured in differences of wave-length, a relation 

 which Preston has recently found to obtain for the natural 

 series of lines in ordinary spectra. 



The object of this note is to point out that it is possible to 

 deduce the Faraday effect from the Zeeman effect by general 

 reasoning, as regards any medium in which the optical disper- 

 sion is mainly controlled by a series of absorption bands for 

 which the Zeeman efiect obeys the above law, without its being 

 necessary to introduce any special dynamical hypothesis. For 

 this law ensures that the effect of the magnetic field on the 

 periods of the corresponding free vibrations of the molecules is 

 the same as that of a bodily rotation, say with angular velocity 

 CO, round its axis ; ^ while the complete circular polarisations of 

 the Zeeman doublets, viewed in the direction of the axis, show 

 that their states of vibration are .symmetrical with respect to 

 that axis. Thus, Cl being the angular velocity of the displace- 

 ment vector in a train of circularly polarised waves traversing 

 the medium along the axis, the state of synchronous vibration 

 wliich it excites in the molecules will have exactly the same 

 formal relation to this train when the magnetic field is off as it 

 would have to a train with very slightly different angular 

 velocity n + a when the magnetic field is on, the sign being 

 different according as the train is right-handed or left-handed. 

 Now, change of this angular velocity Cl means change of period 

 of the light : thus the propagation of a circularly polarised wave- 

 train when the field is on is identical with that of the same 

 wave-train when the period is altered by its being carried round 

 with angular velocity + w and there is no influencing magnetic 

 field. 



This last result has been employed by H. Becquerel as a 

 single hypothesis (suggested by Maxwell's notion of a magnetic 

 field in this connection as a vortex in the medium) from which 



^ Communication to the Cambridge Philosophical Society, March 6, by 

 J. Larmor, F.K.S. 



- The circumstance that the mean of the two disturbed periods is equal 

 to that of the undisturbed line shows that no effect of constitutive type is 

 involved in addi 



NO. 1538, VOL. 59] 



to deduce quantitatively both the Zeeman effect and the Fara- 

 day effect, and thus correlate them (" Sur une interpretation ap- 

 plicable au phenomene de Faraday et au phenomene et Zeeman " 

 — Comptes roiidiis, November 8, 1897). He shows, employing 

 chiefly the quantitative results of his own previous experim.ental 

 investigations, that the hypothesis is capable of providing a 

 satisfactory general view of the whole range of the phenomena, 

 and in particular that it leads immediately to a simple law of 

 dispersion for the Faraday effect, namely rotatory power pro- 

 portional to \dnjdK where « is the refractive index correspond- 

 ing to wave-length A measured in vacuum, a law which is in 

 good agreement with Verdet's results for carbon disulphide and 

 creosote. 



The preceding argument provides a general dynamical justifi- 

 cation of this hypothesis, for the case of all metiia in which the 

 ordinary gradient of dispersion is mainly controlled by one or 

 more powerful absorption bands beyond the visible spectrum, 

 for all of which the Zeeman constants are the same : it also 

 shows that Becquerel's hypothesis has an approximate validity 

 when these constants are nearly the same for all the effective 

 bands. In the immediate neighbourhood of any single band 

 the dispersion is anomalous, and is controlled practically by that 

 band alone ; the application will then be exact, and in Bec- 

 querel's hands it has given a complete account of the excessive 

 and anomalous rotation first observed by Macalusoand Corbino 

 in sodium vapour for light adjacent to the D lines. As was to 

 be anticipated, these simple general conclusions are consistent 

 with the results of the more special dynamical investigations of 

 FitzGerald and Voigt. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Dr Robert Muir, at present professor of pathology in 

 the University of St. Andrews, has been appointed to the 

 chair of Pathology in the University of Glasgow. 



By the will of the late Miss Elizabeth Brown, who died on- 

 March 5, the British Astronomical Association will receive her 

 observatory at Further Burton, with the astronomical instru- 

 ments in it, and the sum of 1000/. 



Science states that Mr. John D. Rockefeller has offered 

 100,000 dollars to Denison University, Granville, O. , if the 

 friends of the institution will, within the next year, raise the 

 sum of 150,000 dollars. 



The British Child-Study Association has issued the first 

 number of a magazine entitled The Paidologist, which is to be 

 published three times a year, and \\"iU be concerned with the 

 physical and psychical aspects of child-life. The aims of the 

 Association are both scientific and educational ; and the new 

 magazine is intended as a medium in which the results of re- 

 search on child psychology shall be recorded, and practical 

 suggestions which will assist in the evolutionary progress of the 

 race shall be described. 



With reference to the Board oi Education Bill, the Council 

 of the Association of Technical Institutions has unanimously 

 adopted the following resolutions : (i) In reference to Sectiot^ 

 2 of Clause 3, '* That, inasmuch as in some counties and in most 

 county boroughs the funds available are already fully appro- 

 priated for the purposes of technical education it is not, in the 

 opinion of this Council, desirable that these funds should be 

 applied to the payment of the expenses of inspecting schools- 

 under this Section. " (2) " That, in the opinion of this Council, 

 having regard to the fact that the funds assigned under the pro- 

 visions of the Technical Instruction Acts are not more than 

 adequate for the maintenance and development of technical 

 education, it is essential that for the further purposes of 

 secondary education additional funds be provided." It has also 

 decided to take steps to endeavour to secure that the interests 

 of technical education shall be adequately represented on the 

 consultative committee named in Clause 4 of the Bill. 



The Commissioners appointed under the University 01 

 London Act, 189S, have given notice that they are now pre- 

 pared to consider applications from duly qualified teachers and 

 lecturers giving instruction of the University type in public 

 educational institutions situate within a radius of thirty miles 

 from the University buildings, who desire to be recognised a.s 

 teachers of the University. By a "public educational institu- 



