April 29, 1922J 



NATURE 



557 



P'rom the foregoing it seems probable that ordinary'- 

 magnetic attraction and hysteretic repulsion determine 

 between them the behaviour of particles in the field 

 of an alternating-current magnet. Of these two factors 

 the former is fairly understood ; it remains to indicate 

 one or two points concerning the latter. Hysteretic 

 repulsion is low and attraction relatively high when 

 the frequency of alternation is low, and vice versa ; 

 Mr. Mordey found that, with an increase of frequency 

 from 25 to 75 periods, the speed at which the material 

 was repelled increased approximately as the square of 

 the increase in frequency. At higher frequencies, 

 however, repulsion appears to be again inactive ; Mr. 

 Mordey, for example, found that both at 150 periods- 

 and at 350, repulsion was not manifest but attraction, 

 even haematite remaining over the poles. He used 

 relatively low inductions, 560 to 2000, these being 

 more proper to alternating current than the higher 

 inductions associated with direct current in ordinary 

 magnetic separation. 



The continuous repulsion of the ferriferous particles 

 across the stream is forceful and unhesitating, whether 

 these particles be dry or borne in water ; it is assisted 

 by an upward repulsion which frees them from entangle- 

 ment with associated gangue, and gives them power to 

 climb an inclination or even the sides of the container. 

 At the same time, however, these particles, and particu- 

 larly those of magnetite, tend to be held strongly in 

 the plane of their movement, so that unless the field 

 be properly adjusted transverse walls or banks form 

 such as hinder the escape of gangue. 



To make use of this discovery Mr. Mordey has in 

 mind a shallow inclined launder down which the 

 material would flow in the condition of an ore-pulp. 

 With poles running the length of this launder the 

 ferriferous particles would be driven to one side, to 



be collected separately at the bottom, the gangue 

 particles keeping a straight path. 



It is interesting that only iron minerals have so 

 far been found capable of making the transverse 

 movement, such moderately magnetic minerals as 

 ilmenite and wolframite not moving ; it is also of 

 interest that, though magnetite moves more strongly, 

 haematite can hardly be said to be outclassed ; 

 further, a small contamination with iron oxide causes 

 other minerals to move, wolframite and cassiterite, for 

 instance. 



Obviously, therefore, though magnetic susceptibility 

 is doubtless involved it does not enter unfettered ; 

 as already stated, it is associated with hysteretic 

 repulsion. That the repulsion may be due to eddy 

 currents set up in the particles appears to be excluded 

 by the fact that the conductivity of haematite is not 

 high enough to permit any pronounced development 

 of such currents ; moreover, particles of metallic 

 aluminium, the conductivity of which is very high, 

 are not repelled. 



It is to be hoped that this process of magnetic 

 separation may so develop that deposits, such as that at 

 Dunderland, Norway, which contain much haematite 

 in addition to magnetite, and others consisting largely 

 ' of granular haematite, may be successfully treated. 

 In view of the many deposits coming within these 

 descriptions, and of the fact that the present means 

 of magnetic separation, good as they are for dry work, 

 fail entirely to separate feebly-magnetic minerals from 

 a water-borne pulp, any endeavours to realise this 

 hope will be viewed by all with the greatest sympathy 

 and interest. The ordinary magnetic concentration of 

 magnetite is not an expensive treatment, but the 

 treatment outlined by Mr. Mordey, being simplicity 

 itself, would cost still less. 



Obituary, 



Prof. J. C. Branner. 



PROF. JOHN CASPER BRANNER, president 

 emeritus of Stanford University, California, 

 died at Palo Alto, California, on March i, in his seventy- 

 second year. He was a geologist of stimulating activity, 

 and was attracted to Brazil as a young man in 1874 

 through his master at Cornell, C. F. Hartt. In 1875 

 he succeeded Hartt as director of the Imperial Geological 

 Commission in Brazil, and, on the establishment of the 

 republic, continued his observations in that country 

 on various expeditions from time to time. In 1885 

 he was appointed professor of geology in Indiana 

 University, and in 1892 to the similar post in the 

 newly founded Stanford University. He won a con- 

 siderable position 'as an economic botanist, and his 

 geological papers cover a wide and practical field. 

 His " Outlines of the Geology of Brazil," the second 

 edition of which was published in the Bulletin of the 

 Geological Society of America as recently as 1920, 

 has been noticed in Nature, vol. 106, p. 58. This 

 very useful summary includes a geological map of the 

 whole country on the scale of i : 5,000,000. 



Many European geologists will remember Branner 

 at the International Geological Congress in Zurich in 

 1894, and all who met him nuist have been won by 



NO. 2739, VOL. 109] 



his strong personality and his equally strong and 

 manly presence. It is characteristic of his outlook 

 that in his most recent treatise he hopes that his work 

 may be of service to the Brazilian people, " to whom 

 I am strongly attached, and in whose welfare I am 

 deeply interested." 



We owe some of the facts and dates in the foregoing 

 notice to an appreciative article by Dr. David Starr 

 Jordan in Science for March 31, and to an obituary 

 notice in the American Journal of Science for April. 



Dr. Andrew McWilliam, C.B.E. 

 The death of Dr. Andrew McWilliam, which occurred 

 on April 5, came as a shock to a large circle of friends 

 and former pupils, and deprives the steel world of a 

 metallurgist of great knowledge and wide experience. 

 A native of Galloway, Dr. McWilliam was educated at 

 Allan Glen's School, Glasgow, and at the Royal School 

 of Mines, of which he became an Associate. On leaving 

 South Kensington, he entered the Sheffield Technical 

 School, afterwards incorporated with the University 

 of Sheffield, but later he left to take up in succession 

 several outside posts. Returning to Sheffield, he was 

 first appointed lecturer, and then assistant professor, 



