March \ 



:884] 



NATURE 



467 



present, and that they are retreating not so much througli co-:nic 

 or telluric causes as through meteorological changes depending 

 partly on the prolonged action of man on the earth. — On the 

 molecular velocities of gaseous bodies (continued), by Arnoldo 

 Violi. — Experimental studies on Thapsia resin, by Francesco 

 Canzoneri. — Distribution of the spots, faculse, eruptions, and 

 protuberances on the surface of the sun, deduced from the ob- 

 servations made at the Observatory of the Collegio Romano 

 during the year 1882, by Pietro Tacchini. — Official return of 

 the archa;ological discoveries made at Este, Bologna, Rome, 

 Bolsena, Albano, and some other parts of Italy during the 

 November of 1883, by S. Barnabei. — Meteorological observa- 

 tions made at the Observatory of the Campidoglio during the 

 month of November, 1SS3. 



January 6, 1S84. — Notice of Prof. Carlo De Stefani's work on 

 the " Lower Lias Formation of the Northern Alps," by S. 

 Taramelli. — New determination of the optical characters of 

 Christianite (anorthite) and Phillipsite (variegated copper ore), 

 by Alf.'edo des Cloizeaux. — Note on the e.-jistence of two disliact 

 optical axes in the Gismondine crystals (two illustrations), by the 

 same author. — On the temperature corresponding to the Glacial 

 perioJ (continued), by Pietro Blaserna. — Some observations of 

 the eighth satellite of Saturn, by E. Millosevich. — Meteoro- 

 logical observations made at the Observatory of the Campidoglio 

 during the month of December, 1883. 



Reiiie d' Aitthropologie de Paris, No. I, 1884, contains : Con- 

 cluding part of Dr. P. Broca's " Description des Circonvolutions 

 Cerebrales de 1' Homme d'apres le Cerveau Scheniatique," com- 

 pleted by Dr. Pozzi. The latter writer draws special attention 

 to the third frontal circonvolution in man, which was first 

 definitely shown by Broca to be the seat of the organ of speech. 

 This function, in thirteen out of fourteen cases, is associated 

 with the left frontal, and in one out of fourteen with the right 

 frontal, as has been proved by loss of the faculty of speech, 

 known as "aphasia," or, according to the writer, more correctly 

 as "aphemia," which is due to lesions of that portion of the 

 brain. Dr. Pozzi sugget.s that, in deference to the scientific im- 

 portance of Broca's discovery, this special convolution should 

 henceforth be di-tinguished by his name. — The continuation of 

 M. Mathias Duval's lectures on " Le Transfonnisme," in which 

 the writer treats specially of heredity and natural selection, 

 drawing his materials, as in the earlier parts, almost exclusively 

 from English sources. — " Les Cafres et plus specialement les 

 Zoulous," by Elie Rectus. This is the first of a series of papers 

 intended by the author to elucidate the history of primitive 

 peoples. 



Rivista Scimtifico-IndustriaU, Flan-nct, December 15-31, 

 1883. — Account of the economic earthquake-warners constructed 

 by the brothers Brassart of the Roman Central Meteorological 

 Bureau (two illustrations), by E. Brassart. — De Tromelin's new 

 aperiodical galvanometer. — On the electric resistance of porce- 

 lain, sulphur, and some other non-conducting substances. — On 

 the measurement of electromotor forces. — On the determination 

 of the work executed and absorbed by a dynamo. — Contribution 

 to palKontological studies in Southern Italy, by Michele del 

 Lupo. 



Raidkonti del R. Istituto Lombardo, .Milan, January 10, 18S4. 

 — On numbers irreducible by complex numbers (concluded), by 

 Prof. C. Formenti. — Contribution to the physiology of the 

 enteric juice, by Prof. L. Solera. — Clinical demonstration of a 

 lymphatic infiltration of mechanical origin in the cornea ; pre- 

 liminary notice, by Dr. R. Rampoldi. — On the declaration of 

 bankruptcy at the instance of the creditors, in the new Italian 

 Commercial Code, by L. Gallavresi. — Attenuating and aggra- 

 vating circumstances in the Criminal Code (concluded), by Prof. 

 A. Buccellati. 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, February 21. — "On an Explanation of 

 Hall's Phenomenon." By Shelford Bidwell, M.A., LL.B. 



Mr. E. H. Hall's papers, giving a full account of his well- 

 known discovery, are printed in the Philosophical Magazine 

 for March iSSo, November iSSo, September 18S1, and May 

 18S3. His original experiment was as follows : — A strip of gold 

 leaf was cemented to a plate of glass and placed between the 

 poles of an electromagnet, the plane of the glass being perpen- 



dicular to the magnetic lines of force. The current derived 

 from a Bunsen cell was pased longitudinally through the gold, 

 and, before the electromagnet was excited, two equipotential 

 points were found by trial near opposite edges of the gold leaf, 

 and about midway between the ends : when these points were 

 connected with a galvanometer there was of course no deflection. 

 A current f ro)U a powerful battery being passed through the coils 

 of the magnet, it was found that a galvanometer deflection oc- 

 curred, indicating a difference of potential between the two 

 points, the direction of the current across the gold leaf being 

 opposite to that in which the gold leaf itself would have moved 

 across the lines of force had it been free to do so. On reversing 

 the polarity of the magnet the direction of the transverse elec- 

 tromotive force was reversed ; and when the magnet was demag- 

 netised the two points reverted to their original equipotential 

 condition. 



Subsequent experiments showed that the direction of the eflfect 

 differed according to the metal used. Thus with silver, tin, 

 copper, brass, platinum, nickel, aluminium, and magnesium, the 

 direction of the transverse electromotive force was found to be 

 the same as in the case of gold : with iron, cobalt, and zinc the 

 direction was reversed, and with lead there was no sensible effect 

 in either direction. 



Hall's results may be expressed by saying that the equipoten- 

 tial lines across the strip are rotated in a definite direction with 

 respect to the lines of force. This effect was attributed by him 

 to the direct actiin of the magnet on the current; and very 

 great importance has been attached to the phenomenon in_ conse- 

 quence of the opinion expressed by Prof Rowland and others 

 thit it is connected with the magnetic rotation of the plane of 

 polarisation of light, and thus furjiishes .additional evidence of 

 an intimate relation between light ani electricity. 



A number of experiments made by the author convinced him, 

 however, that no direct action of the kind supposed was ever 

 produced, and he ultimately found that Hall's phenomenon 

 might be completely expHinei by the joint action of mechanical 

 strain and certain thermo-electric effects. 



The strain is produced by electro-magnetic action. It will be 

 convenient to refer to the metallic plate or strip (which for the 

 purposes of this explanation may be assumed to be rectangular) 

 as if it were an ordinary map, the two shorter sides being called 

 respectively west and east, and the two longer north and south. 

 Let the south pole of an electro-magnet be supposed to be 

 beneath the strip, and let the strip be traversed by a current 

 jiassing through it in a direction from west to east. Then the 

 strip will tend to move across the lines of force in the direction 

 from south to north. Since, however, it is not free to move 

 bodily fro n its position, it will be strained, and the nature of 

 the strain will be somewhat similar to that undergone by a 

 horizontal beam of wood which is rigidly fixed at its two ends 

 and supports a weight at the middle. Imagine the strip to be 

 divided into two equal parts by a straight line joining the middle 

 points of the west and east sides. Then in the upper or northern 

 division the middle district will be stretched and the eastern and 

 western districts will be compressed, while in the lower division 

 the middle part will be compressed and the two ends will be 

 stretched. If now a current is passing through the plate from 

 west to east, the portion of the current which traverses the 

 northern division will cross first from a district which is com- 

 pressed to one which is stretched, and then from a district which 

 is stretched to one which is compressed ; while in the southern 

 division the converse will be the case. And here the thermo- 

 electric effects above referred to come into play. 



Sir '\Vm. Thomson, in 1856, announced the fact that if a 

 stretched copper wire is connected with an unstretched wire of 

 the same material, and the junction heated, a thermo-electric 

 current will flow from the stretched to the unstretched wire 

 through the hot junction, while if the wires are of iron, the 

 direction of the current is from unstretched to stretched. From 

 this it might be inferred that a current would flow through the 

 heated junction from an unstretched or free copper wire to a 

 longitudinally compressed copper wire, and from a longitudinally 

 compressed iron wire to a free iron wire; and experiment shovvs 

 this to be the case. A fortiori therefore the direction of the 

 current through the heated junction will be from stretched to 

 compressed in the case of copper wire, and from compressed to 

 stretched in the case of iron. If therefore a current is passed 

 from a stretched portion of a w ire to a compressed portion, heat 

 will (according to the laws of the Peltier effect) be absorbed at 

 the junction if the metal is copper and will be developed at the 



