82 



NATURE 



[April 3, 19 19 



Indian timbers for divers uses would require years 

 of work conducted by a number of investigators. 

 Mr. Pearson's is pioneer work rather aiming to 

 arrive at some practical conclusions within a time 

 corresponding with the urgency of India's needs. 

 But it would appear eminently desirable to 

 accelerate and extend Mr. Pearson's work by the 

 employment of a staff of investigating experts, 

 in view not only of the results that he has already 

 obtained, but also of the possibility that in 

 India it may often be commercially more profit- 

 able to cultivate rapidly growing timber trees 

 the wood of which is perishable and of low 

 quality, but capable of being cheaply preserved by 

 antiseptic treatment, than to grow trees of slower 

 growth the timber of which is superior in quality 

 and in durability. Percy Groom. 



PROF. RIGHI'S RESEARCHES. 

 I Fenomeni Elettro-Atomici sotto I'Azione del 



Magnetismo. By Prof. A. Righi. Pp. xvi + 435. 



(Bologna : Nicola Zanichelli, n.d.) Price 



17.50 Hre. 

 TN this volume Prof. Righi gives a summary of 

 -*- his researches on the effect of a magnetic field 

 on the electric discharge. These researches 

 have been mainly concerned with three effects : 

 the change in the potential required to start 

 the discharge, the change in the appearance 

 of the charge at low pressures, and the 

 mechanical forces acting on bodies in the 

 neighbourhood of the discharge. In all three 

 branches of his study he has recorded a 

 large number of interesting and suggestive facts, 

 which deserve the close attention of all students 

 of physics ; if they have not received the attention 

 they deserve, it is largely because Prof. Righi has 

 tended to describe his work in terms of theories 

 which others who have pursued parallel investiga- 

 tions have been unable to accept. 



A review is not a suitable medium for scientific 

 discussion, and any detailed criticism of Prof. 

 Righi 's views would be out of place. But perhaps 

 it may be useful to record some questions which 

 inevitably occur to a reader of the chapter which 

 deals with the effect of the magnetic field on the 

 discharge potential, for this matter has been dis- 

 cussed less thoroughly than the theory of " mag- 

 netic rays " on which Prof. Righi bases his inter- 

 pretation of the second group of phenomena. 



Prof. Righi found that a magnetic field, whether 

 parallel or perpendicular to the electric field, may 

 produce either an increase or a decrease lin the 

 discharge potential. He asserts that the 

 "accepted theory," which attributes the effect of 

 the magnetic field to a change in the path of the 

 ions, is inadequate to account for such a differ- 

 ence in the sign of the effect. As a matter of fact, 

 there is no accepted theory which predicts satis- 

 factorily the discharge potential in terms of the 

 paths of the ions, even when there is no magnetic 

 field. Changes in the discharge potential due to 

 added electric fields (such as are produced by 

 bringing an insulated body near to one electrode of 

 NO. 2579, VOL. 103] 



a tube at low pressure) are at present inexplicable 

 except in the most vague and general way ; so long 

 as such changes remain unexplained, it is quite 

 impossible to prove that the change in the paths 

 of the ions produced by a magnetic field must 

 affect the discharge potential in one sense rather 

 than the other. Prof. Righi, on the other hand, 

 asserts that a theory which considers only the 

 path of the ions after they are separated must 

 predict that a transverse magnetic field will pro- 

 duce an increase in the discharge potential. In 

 order to explain the occurrence of a decrease in 

 certain conditions, an additional hypothesis is 

 required; that which he suggests is based on the 

 action of the field on the atom before it is ionised ; 

 he suggests that the field increases the radius of 

 the electronic orbit in the atom, and so decreases 

 its stability. 



The first question which arises naturally is how 

 Prof. Righi arrives at a result so directly contrary 

 to that on which Langevin's theory of diamag- 

 netism is based ; according to the new theory, all 

 atoms ought to be paramagnetic. The second is 

 why he assumes, without proof, that an increase in 

 the radius of the orbit means decreased stability. 

 The third is whether he has attempted to calculate 

 numerically the change predicted by his theory in 

 the orbit ; if he will do so, he will find that the 

 change in the orbit produced by such fields as 

 are concerned here is much too small to be likely 

 to lead to any important change in the energy 

 of ionisation. The fourth is why he has not 

 attempted to apply one of the usual methods for 

 measuring the energy required for ionisation to 

 test his theory — and so on. 



To these questions no answers are given in the 

 volume before us. The author seems to us gene- 

 rally to be apt to seize with too great readiness 

 on any explanation which will account for facts 

 immediately under his notice, without considering 

 with due care hov/ it may fit in with facts less 

 immediately obvious, and too ready to be content 

 with qualitative explanations when quantitative 

 explanations are required ; for the same features 

 are to be found in his treatment of the other two 

 effects which he has studied, though they are less 

 noticeable in his work on the " magnetic rota- 

 tions " of bodies near or immersed in the dis- 

 charge. They are less noticeable because the 

 explanations which he offers are less novel. The 

 phenomena which he describes are, as he recog- 

 nises, direct consequences of the fundamental laws 

 of electromagnetism ; the criticism here would be 

 rather that he treats individually and with unneces- 

 sary detail facts which are all illustrations of a 

 single recognised principle. In particular, the 

 explanation which is offered of the Hall effect, on 

 the analogy of the "magnetic rotations," appears 

 to differ in no essential way from that expounded 

 in many text-books. 



Perhaps a criticism of this kind is unjust, for 

 Prof. Righi explains that he is not writing for the 

 expert, but for the reader who wants to bring 

 his knowledge of physics up to date. He inserts 

 an introductory chapter for the benefit of such a 



