March 25, [ 



NA TURE 



485 



it meant a permanent diamagnetic polarity, that is to say, "a 

 diainagnet" which seemed an incomprehensible result. My 

 suspicions were at once aroused, however, as to the possibility 

 of a transverse ordinary magnetisation ; and subsequent expe- 

 rience, on the whole, confirms this explanation. 



All the substances were then tried over again for permanent 

 magnetic polarity, and not one of them has failed to show it. A 

 piece of wood, for instance (or any other substance), which 

 points axially between the poles, instantly reverses its position, 

 turning through iSo°, when the magnet is reversed. But the 

 reversal must be done with the weak current only : anything 

 like a strong current, e.g. that from two or three secondary cells, 

 instantly destroys and reverses the permanent magnetism, and no 

 abnormal behaviour is then detected. Some substances, how- 

 ever, retain it better than others. The permanent magnetism 

 requires a strong current to excite it, and a very weak reverse 

 current to detect it. Without these conditions it would certainly 

 have been overlooked. It does not seem to matter whether a sub- 

 stance is magnetic or diamagnetic, it always reverses its position 

 or nearly reverses it when the weak reverse current is applied. 



The piece of copper was next lidd long-ways in the field, and 

 a strong current applied. On now han^jing it at 45" in the 

 field, and testing it by a weak current, it at once returned to its 

 axial position (though the copper was electrolytically " pure " and 

 decidedly diamagnetic) ; on reversing the weak current, it at 

 once turned through 180°, setting axially the other way, thus 

 behaving exactly like a weak magnet. When a strong current 

 was applied this behaviour was lost, the piece set itself nearly 

 equatorially again, and the residual axial magnetism was either 

 masked or lost. The reaction of induced currents natur.ally 

 makes the examination of conducting masses rather troublesome. 



During Christmas week Sir Wm. Thomson happened to pay 

 the laboratory a hurried visit, and I showed him a piece of pitch- 

 pine between the poles behaving exactly like a weak compass- 

 needle ; "a wooden magnet," as he at once called it. He was 

 good enough to suggest a better mode of arranging the experi- 

 ment for my original purpose of looking for the conversion of 

 magnetism into dianir.gnetism : an arrangement which I have 

 since adopted. So far, however, the results in this direction are 

 very preliminary. 



In all these experiments there is one flaw ; and it is partly 

 owing to this flaw that I have regarded them as unfit for pub- 

 lication. Indeed, I only send this note now because of the 

 publication of Dr. Tumlirz's result. His experiment with quartz 

 is very like one of mine, and it is very clearly and neatly de- 

 scribed in his paper. But the same flaw, or what appeared to 

 me to be such, ^eems to extend to his ca e also. What guanan- 

 tee is there that no trace of iron is present, — perhaps as mere 

 dirt, more likely as an infinitesimal ingredient of the substance? 

 Several of the pieces of coke I used had been boiled for weeks 

 in several lots of hydrochloric acid, and the last few washings 

 gave no ferrocyanide coloration ; but I have no doubt the coke 

 yet contains iron. Possibly the other substances do too. 



Suppose, now, asubstance contains a trace of iron, which iron 

 is susceptible of permanent magnetism, no matter how feeble : 

 then, no matter whether the substance itself be j aramagnetic 

 or diamagnetic, in an intense field its own properties will alto- 

 gether overpower those of the trace of iron, for this trace may 

 be considered as magnetically saturated and done for at once. 

 But suppose the substance next finds itself in a very weak field : 

 the induced magnetism and the force depending on it, since they 

 vary as the squnre of the field, are vanishing quantities ; any 

 trace of residual or permanent magnetism has it all its own 

 way. 



What way is there of proving that not a trace of iron exists 

 in a body? Chemical tests are surely futile compared with the 

 test of a magnet. I see at present no way out of the difficulty. 



And would not the same difficulty recur in connection with my 

 original notion ? I believe it would. Suppose I succeeded in 

 finding a substance, paramagnetic in a weak field, diamagnetic 

 in a strong one ; it would be open to any one to object that the 

 paramagnetism was due to a trace of magnetic impurity : that 

 this impurity, being intrinsically highly susceptible, causes all 

 the observed action in a weak field, but that it soon becomes 

 saturated as the field increases in strength, and that then its 

 force is altogether overpowered by the main bulk of less sus- 

 ceptible substance, whose saturation-point, if existent at all, is 

 miles higher up. Tnis substance then regulates the behaviour 

 of the body, and, according as it is diamagnetic or m.ignetic, 

 the whole body behaves diamagnetically or magnetically. 



Notwithstanding the prevision of this difficulty, I determined 

 to try the experiments, not knowing what might come of them, 

 and thinking that a body which could be made magnetic or dia- 

 magnetic at pleasure would be of some interest, however its 

 beh.aviour might be explained. I even thought of artificially 

 constructing such a body by incorporating a trace of iron in a 

 lump of bismuth, or by using semi-purified commercial bismuth. 

 I have not done this yet, however, and accordingly do not know 

 if it be possible. 



Tliat which has come, so far, of these experiments, viz. the 

 apparent existence of magnetic retentivity in all matter, is in 

 itself n)t an improbable result; rather, one might say it is 

 a probaljle one ; and although it may be poisiblej to explain 

 it by a trace of iron impurity, it by no means follows that this 

 disappointing sort of explanation is the correct one. 



The singular fact which most strongly suggests the need for 

 some such explanation is that diamagnetic bodies are capable of 

 ordinary permanent magnetism. It is true that on the Weber- 

 Ampere theoiy the specific molecular current of a diamagnetic 

 substance need not be zero but may have a small positive value, 

 which is easily destroyed and reversed by a powerful field, but 

 which yet may endow the substance with magnetic properties 

 in a weak field. But the worst of it is that I have never been 

 able to detect any trace of paramagnetic property, in a dia- 

 magnetic substance, other than this permanent or residual 

 polarity excited by immersion in a strong field. 



The only suggestion I can make is the following. 



Let the molecular channels in a diamagnetic substance be 

 not wholly free from resistance, though their resistance must be 

 very small ; let induced currents be excited in these channels 

 by immersion in a magnetic field, and let them have time to 

 dissipate a little energy and begin to die away before the field 

 is removed. On now destroying the field, the inverse induction 

 will more than destroy the previously induced currents, and 

 will leave a residue of opposite current in the molecules ; the 

 body will therefore behave as a weak magnet, until these re- 

 sidual currents die away. 



I must examine more carefully the excitation and decay of the 

 permanent magnetism with time. With wood it seems to be a 

 question of hours. 



These dissipation experiments are very important and should 

 be seriously attempted in several directions with proper appli- 

 ances and funds. Thus : gas molecules appear to be perfectly 

 elastic, or rather their impact coefficient of restitution is sup- 

 posed to be unity, but if a box of gas be shut up in infinitely 

 adiabatic cotton-wool for a century, will it have gone colder? 



Again, iron molecules are supposed to be infinitely con- 

 ducting, and their Ampere currents seem permanent ; but if the 

 moment of a bar were measured from time to time against 

 gravity, when a given current circu'ated in a given helix round 

 it, would it be found that age impaired its strength ? 



Once more, bismuth is supposed to be diamagnetic by reason 

 of its non-resisting channels ; but suppose a piece of bismuth is 

 left in a constant magnetic field for a year, will it have lost some 

 of iis diamagnetic property ? and when taken out will it be found 

 magnetic for a time ? 



It may be remarked that, whereas it is certain (on Ampere's 

 theory) that iron molecules are almost infinitely conducting, we 

 have no siuiikar assurance for bismuth ; it is even possible to sur- 

 mise that a body may tend to become diamagnetic in proportion 

 as it chokes off its own molecular currents, while magnetic 

 bodies are such as retain them perennially without apparent 

 loss. If such were the case, diamagnetism would sometimes 

 improve with age. 



I am, of course, aware that there is another, and merely 

 differential, theory of diamagnetism, but this leaves magnetism 

 itself wholly explained ; whereas, directly the Ampere theory of 

 magnetism is even provisionally accepted, the Pruth of infinite 

 molecular conductivity is already crossed, and the Weber theory 

 of diamagnetism follows as a natural and indeed inevitable con- 

 sequence. Oliver Lodge 



University College, Liverpool, March 15 



Dissociation and Contact-Action 



In a recent issue (Nature, vol. xxxiii. pp. 350-51) you drew 

 attention in your " Chemical Notes " to some recent researches 

 of M. Konovaloff on " contact-actions," and to the suggestion 

 made by him that " the dissociation (in the cases referred to) 

 w.as a consequence of the contact-action of the solid body." On 



