NATURE 



221 



THURSDAY, JANUARY 21, 1875 



DR. LLOYD'S " TREATISE ON MAGNETISM" 



A Treatise on Magnetism, General and Terrestrial. By 



Humphrey Lloyd, D.D., D.C.L., Provost of Trinity 



College, Dublin. (London : Longmans and Co., 1S74.) 



AN observational science like meteorology or terrestrial 

 magnetism is placed in some respects at a disad- 

 vantage when compared with the more experimental 

 branches of physical inquiry. It is often difficult to 

 obtain a good and readable account of that which 

 has been done. The reason of this is, that those 

 who are personally engrossed with the science have 

 to deal with such large masses of figures and precise 

 measurements that they are frequently unable to spare 

 the time necessary to give a good historical account 

 of their favourite research. Those again who are the 

 historians of science find it a very formidable task to 

 bring themselves en rapport with all that has been done 

 in such a subject as terrestrial magnetism — in fine, there 

 is not sufficient inducement to undertake the task. No 

 doubt, when such a science is more advanced and has 

 attained a position like that of astronomy, it will find 

 plenty of historians ; but in its infancy, and when a good 

 resume of the progress already made is of peculiar value, 

 it has comparatively few friends. Now these are precisely 

 the circumstances when a Government or a University is 

 able to interfere with very great effect, and with respect to 

 terrestrial'magnetism this opportunity has been admirably 

 used by Trinity College, Dublin. The Rev. Dr. Lloyd tells us 

 in his preface that the Dublin Magnetic Observatory was 

 founded and placed under his superintendence by the 

 governing body of Trinity College in 1838. This college 

 has been peculiarly fortunate in having chosen as an 

 observer the eminent physicist who is now its provost, and 

 who, besides leaping much fame as a practical magne- 

 tician, has at length found leisure to present us with the 

 much-required treatise on terrestrial magnetism. 



The first part of this work refers to the general pheno- 

 mena of magnetism, and contains one of the clearest 

 accounts of the elementary laws of this subject which we 

 have ever read. Some of the experiments recorded we 

 do not remember to have seen anywhere else. 



One of the most interesting prehminary chapters is that 

 on the effects of temperature. It is well known that heat 

 has a very peculiar effect upon all magnets. The following 

 paragraph from Dr. Lloyd's work will explain the particu. 

 lars of this action : — 



" Heat is also found to weaken the coercitive force of 

 iron, and therefore to facilitate its magnetisation and de- 

 magnetisation. When a bar of iron is heated and exposed 

 to the inductive action of a strong magnet, the magnetism 

 developed is augmented. This effect increases up to a 

 dull red heat, at which it is a maximum. At a bri;j;ht red 

 heat the capability of induction ceases altogether. Cast- 

 iron and steel present the same results. The maximum 

 force imparted to soft iron has been found by M. Ed. 

 Becquerel to be ro4, that imparted at the ordinary tem- 

 perature of the air being unity ; and it is a remarkable 

 circumstance that the maximum force induced in east-iron 

 and in steel is precisely the same as that of soft iron, 

 although at ordinary temperatures their induced magne- 

 tisms are very different. It appears from these facts that 

 the coercitive force of these bodies vanishes altogether at 

 a dull red heat " (p. 23). 

 Vol. XI. —No. 273 



Although it may be convenient to speak of the mag- 

 netic fluid, yet we think there can be little doubt that in a 

 magnet we have directed molecular motion of some kind. 

 This, we believe, is the hypothesis held by Sir W. Thom- 

 son and other physicists. The author of this notice has 

 ventured to bring forward certain views as to the action 

 of heat in destroying all directed motion. We know, for 

 instance, that the conduction of electricity and of heat, 

 two forms of directed molecular motion, is more resisted 

 at high than at low temperatures. The analogy urged to 

 explain this was that of a carriage or train in motion, on a 

 road lined with passengers who were constantly entering 

 at the one side while they were passing out of it at the 

 other. 



A stream of passengers of this nature would have the 

 effect of bringing the directed motion of the train ulti- 

 mately to rest. Now, heat may affect directed molecular 

 motions in the same way, carrying into the train matter 

 which does not partake of the motion of the train, and 

 carrying out of the train matter which does partake of 

 this motion, and so weakening the velocity of the train. 

 Even visible directed motion may be influenced in this 

 wa)-, and it does not seem improbable that the etherial 

 medium may act after this fashion in stopping. the differen- 

 tial motions of the universe. 



Now the question arises, is it likely that we have any 

 action of this nature traceable in the effects of heat upon 

 magnetism ? Let us again quote from Dr. Lloyd as to 

 certain peculiarities of the action of heat : — 



" When the heat applied to a steel magnet is moderate 

 — when, e.g., it does not exceed that of boiling water — 

 pait of the magnetism which had disappeared on the 

 increase of temperature reappears when the original tem- 

 perature is restored. It follows from this that heat pro- 

 duces two effects, which (in the present state of our 

 knowledge) must be considered as distinfct. 



" Like mechanical action, it permanently destroys a 

 portion of the existing magnetism by enabling the two 

 magnetisms which had been separated in each molecule 

 to recombine. And, on the other hand, it renders latent, 

 or neutralises, another portion of the same magnetism, 

 which portion reappears again when the temperature is 

 reduced to its original state. 



■' This two-fold operation of heat, although fully recog- 

 nised as a fact, has not been sufficiently considered in 

 reference to the cause. There seems reason to believe 

 that the two effects, so dissimilar in their conditions, are, 

 in fact, referable to distinct causes ; and that while the 

 permanent loss of magnetism is a dynamieal effeet due to 

 \.\\^ moleeular movement \vi^\i\<:\\\\&'iX is knoivn to con- 

 sist, the recoverable portion is probably to be ascribed to 

 the dilatation of the body and to the diminution of the 

 reciprocal action of the magnetic elements consequent 

 upon their increased distance." 



We quite agree with Dr. Lloyd in his remarks on these 

 two effects of heat, and would venture to supplement 

 them with a suggestion as to the possibility of regarding 

 the dynamical ej}ect of heat as due to the introduction of 

 new matter — new passengers, as it were — into the directed 

 train of magnetic motion. If this view be tenable, we 

 may perhaps imagine that a permanent loss of mag- 

 netism will be occasioned by every change of tempera- 

 ture of the magnet, whether this be from a lower tempe- 

 rature to a higher or from a higher temperature to a lower ; 

 in fact, as far as we can see, all the experiments hitherto 

 made are just as compatible with this supposition as with 



