June 23, 1904J 



NA TURE 



171 



Further, in a paptr publi--lu-d in 1SS2. he ^uf^'^ests 

 that a further reason for the discrepancy as regards 

 glass mav be the fact that his experiments seen, to 

 show that glass would exhibit anomalous dispersion 

 if the spectrum could be examined below the visible 

 ra\s. Vet another reason for the discrepancy also 

 suggested itself to Dr. Hopkinson, and this \vas the 

 effect of residual charge, for he regarded capacity, 

 residual charge, and dielectric conductivity, all as 

 cirdinarilv known, as parts of one continuous pheno- 

 menon, and he assumes that we may add the effects 

 nf simultaneously, or successively, applied electro- 

 motive forces, and that residual charge is proportional 

 to the electric forces producing it. He thus gets the 

 expression ;- - 



- n 

 where r, is the displacement at a time /, x the P.D. 

 applied at a time u> before /. and \/'(o)) is a function of w 

 only. 



The experimental verification of this formula, and 

 the detailed examination of various substances such as 

 gl.iss. ice, and castor oil, from this point of view are 

 described in two exceedingly interesting papers. The 

 influence of temperature on the phenomena is also 

 examined. It appears from these experiments that the 

 \alue of the specific inductive capacity of a substance 

 depends on the time of contact. Thus Dr. Hopkinson 

 found that the specific inductive capacity of ice when 

 measured for periods of i/iooth to i/ioth second in- 

 creases both with rise of temperature and with increase 

 of time, and its value is of the order 80, but when 

 measured for periods such as i/io" second its value is 

 about 3, and he adds, " ^^'e conclude that the great 

 deviation of ice from Maxwell's law is due to residual 

 charge, which comes out between frequencies 10,000 

 .uul 100." 



We may mention here that there are some misprints 

 in the papers just referred to. On p. 15 equation (6) 

 |X\//;y)-U| should be Xjv/'fO-BI ; on p. 107, in equa- 

 tion (8), \"-/.v should be \7.x ; on p. 113, a slight exten- 

 sion of the upper radius of the lower right-hand quad- 

 rant makes it appear as if this quadrant were joined 

 to the right-hand top quadrant ; and on p. 120, dyi'dt — ''^t 

 should be ify,:iil = X^[t), and the reasoning that is given 

 relatively to \^(/) really applies to X\jr{/). 



The magnetic papers commence with one on the 

 magnetisation of iron, its value being greatly enhanced 

 bv the chemical analyses which are given of the speci- 

 mens e.xperiniented on. An exact definition of coercive 

 force is given, and the ascending and descending curves 

 are found for ;i l;irge number of samples by the split 

 bar and voke method. .Attention is also directed to the 

 wav in which a small quantity of manganese changes 

 the magnetic properties of iron; thus iron with 12 per 

 cent, of manganese is practically non-magnetic. 



The papers dealing with the effects of temperature 

 on the magnetic properties of iron contain some very 

 striking results. It had long been known that iron 

 or steel became non-magnetic when raised to a suffi- 

 ciently high temperature, viz. about 780° C. This Dr. 

 Hopkinson calls the critical temperature, and he shows 

 that for small magnetising forces the magnetisation of 

 NO. 1808, VOL. 70] 



iron inceases with rise of temperature until it ap- 

 proaches the critical temperature, but, on further heat- 

 ing, the magnetisation very suddenly almost entirely 

 disappears. It is also shown that as conjectured by 

 Barrett, recalescence occurs at the critical temperature, 

 and that the quantity of heat liberated in recalescence 

 is comparable with the heat required to melt bodies ; 

 and, as a further proof of the connection between, 

 recalescence and the disappearance of magnetism. Dr. 

 Hopkinson shows that no liberation of heat takes place 

 in a non-magnetisable manganese steel when experi- 

 mented on in the same way as hard steel and iron. 



Most remarkable results were found with certain 

 alloys of iron and nickel. Thus it was found that a 

 specimen containing 25 per cent, of nickel could exist 

 in two different and quite stable states through a range 

 of temperature from a little below freezing tO' 

 5So° C, one state being non-magnetisable, the other 

 magnetisable. Other physical properties of this alloy 

 were found to change with its magnetic properties; 

 thus its mechanical strength, its extensibility, its elec- 

 tric resistance, its density, are all different in the two 

 states. From the memoir attached to the first volume 

 we learn that Dr. Hopkinson tried if other substances, 

 such as chromium and manganese steel, would behave 

 in a similar manner when experimented on in the 

 same way al the nickel and iron alloy, but none of 

 them showed any sign of becoming magnetic, although 

 cooled in solid carbonic acid. 



Next follow papers on magnetic viscosity and on 

 the propagation of magnetisation of iron as effected by 

 the electric currents in the iron. It will be remem- 

 bered with what interest the cxpi'rimenls on the 

 latter subject were witnessed, ;md how, when the cur- 

 rent was sent round the coil magnetising the block of 

 iron, the ballistic galvanometers attached to the various 

 search coils embedded in the mass deflected with con- 

 siderable intervals one after the other ;is the magne- 

 tisation reached their respective search coils. 



Lastly come papers on the rupture of iron w ire by a 

 blow, on the mathematical theory of Tartini's beats, 

 on the effect of internal friction on resonance, on the 

 optical properties of a titano-silicic glass, on the quasi- 

 rigidity of a rapidly moving chain, on the torsional 

 strain which remains on a twisted glass fibre after 

 release from twisting stress, on the stresses caused in 

 an elastic solid by inequalities of temperature, and 

 various others. 



At the present time, when so much attention is being 

 given to the development of applied education in this 

 country, it is instructive to look back on this ideal 

 technical teacher, this translator of abstract mathe- 

 matics into concrete industrial achievements, a man 

 who was so able that he was quite simple and modest — 

 for only mediocrity requires " side " — who sometimes 

 spoke of things as having happened to be carried out 

 by himself, as if it were a matter of mere chance that 

 they had not been originated and accomplished by any- 

 one else. What irony of fate when so many holiday- 

 making -Alpine tourists, whose only possible claim to 

 notice consists in their having made some ascent a 

 little earlier in the season or gone a little higher than 

 someone else, return scatheless year after year, that 

 the man who was doing, and had done, great work in 



