270 



NATURE 



\yan. 23, 1690 



magnetism.^ I am not aware that anything corre- 

 sponding with recalescence has been observed in the 

 case of nickel. Experiments have been tried, and gave 

 a negative result, but the sample was impure ; and 

 the result may, I think, be distrusted as an indication of 

 what it would be in the case of pure nickel. The most 

 probable explanation in the case of iron, at all events, 

 appears to be that when iron passes from the magnetic to 

 the non-magnetic state it experiences a change of state of 

 comparable importance with the change from the solid to 

 the liquid state, and that a large quantity of heat is ab- 

 sorbed in the change. There is, then, no need to suppose 

 chemical change ; the great physical fact accompanying 

 the absorption of heat is the disappearance of the capacity 

 for magnetization. 



What explanations have been offered of the phenomena 

 of magnetism .? That the explanation must be molecular 

 was early apparent. Poisson's hypothesis was that each 

 molecule of a magnet contained two magnetic fluids, 

 which were separated from each other under the influence 

 of magnetic force. His theory explained the fact of mag- 

 netism induced by proximity to magnets, but beyond this 

 it could not go. It gave no hint that there was a limit to 

 the magnetization of iron — a point of saturation ; none of 

 hysteresis ; no hint of any connection between the mag- 

 netism of iron and any other property of the substance ; 

 no hint why magnetism di sappears at a high temperature. 

 It does, however, give more than a hint that the perme- 

 ability of iron could not exceed a limit much less than its 

 actual value, and that it should be constant for the mate- 

 rial, and independent of the force applied. Poisson gave 

 his theory a beautiful mathematical development, still 

 useful in magnetism and in electrostatics. 



Weber's theory is a very distinct advance on Poisson's. 

 He supposed that each molecule of iron was a magnet 

 with axes arranged at random in the body ; that under 

 the influence of magnetizing force the axes of the little 

 magnets were directed to parallelism in a greater degree 

 as the force was greater. Weber's theory thoroughly 

 explains the limiting value of magnetization, since no- 

 thing more can be done than to direct all the molecular 

 axes in the same direction. As modified by Maxwell, or 

 with some similar modification, it gives an account of 

 hysteresis, and of the general form of the ascending curve 

 of magnetization. It is also very convenient for stating 

 some of the facts. For example, what we know regarding 

 the effect of temperature may be expressed by saying that 

 the magnetic moment of the molecule diminishes as the 

 temperature rises, hence that the limiting moment of a 

 magnet will also diminish ; but that the facility with 

 which the moLcules follow the magnetizing force is also 

 increased, hence the great increase of /n for small forces, 

 and its almost instantaneous extinction as the temper- 

 ature rises. Again, in terms of Weber's theory, we can 

 state that rise of temperature enough to render iron non- 

 magnetic will not clear it of residual magnetism. The 

 axes of the molecules are brought to parallelism by the 

 force which is impressed before and during the time that 

 the magnetic property is disappearing ; they remain 

 parallel when the force ceases, though, being now non- 

 magnetic, their effect is nil. When, the temperature 



' I have only recently become acquainted with the admirable work of 

 M. Osmond on recalescence. He has examined a great variety of samples 

 of steel, and determined the temperatures at which they give off an excep- 

 tional amount of heat. Some of his results are apparent on my own curves, 

 though I had assumed them to be mere errors of observation. For example, 

 refernng to my Koyal Society paper, there is, in Fig. 38, a hint of a second 

 small anomalous point a little below the larger one. And. compari'ig 

 Figs. 38 and 38A, we see that the higher the heating, the lower is the point 

 of recalescence ; both features are bn ught out by M. Osmond The 

 double recalescence observed by M. Osmmd in steel with a moderate 

 quantity of carbon I would explain provisionally by supposing this steel to 

 be a mixture of two kinds which have different critical temperatures. 

 Although M. Osmond's metlicd is admirable for determining the tempe-ature 

 of recalescence, and whether it is a single point or multiple, it is not 

 adapted to determine the quantity of heat liberated, as the small sample 

 used is inclosed in a tube of considerable mass, which cools down at the 

 same time as the sample experimented upon. 



falling, they become again magnetic, the effect of the 

 direction of their axes is apparent. But Weber's theory 

 does not touch the root of the matter by connecting the 

 majjnetic property with any other property of iron, nor 

 does it give any hint as to why the moment of the mole- 

 cule disappears so rapidly at a certain temperature. 



Ampere's theory may be said to be a development of 

 Weber's : it purports to state in what the magnetism of 

 the molecule consists. Associated with each molecule is 

 a closed electric current in a circuit of no resistance ; 

 each such molecule, with its current, constitutes Weber's 

 magnetic molecule, and all that it can do they can do. 

 But the great merit of the theory— and a very great one 

 it is — is that it brings magnetism in as a branch of elec- 

 tricity ; it explains why a current makes a magnetizable 

 body magnetic. It also gives, as extended by Weber, an 

 explanation of diamagnetism. It, however, gives no hint 

 of connecting the magnetic properties of iron with any 

 other property. Another difficulty is this : When iron 

 ceases to be magnetizable, we must assume that the mole- 

 cular currents cease. These currents represent energy. 

 We should therefore expect that, when iron ceased to be 

 magnetic by rise of temperature, heat would be liberated ; 

 the reverse is the fact. 



So far as I know, nothing that has ever been proposed 

 even attempts to explain the fundamental anomaly. Why 

 do iron, nickel^ and cobalt possess a property which we 

 have found nowhere else in nature ? It may be that at 

 lower temperatures other metals would be magnetic, but 

 of this we have at present no indication. It may be that, 

 as has been found to be the case with the permanent 

 gases, we only require a greater degree of cold to extend 

 the rule to cover the exception. For the present, the 

 magnetic properties of iron, nickel, and cobalt stand as 

 exceptional as a breach of that continuity which we are in 

 the habit of regarding as a well proved law of Nature. 



NOTES ON A RECENT VOLCANIC ISLAND 

 IN THE PACIFIC. 



IN 1867, H.M.S. Falcon reported a shoal in a position 

 in about 20^ 20' S., and 175° 20' W., or 30 miles west 

 of Namuka Island of the Friendly or Tonga Group. 



In 1877 smoke was reported by H.M.S. Sappho to be 

 rising from the sea at this spot. 



In 1885 a volcanic island rose from the sea during a 

 submarine eruption on October 14, which was first re- 

 ported by the Janet Ntchol, a passing steamer, to be 2 

 miles long and about 250 feet high. 



The U.S.S. Mohican passed it in ] 886, and from calcula- 

 tion founded on observations in passing, gave its length 

 as ij*o miles, height 165 feet. The crater was on the 

 eastern end, and dense columns of smoke were rising 

 from it. 



In 1887 the French man-of-war Decres reported its 

 height to be 290 feet. 



In the same year an English yacht, the 5y<^//, passed it, 

 and a sketch was made by the owner, H. Tufnell, Esq., 

 which is here produced. 



The island has now been thoroughly examined and 

 mapped, and the surrounding sea sounded by H.M. 

 surveying-ship Egerta, Commander Oldham. 



It is now ly^j mile long, and yjj of a mile wide, of the 

 shape given in the accompanying plan. The southern 

 portion is high, and faced by cliffs on the south, the 

 summit of which is 153 feet above the sea. A long flat 

 stretches to the north from the foot of the hill. 



The island is apparently entirely formed of ashes and 

 cinders, with a few blocks and volcanic bombs here and 

 there, especially on the verge of the hill. 



Under the action of the waves, raised by the almost 

 constant south-east winds, this loose material is being 

 rapidly removed ; continual landslips take place, and 

 Commander Oldham is of opinion that the original 



