METEORIC AND ARTIFICIAL NICKEL-IRON ALLOYS. 
69 
The distribution in space of the chemical constituents of a homogeneous alloy is 
analogous to that which exists in a solution, and the redistribution on crystallisation 
is the same phenomenon in each case, although the conditions under which the 
redistribution occurs in a fluid medium must be different from those in which it takes 
place when the medium is solid. Experiment has shown, however, that interdiffusion 
of metals can take place even at low temperatures, and hence to postulate the 
interdiffusion of the nickel and iron, as in the above theory, does not involve the 
introduction of an ad hoc hypothesis. 
The theory involves the result that in any nickel-iron alloy—assuming the 
magnetisability to be a function of the state of crystallisation—the magnetism will 
not appear and disappear at one definite temperature. Thus in alloys containing 
dp, 9s per cent, of nickel, crystallisation will begin at temperatures corresponding to 
p', s' respectively, and will not be complete until temperatures corresponding to p, s 
are reached. 
Hence over the range of temperature represented by pp in the one case and by s's 
in the other the amount of magnetic material will be increasing. In only one alloy, 
viz., that corresponding with C, in which the composition of the crystals is the same as 
that of the solid solution, will it be possible for the magnetism to disappear or reappear 
completely at one temperature. The data given by Osmond {Joe. cit.) are qualitatively 
in agreement with this deduction. The curves which he gives show that in the alloy 
containing 7675 per cent, of nickel the magnetism reaches its full value very rapidly, 
after the temperature of reappearance is reached, just as it does in pure iron. It is 
further clear on inspection of the curves (‘ Rev. Gen.,’ vol. XIV., pp. 868 and 869, 1903), 
that the range over which the permeability reappears becomes wider as the percentage 
of nickel increases from A or diminishes from C. Similarly with percentages above 
C, the range over which the magnetism reappears at first widens, but, as D is 
approached, again becomes restricted [cf. also Section VI., § 8, p. 88). 
§ 5. The connection between permeability and the state of crystallisation is directly 
suggested in an experiment by Guillaume (‘Comptes Rendus,’ vol. 134, p. 596). 
A non-magnetic specimen of nickel steel, part of the surface of which was polished, 
was cooled down, with the result that visible crystals were developed and that the 
material become magnetic. According to Guillaume (‘ Journ. Iron and Steel Inst.,’ 
vol. II., p. 297, 1905), the first observation of this kind was made by Osmond. 
By observation of the changes of volume (which are accompanied by the appearance 
of magnetism) in certain nickel-iron alloys, Guillaume has shown (see ‘ Rev. Gen.,’ 
1903, loc. cit.) that these changes agree quantitatively with the view that the presence 
of the nickel merely alters the temperature at which the transition of the iron in the 
alloy, from the non-magnetic to the magnetic state, takes place. The simplest 
interpretation of his results is that the nature of the change is the same in all the 
alloys, while the temperature at which the change begins, and the rate at which it 
proceeds, depends upon the percentage of nickel. The experiments are necessarily 
