92 
MR. S. W. J. SMITH ON THE THERMOMAGNETIC ANALYSIS OF 
Nickel Steel (see Curves, fig. 22). v (94), w (95), x (96). 
Here, after a single alternation over about 60° C., the permeability x is about 
23 per cent, greater than v, and the subsequent air temperature permeability (97) is 
about 16 per cent, less than that after continuous cooling (85). 
It is important to notice that even in a case like v x %v Y x x , p 1 , in which it is assumed 
that all the solution remaining at- p x is simultaneously labile, just as when the cooling 
is uninterrupted, the permeability may be less below jh than after continuous cooling. 
For, in the former case, if the cooling has not taken place sufficiently slowly, the 
portions of the solution of which the crystallisation has been retarded (by the 
deposition from them of crystals less rich in nickel than themselves) may give rise 
eventually to crystals containing an appreciably greater percentage of nickel than is 
contained by the original nuclei which they surround. 
Thus interrupted cooling may, on account of the incomplete equilibrium due to 
difficulty of diffusion, be followed by a less homogeneous crystallisation of the material 
as a whole than when the cooling is continuous. If this does occur, the permeability 
will be less in the less homogeneous material, because (ef Hopkinson, ‘Hoy. Soc. Proc.,’ 
loc. cit., p. 70, above) the permeability of a nickel-iron alloy in a given weak field 
decreases as the percentage of nickel increases towards 25. 
§ 12. Owing to the comparative lowness of the temperatures at which the last- 
stages of the crystallisation occur, it is probable that the greatest degree of homogeneity 
will not be attained unless the cooling is extremely slow and that, if the cooling is 
comparatively rapid (occupying only a few hours), the formation of mixed crystals 
will be incomplete at ordinary temperatures. A subsequent alternation of temperature 
between that of the air and a higher temperature (below that at which solid solution 
begins to re-appear) will then produce a perceptible effect upon the permeability- 
temperature curve. It will accelerate the passage of the material into the state of 
final equilibrium, to be expected theoretically, in which the crystallisation is complete 
and the composition of the crystals is uniform. 
The experiments 97 to 107 (nickel steel, p. 47) may be cited as an example of the 
effects of such alternation. The data are shown graphically in fig. 22 (nickel steel). 
Precisely similar effects are shown also in figs. 17, 19, and 20. From each of the 
curves it seems that, during reheating, little effect is produced until the temperature 
exceeds 450° C. ; but, beyond this, there is a considerable accession of permeability 
(cf. Section VII., § 11, p. 101). The effect is most noticeable in those cases in which 
the previous value of the permeability at the air temperature is low. 
Section VII.— The Composition of the Sacramento Meteorite as Determined 
BY THERMOMAGNETIC ANALYSIS. 
§ 1. A comparison of the thermomagnet-ic curves of the meteoric iron with those 
of the artificial alloy, and with the data given by Osmond and others, shows, in the 
