Strong Fields at Different Temperatures. 305 



field of the Ruhmkorff, as the constant permeability at first 

 led me to expect, but tended towards a limit. The largest 

 rotation obtained was about ^ of the maximum for iron. All 

 this appears to me to show the material to be essentially he- 

 terogeneous, relatively strongly magnetic layers being inter- 

 posed between the feebly magnetic mass. Probably the 

 structure on the whole is laminar, and so fine-grained that to 

 ordinary chemical mass-tests it appears perfectly homogeneous. 

 The ultimate heterogeneousness is only revealed by the mag- 

 neto-optical method, in which the action is restricted to a 

 surface patch a few millim. in diameter and a fraction of a 

 wave-length in thickness. Microscopical examination of a 

 mirror showed a faint indication of streaks in the polished 

 surface, and the specks of rust alluded to appeared to be 

 arranged in definite configurations. The crystallographic 

 method of corrosive etching was now resorted to : a drop of 

 dilute nitric acid produced a peculiar network of microscopical 

 surface figures, not revealed by a common steel mirror after 

 the same treatment. This affords another proof for the hetero- 

 geneousness of manganese steel, which thus appears to be 

 rather an unsatisfactory alloy for the physicist to deal with*, 

 however interesting it may be to the engineer. 



I believe great care should be taken in drawing conclusions 

 from the greater or less attraction of manganese steel filings 

 by strong magnets, this attraction proving an altogether false 

 criterion. I have not been able to file my samples ; but 

 small particles scratched off with a glass-cutting diamond 

 were readily attracted by any magnet. A piece of wire was 

 then drawn out to J- millim. diameter, and a number of small 

 bits, about 1 millim. long, cut off. These also behaved like a 

 heap of filings, and were attracted by a magnet as long as 

 separate. But on kneading them together with very little 

 wax, the lump was as unaffected by the magnet as a larger 

 piece of wire or bar. 



I think all this is only due to the diminutive dimensions 

 of the particles, and to nothing else, because they may thus 

 come very near the edges of the steel magnet. Here the 

 space-variation of .0 2 , on which the attraction depends, pro- 

 bably is considerable. In connexion with this it may be re- 

 membered that mineralogists extract ferruginous minerals, 



* In fact the ordinary theory of magnetic induction ceases to be ap- 

 plicable to heterogeneous solids ; all results obtained by the magneto- 

 meter, isthmus, Quincke's, or any other method must therefore be re- 

 garded as untrustworthy. If, for example, the laminae are, on the whole, 

 perpendicular to the held, the constant permeability, found by Messrs, 

 Ewing and Low, comes out quite naturally. 





