304 H. E. J. G. du Bois on Magnetization in 



W. Thomson in 1855*. The relation assumed to exist be- 

 tween the optical phenomena and Hall's effect (as extended 

 to displacement currents) by Messrs. Rowland, H. A. Lorentz, 

 and van Loghemf appears in a certain sense as one of its 

 particular cases. In conclusion, it may be remarked that the 

 above phenomena are not actions at a distance, but actually 

 occur on the selfsame spot as the magnetization, which thus 

 appears to be their common cause, and to have a real physical 

 existence. 



Appendix. 

 On Manganese Steel. 



The optical method having proved a valuable aid in the 

 case of magnetite, it occurred to me to apply it for scruti- 

 nizing the behaviour of manganese steel. A piece of bar and 

 some wire was kindly supplied to me by Mr. Hadfield, of the 

 Hecla Works, Sheffield. Chemical analysis, for which I am 

 again indebted to Dr. Serda, gave 87*1 per cent. Fe, 11*8 per 

 cent. Mn, "3 per cent. Si, and some C. It is susceptible of 

 an excellent polish, rather better than that on ordinary steel. 

 However, I find it somewhat more liable to rust ; after some 

 months the polished surface showed small specks of oxide 

 under the microscope. Rough magnetometric measurement, 

 with the strong fields of the coil, gave an apparently con- 

 stant susceptibility of only '001, or permeability 1*013 %\ 

 about 25 per cent, of the magnetization being residual. 



On magneto-optic examination, in the manner above de- 

 veloped, negative rotation was found in every case. But its 

 numerical value varied considerably according as different 

 patches of the same polished surface were made to reflect, or 

 as different mirrors, ground on to the same piece, were tried. 

 The rotation on a given patch was not proportional to the 



* Sir W. Thomson, Reprint of Papers, ii. p. 178. 



t See Wied. Beiblatter, viii. p. 869 (1884). 



X Literature on p. 34 of Mr. Hadfi eld's paper read before the Iron 

 and Steel Institute, Edinburgh, 1888 ; further, Barrett, Proc. Roy. Soc. 

 Dubl. vi. p. 107 (1888). Results of experiments made at the German 

 marine observatory of ^Vilhelm shaven are given in Ann. d. Hydrogr, 

 xvii. p. 177 (1889), though not in absolute measure ; however, on con- 

 sidering the data given, the susceptibility may be estimated at "005 for 

 the thin bar, and -0015 for the thick bar used. Ewing and Low, he. cit., 

 find the permeability much larger, viz. P4, and constant for magnetizing 

 fields between 2000 and 10,000 C.G.S. This seems to show that the 

 magnetic properties of manganese steel are exceedingly variable from one 

 sample to another. [The same conclusion is arrived at by Dr. Paul Meyer, 

 in an article published quite recently (Elektrotechnische Zeitschrift, x. 

 p. 582 (1889) after the reading of the present paper.] 



