474 Scientific Proceedings, Royal Dublin Society. 



noticed that the permanent magnetic loss by heating to 250° and 

 then cooling is much greater with ordinary than with manganese 

 steel. If the latter could have a high moment conferred upon it, 

 it would be incomparably the best material for the construction of 

 magnets and compass needles. Dr. Hopkinson (Phil. Trans., May, 

 1889) has treated a 12 per cent, specimen of manganese steel to a 

 temperature of upwards of 800° C, and finds that even at this 

 temperature, when ordinary steel has entirely lost all magnetic 

 properties, manganese steel still remains slightly magnetic. But 

 the most remarkable effect of temperature on the magnetic state 

 of a body is that which Dr. Hopkinson has lately discovered in an 

 alloy of iron and nickel containing 25 per cent, of the latter metal. 

 At the ordinary temperature this alloy is practically non-magnetic, 

 but on being cooled to a little below the freezing point it becomes 

 magnetic, and remains in this condition even when heated up to a 

 temperature of 580° Cent. Then it suddenly ceases to be magnetic, 

 and remains non-magnetic until cooled again below the freezing 

 point. This nickel alloy has therefore two stable conditions, one 

 magnetic, and the other non-magnetic ; its state being determined 

 by the high or low temperatures to which it was last exposed. 

 Facts of this kind show the great importance of a fuller magnetic 

 study of the different alloys of the magnetic metals : thus slowly 

 may we hope to throw more light on the outstanding enigma of 

 why magnetic properties are confined to the three elements — iron, 

 nickel, and cobalt. 



NOTE ADDED IN PBESS. 



M. Osmond's admirable researches throw considerable light 

 on the nature of manganese steel, which probably owes its non- 

 magnetic character to its heterogeneousness. 



M. DuBois in a recent Paper (Phil. Mag., 1890) has confirmed 

 this view by a magneto-optic method of investigation. Chemical 

 and microscopic examination also exhibit its heterogeneous struc- 

 ture. 



