454 Mr. R. S. Willows on the Variation of the 



explanation adopted by M. Guillaume is very similar to the 

 one given above, a brief abstract of the paper is given here, 

 and the points of resemblance noted. 



M. Guillaume divides the nickel-iron alloys into two classes, 

 (1) those containing less than 25 per cent, nickel, which he 

 calls irreversible alloys, and (2) those containing more than 

 25 per cent, nickel, which he calls reversible. As discovered 

 by Hopkinson, nickel-steels which are not magnetic at ordi- 

 nary temperatures can be rendered magnetic by being cooled 

 in a freezing-mixture. If an alloy which has been so cooled is 

 then heated, it loses its magnetism at about 700°-800° C. If 

 it be then cooled there is a considerable range of temperature 

 in which it is devoid of magnetic properties if it belongs to 

 the first class, in one case from 600° C. to below 0° C, while 

 if it belongs to the second class, its magnetic properties will 

 be very similar at the same temperature no matter whether it 

 is being heated or cooled. At the temperature at which the 

 magnetism began to fall off, Hopkinson (loc. cit.) found an 

 absorption of heat, while there was an evolution of heat at 

 the point where the magnetic properties reappeared. The 

 irreversible alloys were also found to increase in volume when 

 they became magnetic. Guillaume * found that the dilatation 

 by heating depended largely on the composition, rising 

 gradually to a maximum when the amount of nickel present 

 was 24 per cent., after which it fell to a minimum for 36 per 

 cent, of nickel and then slowly rose again. He also found f 

 for a certain alloy of the first class which regained its mag- 

 netic properties on being cooled to 130° after being heated to 

 600°, that it elongated on cooling until —60° was reached ; 

 but if before this lower temperature was reached it was 

 again heated, it expanded proportionally to the temperature, 

 and contracted also proportionally to the temperature on 

 being cooled a second time, until the temperature was again 

 reached at which the re-heating commenced, when it expanded 

 again, following the same curve as it did on first cooling. 

 The amount of the expansion on reheating depended on 

 the stage at which this reheating; be oan, and hence he inferred 

 that this and similar alloys possess an infinite number of states 

 of equilibrium. When a reversible alloy was cooled it con- 

 tracted, but when the temperature was maintained constant 

 this contraction ceased and it expanded again slightly, the 

 amount depending on the stage at which the cooling was 

 stopped. The electrical resistance, according to Guillaume J, 

 alters regularly with temperature. It is remarkable that 



* Compt. Rend. vol. cxxiv. p. 176. f Ibid. vol. cxxvi. p. 739. 



J Ibid. vol. cxxv. p, 2Hb. 



