Dr. J. Hopkinson. 



[May 1, 



CURVE 4. 



the logarithms of the excess of temperature above the room, as in 

 Curve 5. The most remarkable feature in Curve 3 is that the material 

 has two critical temperatures, one at which it ceases to be magne- 

 tisable with increase of temperature, the other, and lower, at which it 

 again becomes magnetisable as the temperatures fall, and that these 

 temperatures differ by about 150 C. Between these temperatures, 

 then, the material can exist in either of two states a magnetisable 

 and a non-magnetisable. Note, further, that the curve for de- 

 creasing temperature returns into that for increasing temperature, 

 and does not attain to the high value reached when the tempera- 

 ture is increasing. From Curve 4 we see that there is absorption 

 of heat about 750 C., and not before ; and from Curve 5 that heat 

 is given off at 632 C., and again at a lower temperature. Com- 

 paring these temperatures with Curve 3, it is apparent that the 



