INTENSITY OF MAGNETIC AND DIAMAGNETIC FORCES. 7^3 



which it exerts upon permanent steel magnets has alone been 

 examined with accuracy. This permanent magnetism is destroyed 

 by a white heat ; it was also found that at this temperature iron 

 ceased to be attracted by a magnet. The observations of M. 

 Pouillet refer to this point; he found that cobalt, even at the 

 highest temperature, remains magnetic ; that, as the heat in- 

 creases, chromium ceases to be magnetic a little below a red 

 heat, nickel at 662° F., and manganese at 68° to 77° F. below 

 zero. After the discovery of diamagnetism, it occurred to me 

 whether the magnetic condition of the body at these limits might 

 not have passed into the diamagnetic state. But Faraday found 

 that white-hot iron was always distinctly magnetic, although but 

 slightly so. He was never able to observe a transition into the 

 diamagnetic state ; nor has he observed any influence upon the 

 diamagnetism of solid and fluid bodies. He merely imagined, 

 quite recently, after having observed that warm air is more 

 strongly diamagnetic than cold air, that heat might increase the 

 diamagnetism of all bodies. The method adopted in our deter- 

 minations of the intensity gives here also the most certain ex- 

 planation. 



60. A hollow hemispherical cup of sheet brass, 56 millims. in 

 diameter, was filled with white sand, and a small piece of sheet 

 iron placed horizontally in such a manner that the sand formed 

 a layer 6 millims. to 8 millims. in thickness above it. Three thin 

 silvered copper wires, which converged superiorly, were fixed to 

 this cup, and supporting the scale-pan, could be suspended to the 

 beam of the balance. The cup with the sand was heated over 

 a coal fire, suspended to the balance, brought into equilibrium, 

 and placed as usual above the approximated round halves of the 

 keeper; the magnetism was then excited by the current of a 

 single Grove's cell, and the weight which was requisite to pull 

 off the cup was determined. These determinations of weights 

 were repeated constantly during the gradual cooling; to avoid 

 the loss of time, this was effected by gradually placing first shot, 

 and then fine sand, both of which were subsequently weighed, 

 into the other pan of the balance until the separation ensued. 

 To determine about the temperature, which at the first separa- 

 tion might be 572° F., the time at which the cup was withdrawn 

 in each case is given in the following table, together with the 

 •weight requisite to produce the separation. I may remark here, 

 that after the fourth separation the cup still hissed when touched 



