and Maximum of Magnetism of Nickel and Cobalt. 323 



have no data for determining which it is at present. If it were 

 possible for 33 to attain a maximum value so that /x should ap- 

 proach to 0, k would be negative, and the substance would then 

 become diamagnetic for very high magnetizing forces*. This is 

 not contrary to observation ; for at present we lack the means of 

 producing a sufficiently intense magnetic field to test this expe* 

 rinientally, at least in the case of iron. To produce this effect 

 at ordinary temperatures, we must have a magnetic field greater 

 than the following — for iron 175,000, for nickel 63,500, and 

 for cobalt about 100,000 (?). These quantities are entirely be^ 

 yond our reach at present, at least with any arrangement of 

 solenoids. Thus, if we had a helix 6 inches in diameter and 3 

 feet long with an aperture of 1 inch diameter in the centre, a 

 rough calculation shows that, with a battery of 350 large Bun^ 

 sen cells, the magnetic field in the interior would only be 15,000 

 or 20,000 when the coils were arranged for the best effect. We 

 might obtain a field of greater intensity by means of electro- 

 magnets, and one which might be sufficient for nickel ; but we 

 cannot be certain of its amount, as I know of no measurement 

 of the field produced in this way. But our principal hope lies 

 in heating some body and then subjecting it to a very intense 

 magnetizing-force ; for I have recently found, and will show pre- 

 sently, that the maximum of magnetization of nickel and iron de- 

 creases as the temperature rises, at least for the two temperatures 

 0° C. and 220° C. I am aware that iron and nickel have been 

 proved to retain their magnetic properties at high temperatures, 

 but whether they were in a field of sufficient intensity at the time 

 cannot be determined. The experiment is at least worth trying 

 by some one who has a magnet of great power, and who will 

 take the trouble to measure the magnetic field of the magnet at 

 the point where the heated nickel is placed. This could best be 

 done by a small coil of wire, as used by Verdet. 



But even if it should be proved that 33 does not attain a maxi- 

 mum, but only 3, it could still be explained by Faraday's theory ; 

 for we should simply have to suppose that the magnetic induc- 

 tion 33 was composed of two parts — the first part, 47r3, being- 

 due to the magnetic atoms alone, and the second, 1q, to those 

 lines of force which traversed the sether between the atoms. To 

 determine whether either of these quantities has a maximum 

 value can probably never be done by experiment \ we may be 

 able to approach the point very nearly, but can never arrive at 

 it, seeing that we should need an infinite magnetizing-force to 

 do so. Hence its existence and magnitude must always be in- 

 ferred from the experiments by some such process as was used 



* See Maxwell's * Treatise on Electricity and Magnetism/ art. S-J4.— 

 J. C. M. 



