58 HENEY A. ROWLAND 



cobalt about 100,000 (?). These quantities are entirely beyond 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 Bunsen 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 

 electromagnets, 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-f orce ; for I 

 have recently found, and will show presently, that the maximum of 

 magnetization of nickel and iron decreases as the temperature rises, at 

 least for the two temperatures 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 try- 

 ing 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 maximum, 

 but only $, it could still be explained by Faraday's theory; for we 

 should simply have to suppose that the magnetic induction 33 was 

 composed of two parts the first part, 4 Trig, being due to the magnetic 

 atoms alone, and the second, >, to those lines of force which traversed 

 the aether between the atoms. To determine whether either of these 

 quantities has a maximum value can probably never be done by experi- 

 ment; 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 inferred 

 from the experiments by some such process as was used in my first 

 paper, where the curve of permeability was continued beyond the point 

 to which the experiments were carried. Neither does experiment up 

 to the present time furnish any clue as to whether it is 33 or $ which 

 attains a maximum. 



As the matter is in this undecided state, I shall hereafter in most 

 cases calculate both $ and * as well as 33 and //, as I am willing to admit 

 that $ may have a physical significance as well as 33, even on Faraday's 

 theory. 



