722 PLUCKER ON THE DETERMINATION OF THE 



ported exerts the most decided influence upon it ; and by varying 

 the mass, this supporting power may be increased a hundred or 

 a thousand times. And how can we determine this mass in dif- 

 ferent magnets so as to be enabled to compare their supporting 

 power ? Moreover, ao long as the magnetic polarity excited in 

 the iron of the keeper or the entire body attracted reacts to the 

 augmentation of the power of the electro-magnet, and lastly, so 

 long as one portion of the attracted body acts upon the other so 

 as to excite magnetism, so long will a comparison of the intensity 

 of the attractive forces, which the magnet exerts upon the differ- 

 ent magnetic substances, be out of the question. I believe, 

 however, that after the previous remarks we may admit without 

 hesitation, that the disturbing influences in question are not 

 present when iron or nickel, in a state of fine division, is uni- 

 formly diffused in not too large quantity through a substance 

 which is but little susceptible of the influence of the magnet, as 

 lard ; or when the solution of a salt of iron or nickel is used. I 

 believe that 1 am justified in assuming that the attraction of the 

 entire mass is then equal to the sum of those attractions, which, 

 when we divide this mass into parts, the magnet would exert 

 upon the parts individually, evert if the other portions were not 

 present. 



But our method of determining the relative magnetic inten- 

 sities of different substances would retain its full value, even if 

 the action of the reciprocally inducing portion of the attracted 

 mass did not vanish ; but the volume and the limits remaining 

 the same, is proportional to that force with which the magnet 

 attracts the different substances. 



13. We may easily become satisfied, by a simple experiment, 

 that the attraction of a compact mass of iron by a magnet is not 

 the sum of those attractions which are emitted by the magnet to 

 the separate parts of the mass, but that the disturbing effects of 

 induction are also present. Thus if we place an iron rod upon 

 the pole of a magnet, a certain w-eight A is requisite to withdraw 

 it. If we then cut the rod into two pieces, and place the lowest 

 piece upon the pole exactly as before, but the upper piece upon 

 any non-rhagnetic support, which keeps it at its former distance 

 from the pole, we can again determine two weights B and C 

 by the balance, which are requisite for the withdrawal of the 

 two parts. We then find, — 



A>B + C. 



