DIFFERENTIAL MAGNETIC ACTION. 
17 
over it ; showing that the tendency of the smoke was outwards in every part of the 
magnetic field occupied by air or gas, and that therefore its motion was due to the 
action of the magnet on it as a diamagnetic, and not to currents of the air, which, 
if existing, would be inwards in one place or direction, and outwards in another. 
2755 . When magnetic or diamagnetic fluids were subject to the magnetic force 
upon a plate of mica over the poles, according to the ingenious arrangement of 
Plucker, they quickly assumed the different forms correspondent to their nature, 
but after that there was no further motion or current in them. The cases are no 
doubt different to those where the whole of the magnetic field is occupied by the 
same medium ; still, as far as it goes, it helps to confirm the conclusion that no cur- 
rents are formed. On putting the same liquids between the poles in glass cells, no 
magnetic currents could be observed in them, though fine particles were introduced 
into the fluids, for the purpose of making such changes of place visible, if they 
occurred. 
2756 . So there is no evidence, either by the action on a ray of light (2727- 2729.), 
or by any expansion or contraction (2750.), or by the production of any currents 
(2754.), that the magnet exerts any direct power of attraction or repulsion on the 
particles of the different gases tried, or that they move in the magnetic field, as they 
are known to do, by any such immediate attraction or repulsion. 
^ ii. Differential magnetic action. 
27b7. Then what is the cause of the diamagnetic change of place ? The effect is 
evidently a differential result, depending upon the differences of the two portions or 
masses of matter occupying the magnetic field, as the air and the streams of other 
gas in it*, or mercury and the tube of air in it (2407.), oi’ water and the piece of bis- 
muth in it (2301.); and though exhibited only in the action of masses, the latter must 
no doubt owe their differences to the qualities of the particles composing them. Yet 
it is to be observed, that no attempt to separate the perfectly mixed particles of very 
different substances has ever succeeded, though made with most powerful magnets. 
Oxygen and nitrogen differ exceedingly, yet no appearance of the least degree of sepa- 
ration occurred in very powerful magnetic fields-l-. In other experiments I have en- 
closed a dilute solution of sulphate of iron in a tube, and placed the lower end of the 
tube between the poles of a powerful horseshoe magnet for days together, in a place 
of perfectly uniform temperature, and yet without the least appearance of any con- 
centration of the solution in that end which might indicate a tendency in the par- 
ticles to separate. 
2758 . The diamagnetic phenomena of the gases, when considered as the differential 
result of the action of volumes of these bodies, may be produced and examined in a 
very useful manner by the employment of soap-bubbles, as follows : — A glass tube was 
fitted with a cap, stopcock and bladder, so that any given gas contained in the blad- 
* Philosophical Magazine, 1847, vol. xxxi. p. 409. f Ibid, p, 416. 
MDCCCLI. D 
