174 Dr. Tyndall on Diamagnetism and MagnecrystalUc Action* 



distant from zero. Tlie current was now gradually raised, but 

 when it reached 35° the beam receded and was held firmly 

 against the stop. When the circuit was broken it left the stop, 

 and, after some oscillations, came to rest at zero. 



These experiments seem fully to bear out the notion of 

 Pliicker. In each case we waited till both forces were in equi- 

 librium ; and it might be thought that if the forces followed the 

 same law, the beam ought not to move. Let us, however, clear 

 the experiment of all mystery ; when the beam was in equili- 

 brium with a current of 10°, let us ask what forces were opposed 

 to the repulsion of the bismuth ? There was first of all the at- 

 traction of the zinc ; but besides this, there was a torsion of 4°, 

 for the position of equilibrium for the beam when the magnet 

 was unexcited was at zero. Let us suppose the magnetism of 

 the zinc at the distance of 4°, and with the current 10°, to be 

 equal to 8° of torsion ; this, added to the 4° already present, will 

 give the force opposed to the bismuth; the repulsion of the 

 latter is therefore equal to 12. Let us now conceive the current 

 raised from 10° to 35°, that is quadrupled. Supposing the 

 magnetism of the zinc to be increased in proportion to the 

 strength of the current, its attraction will now be 32°; this, 

 added to 4° of torsion, which remains constant, makes 36, which 

 is therefore the force brought to bear against the bismuth by a 

 current of 35° under the present circumstances. But the re- 

 pulsion of the bismuth is also quadrupled ; it is now 48. This, 

 opposed to a force of 36, necessarily conquers, and the beam is 

 repelled. 



We thus see, that although the magnetic force on one side, 

 and the diamagnetic on the other side, follow precisely the same 

 law, the introduction of the small constant 4° entirely destroys 

 the balance of action, so that to all appearance diamagnetism 

 increases in a much greater ratio than magnetism. Such a con- 

 stant has probably crept into the experiments of Pliicker; an 

 inadvertency not to be wondered at, when we remember that the 

 force Vas new ^X the time, and our knowledge of the precau- 

 tions necessary to its accurate investigation very imperfect. 



§ 2. On MagnecrystalUc Action. 



Pliicker has discovered, that when a crystal of pure carbonate 

 of lime is suspended in the magnetic field with its optic axis 

 horizontal, the said axis always sets itself equatorial. He attri- 

 buted this action of the spar to a repulsion of the optic axis by 

 the magnet, wholly independent of the magnetism or diamag- 

 netism of the mass of the crystal. It was the product of a new 

 force, which Mr. Faraday has named the optic axis force. 



In the memoirs published by Knoblauch and me, this view is 



