Dr. Tyndall on Diamagnetism and Magnect^ystallic Action, 1 73 



the point which I have ah*eady reached, when I learned that a 

 memoir on diamagnetism by M. Edmond Becqnerel had ap- 

 peared in the May number of the Annates de Chimie et de Phy- 

 sique. In this memoir the views of PlUcker are also contro- 

 verted, and a number of experiments are adduced to prove the 

 identity of the laws which regulate magnetic attraction and dia- 

 niagnetic repulsion. The argument employed by M. Becquerel 

 is the same in principle as that furnished by the foregoing ex- 

 periments. He proves that the repulsion of bars of bismuth, 

 sulphur and wax, increases as the square of the exciting current, 

 and that the attraction of a little bar of iron follows the same 

 law. We have both been guided in our inquiries by the same 

 fundamental thought, though our modes of carrying out the 

 thought are different. 



I have observed many phsenomena, which, without due con- 

 sideration, would lead us directly to Plucker's conclusions ; a few 

 of these I will here describe. The bismuth balls were placed 

 upon the beam, and one core was excited ; on the top of the ball 

 opposite, a particle of iron, not the twentieth part of a common 

 pin-head in size, was fixed. A current of 10 circulated in the 

 helix, and the beam came to rest at the distance of 4° from the 

 zero of the under graduation. I then permitted the current to 

 increase gradually. The magnetism of the iron particle and the 

 diamagnetism of the bismuth rose of course along with it, but 

 the latter triumphed ; the beam was repelled, and finally came 

 to rest against a stop which was placed 9° distant. 



The particle of iron was removed, and a small crystal of car- 

 bonate of iron put in its place; a current of 15° circulated in 

 the helix, and the beam came to rest at about 3° distant from 

 zero. Tlie current was raised gradually, but before it had reached 

 30° diamagnetism conquered, and the beam receded to the stop 

 as before. 



Thinking that this apparent triumph of diamagnetism might 

 be due to the fact, that the crystal of carbonate of iron had be- 

 come saturated with magnetism, and that it no longer followed 

 the law of increase true for a larger piece of the substance, I 

 tested the crystal with currents up to 49° ; the attractions were 

 exactly proportional to the square of the exciting currents. 



Thinking also that a certain reciprocal action between the bis- 

 muth and the crystal, when both were placed together in the mag- 

 netic field, might so modify the latter as to produce the observed 

 result, I removed the crystal, and })laced a cube of the zinc of 

 commerce upon the opposite end of the beam. The zinc was 

 slightly magnetic. Bismuth and zinc were thus separated by an 

 interval of 6 inches; both cores were excited by a current of 

 10°, and the beam, after some oscillations, came to rest at 4° 



