Dr. Tyndall on Diamagnetism and Magnecrystallic Action. 183 



the view of establishing the general fact, that the repulsion in 

 the line of compression is greatest. That this has fallen out in 

 the manner described is a pure accident. I have no doubt what- 

 ever that models might be made in which this difference of action 

 would be double of that exhibited by the crystal. 



The case, however, is not yet free from suspicion ; the gum- 

 water with which it is necessary to bind the powder may possibly 

 exert some secret influence. When isinglass or jelly is com- 

 pressed, we know that it exhibits optical phsenomena similar to 

 those exhibited by crystals ; and the squeezing of the metallic 

 dough may induce a kind of crystalline structure on the part of 

 the gum sufficient to produce the phaenomena observed. 



An experiment to which I was conducted by the following 

 accident will set this doubt, and I believe all other doubts re- 

 garding the influence of compression, completely at rest. Having 

 repeated occasion to refer to the deportment of crystals in the 

 magnetic field, so as to be able to compare this deportment with 

 the attraction or repulsion of the entire mass upon the torsion 

 balance, through the kindness of Professor Magnus, the great 

 electro-magnet of the University of Berlin* was placed in the 

 room where I experimented. One morning a cube of bismuth 

 was suspended between the moveable poles, and not knowing 

 the peculiarities of the instrument, I chanced to bring the poles 

 too near each other. On closing the circuit, the principal clea- 

 vage of the ci-ystal receded to the equator. Scarcely however was 

 this attained, when the poles were observed moving towards each 

 other, and before I had time to break the circuit, they had 

 rushed together and caught the crystal between them. The 

 pressure exerted squeezed the cube to about three-fourths of its 

 former thickness, and it immediately occurred to me that the 

 theory of proximity, if it were true, ought to tell here. The 

 pressure brought the particles of the crystal in the line of com- 

 pression more closely together, and hence a modification, if not 

 an entire reversion of the previous action, was to be expected. 

 Having liberated the crystal, I boiled it in hydrochloric acid, so 

 as to remove any impurity it might have contracted by contact 

 with the iron. It was again suspended between the poles, and 

 completely verified the foregoing anticipation. The line of com- 

 pression, that is, the magnecrystallic axis of the crystal, which 

 formerly set from pole to pole, now set strongly equatorial. I 

 then brought the poles intentionally near each other, and al- 

 lowed them to close once more upon the already compressed 

 cube ; its original deportment was thereby completely restored. 



* A notion of the power of this instrument may be derived from the 

 fact, that the copper hehces alone which surrounded the pillars of soft iron 



weighed 243 pounds. „ i .,_ . . . 



02 



