PLUCKER ON THE THEORY OF DIAMAGNETISM. 341 



is positive or negative, attracts or repels the extraordinary ray ; 

 the same direction will, under the same circumstances, be at- 

 tracted or repelled by the poles of a magnet. 



4. The attraction or repulsion of a direction in the interior of 

 a body which exhibits itself as independent of the attraction or 

 repulsion of the mass is an isolated fact, a paradoxical fact which 

 at first sight appears irreconcileable with the universal laws of 

 mechanics. 



In order better to appreciate the nature and conditions of the 

 described pha^nomenon, let us fix our attention upon a prism of 

 tourmaline. This prism will, in the first place, be attracted by 

 the two poles of the magnet, but when the poles are withdrawn 

 to a distance from each other it is repelled ; it forsakes the axial 

 position and takes up a position perpendicular to it, like a body 

 whose ultimate particles recede from the magnet. We are here 

 naturally led to the assumption of two hostile forces, one of 

 which acts upon the mass of the tourmaline as on a non-crystal- 

 line magnetic body, and tries to set the prism in the axial posi- 

 tion, while the other, which depends on the crystallization, forces 

 the prism into the equatorial position*. We are obliged to as- 

 sume beyond this, that when the distance between the poles in- 

 creases, the former of these two forces decreases more quickly 

 than the latter. This was the way in which I regarded the sub- 

 ject when, in 1847, I stated the following empirical law as the 

 expression of the phaenomena. 



The force which produces the repulsion (or attraction) of the 

 optic axis of the crystal decreases more slowly with the distance 

 than the magnetic attraction or diamagnetic repulsion of the 

 mass of the crystal. 



5. On the same day I observed first the remarkable deportment 

 of a cylinder of coal, and also the deportment of the tourmaline, 

 between the pointed poles of the large electro-magnet. At first 

 I thought that the behaviour of the coal was determined by its 

 structure, and was thus induced to suspend a plate of tourma- 

 line, whose structure depends upon its crystallization, between the 

 pointed poles. After I had examined all the specimens at my 

 disposal I returned to the coal, and found the explanation in the 



* It is by no means asserted that the two forces are independent of each 

 other. 



