ON CRYSTALLIZING BODIES. 21 



than at the south pole. Here we have evidence of an 

 actual turning round of the crystal, in obedience to the 

 directive force of the magnet ; and we have the curious 

 circumstance of a difference in some way, which is not 

 clearly explained, between the two opposite poles. If, 

 instead of an iron or a ferro-magnetic salt, we employ 

 one which belongs to the other, or diamagnetic, class, 

 we have a curious difference in the result. If into a 

 glass dish, fixed on the poles of a strong electro-magnet, 

 we pour a quantity of a solution of nitrate of silver, and 

 place in the fluid, over the poles of the magnet, two 

 globules of mercury (an arrangement by which that 

 arborescent crystallization, called the Arbor Diana, is 

 produced,) we have the long needle-shaped crystals of 

 silver, arranging themselves in curves which would cut 

 the ordinary magnetic lines at right angles.* 



In the first example given we have an eiMbition of 

 magnetic force, while in the last we have a striking 

 display of the diamagnetic power. 



The large majority of natural formations appear to 

 group themselves under the class of diamagnetics. 

 These bodies are thought to possess poles of mutual 

 repulsion among themselves, and which are equally 

 repelled by the magnetic points of convergence. Con- 

 fining our ideas to single particles in one condition or 

 the other, we shall, to a certain extent, comprehend the 

 manifold results which must arise from the exercise of 

 these two modes of force. At present, our knowledge 

 of the laws of magnetism is too limited to allow of our 

 making any general deductions relative to the dispo- 

 sition of the molecules of matter ; and the amount of 

 observation which has been given to the great natural 

 arrangements, is too confined to enable us to infer more 



* For illustration of this I must refer to my own Memoir, Re- 

 searches on the Influence of Magnetism and Voltaic Electricity on 

 Crystallization, and other conditions of matter, in the Memoirs of 

 the Geological Survey of Great Britain, &c., vol. i. 



