306 . Royal Institution. 



axial, the diamagnetic equatorial; and by attention to this the 

 magnetic body is made to behave like a homogeneous diamagnetic 

 body, and the diamagnetic body like a homogeneous magnetic 

 body. Diamagnetic bars and discs were also examined, and a 

 deportment precisely complementary to that of the magnetic bars 

 and discs was exhibited. A magnetic disc set its thickness from 

 pole to pole and consequently its horizontal diameter equatorial ; a 

 diamagnetic disc set its thickness equatorial and its horizontal 

 diameter from pole to pole. Two bodies of the same exterior form 

 and of the same colour, were suspended simultaneously in the fields 

 of two electro-magnets, and both the latter were excited by the 

 same current; the eye could detect no difference of deportment. 

 Both bodies possessed the shape of calcareous spar, and both set 

 the crystallographic axis equatorial. One body however was com- 

 posed of wax, while the other was a true crystal. In the same 

 way a crystal of carbonate of iron exhibited a deportment precisely 

 the same as that of a model formed of magnetic dust. The explana- 

 tion of these phaenomena may be given in a few words. In the con- 

 struction of the models, the magnetic or diamagnetic dust was formed 

 into a kind of dough and pressed between two glass plates ; the 

 same process was applied to the wax; and it is a universal law, 

 that in diamagnetic bodies the line along which the density of the 

 mass has been increased by compression, sets equatorial, and in mag- 

 netic bodies axial. A reference to this principle will instantly render 

 plain all the experiments we have described. In those cases where 

 the same artificial bar set at one time axial and at another time 

 equatorial, the deportment depended on the circumstance whether 

 the line of compression was vertical or horizontal. When vertical 

 its directive power was annulled, and the action was determined by 

 the exterior form of the body ; but when horizontal, its directive 

 action came into play and determined the position of the mass. 

 The magnetic bar, for example, suspended with its line of pressure 

 vertical, set axial, but with its line of pressure horizontal, it set 

 equatorial ; for the pressure was exerted at right angles to its length. 

 This action is so general that it is diflScult to find a body so perfectly 

 homogeneous as not to exhibit it in some degree. Ipecacuanha 

 lozenges and Carlisle biscuits were suspended in the magnetic field, 

 and exhibited a most striking directive action. The materials in 

 both cases were diamagnetic ; but owing to the pressure exerted in 

 their formation their largest horizontal dimensions set from pole 

 to pole, the line of compression being equatorial. 



Let us endeavour to arrive at the precise logical import of these 

 experiments. Let us suppose that before ever a crystal had been 

 suspended in the magnetic field, we were acquainted with the fact 

 that a slight change of density in any direction is accompanied by 

 such modifications of the magnetic force as those above described : 

 — that we know that flour, bran, soap, shale, magnetic dust, diamag- 

 netic dust, &c., all exhibited this directive action, — that it is in fact 

 a universal law of matter ; and then let us imagine some fortunate 

 experimenter hanging a crystal between the poles and observing a 



