SECT. XXXIV. INFLUENCE OF COMPRESSION. 351 



He concludes that the arrangement of the molecules must be inti- 

 mately connected with paramagnetism or diamagnetism itself, 

 since the effect of that arrangement is equally sensible in bismuth 

 and iron, although the diamagnetism of the former is 25,000 

 times weaker than the paramagnetism of the latter. 



The diamagnetism of conducting substances and metuls, such as 

 gold, silver, and copper, is augmented by division. Compression 

 has also a great effect on magnetic action. For example, a bar 

 of soft iron sets with its longest dimensions from pole to pole of 

 a magnet, but a bar of compressed carbonate of iron-dust, whose 

 shortest dimensions coincide with the line of pressure, sets 

 equatorially. A bar of bismuth whose plane of principal cleavage 

 is parallel to its length sets equatorially, but a bar of compressed 

 bismuth dust, whose shortest dimensions coincide with the line 

 of pressure, or a bar of bismuth whose principal planes of cleavage 

 are transverse to its length, sets with its length axially. The 

 antithesis is perfect whether the bars are under the influence of 

 a magnet or electro-magnet. For since the diamagnetic force is 

 inversely as the conducting power of a body for electricity, and 

 that the latter is a maximum in the direction of the planes of 

 principal cleavage, therefore when these planes are parallel to 

 the axis of the bismuth bar it sets equatorially ; but as the con- 

 ducting power is augmented when the bismuth dust is com- 

 pressed in the direction of the force, the diamagnetic power is 

 diminished, and the bar sets axially. Again, since the para- 

 magnetic force augments with the conducting power, the action 

 of the magnet on the iron is antithetic to that on the bismuth. 



The action of an electro-magnet on copper is strongly con- 

 trasted with that which it exerts on iron or bismuth. For when 

 a copper bar suspended by a thread revolves before its pole, it is 

 brought to a dead halt as soon as the electric current acts upon it, 

 and maintains its position with considerable tenacity, for it does 

 not return when pushed out of it, but keeps its new place with 

 stiffness ; however, as soon as the electric current ceases, there is 

 a strong revulsion, the bar revolving the contrary way. Even 

 when swinging with considerable force it may be caught and 

 retained in any position at pleasure, but there is no revulsion 

 when it is arrested either in the axial or equatorial position ; at 

 any angle between these two, but especially midway, the elec- 

 tricity will make it move towards the axis, but it is arrested 



