132 Prof. Tyudall on the relation of Diamagnetic Polarity 



excited substances are less intense, the dijfei-ence of these forces, 

 owing to the speedier diminution of the force with the distance, 

 is greater than in the centre of the field. It is therefore an error 

 to infer, that, because the attraction of the mass is greater at 

 the edges and corners than in the centre of the field, the mag- 

 netizing force of the former must thei'efore be more intense than 

 that of the latter*. 



There is another interesting and delicate experiment of Mr. 

 Faraday's to which I am anxious to apply the principle of dia- 

 magnetic polarity : the experiment was made with a view of 

 proving that " the magnecrystallic force is a force acting at a 

 distance." " The crystal," writes Mr. Faraday, " is moved by 

 the magnet at a distance, and the crystal can also move the mag- 

 net at a distance. To produce the latter result, I converted a 

 steel bodkin, 3 inches long, into a magnet, and then suspended 

 it vertically by a cocoon filament from a small horizontal rod, 

 which again was suspended by its centre and another length of 

 cocoon filament, from a fixed point of support. In this manner 

 the bodkin was free to move on its own axis, and could also 

 describe a circle about 1^ inch in diameter ; and the latter motion 

 was not hindered by any tendency of the needle to point under 

 the earth's influence, because it could take any position in the 

 circle and yet remain parallel to itself. 



" When a crystal of bismuih was fixed on a support with the 

 magnecrystallic axis in a horizontal direction, it could be placed 

 near the lower pole of the magnet in any position ; and being 

 then left for two or three hours, or until by repeated examination 

 the magnetic pole was found to be stationary, the place of the 

 latter could be examined, and the degree and direction in which 

 it was afi'ected by the bismuth ascertained The effect pro- 

 duced was small ; but the result was, that if the direction of the 

 magnecrystallic axis made an angle of 10°, 20", or 30° with the 

 line from the magnetic pole to the middle of the bismuth crystal, 

 then the pole followed it, tending to bring the two lines into 

 parallelism ; and this it did whichever end of the magnecrystallic 

 axis was towards the pole, or whichever side it was inclined to. 

 By moving the bismuth at successive times, the deviation of the 

 magnetic pole could be carried up to 60°. The crystal, there- 

 fore, is able to react upon the magnet at a distance. But though 

 it thus takes up the character of a force acting at a distance, still 

 it is due to that power of the particles which makes them cohere 

 in regular order, and gives the mass its crystalline aggregation ; 

 which we call at other times the attraction of aggregation, and 

 so often speak of as acting at insensible distances." 



* Some important consequences resulting from this expeiinicut are in- 

 tended for a future communication. 



