NATURAL PHILOSOPHY. 127 



that complementary effects might be here exhibited, and a new an- 

 tithesis thus established between magnetism and diamagnetism. 



No explanation of this action has, to my knowledge, been offered ; 

 and I would now beg to propose one, which seems to be sufficient. I 

 place this large flat magnet upon the table ; over it I put a paper 

 screen, and on the screen I shake iron filings. You know the beau- 

 tiful lines in which those filings arrange themselves, lines which have 

 become classical from the use made of them in this Institution ; for 

 they have been guiding-threads for Faraday's intelligence while ex- 

 ploring the most profound and intricate phenomena of magnetism. 

 These lines indicate the direction in which a small magnetic needle 

 sets itself when placed on any of them. The needle will always be a 

 tangent to the magnetic curve. A little rod of iron, freely suspended, 

 behaves exactly like the needle, and sets its longest dimension in the 

 direction of the magnetic curve. In fact, the particles of iron filings 

 themselves are virtually so many little rods of iron, which, when they 

 are released from the friction of the screen by tapping, set their long- 

 est dimensions alonuj the lines of force. Now, in this bar ma;net the 



i ^^ 



lines ot force run along the magnet itself, and were its particles capa- 

 ble of free motion they also would set their longest dimensions 

 parallel to thelines of force, that is to say, parallel tj the length of 

 the magnet. This, then, is the explanation which I would offer of the 

 lengthening of the bar. The bar is composed of irregular crystalline 

 granules ; and, when magnetized, these granules tend to set their long- 

 est dimensions parallel to the axis of the bar. They succeed, partially, 

 and produce a microscopic lengthening of the bar, which, suitably mag- 

 nified, has been rendered visible to you. 



But can we not bring a body with movable particles within an elec- 

 tro-magnetic coil ? We can ; and I will now, in conclusion, show you 

 an experiment devised by Mr. Grove, which bears directly upon this 

 question, but the sight of which, I believe, has hitherto been confined 

 to Mr. Grove himself. At all events, I am not aware of its ever 

 having been made before a large audience. I have here a cylinder 

 with glass ends, and it contains a muddy liquid. This muddiness is 

 produced by the magnetic oxyde of iron which is suspended mechanic- 

 ally in water. Round the glass cylinder I have coiled five or six 

 layers of covered copper wire ; and here is a battery from which a 

 current can be sent through the coil. First of all, I place the glass 

 cylinder in the path of the beam from our electric lamp, and, by 

 means of a lens, cast a magnified image of the end of the cylinder on 

 the screen. That image at present possesses but feeble illumination. 

 The light is almost extinguished by the suspended particles of mag- 

 netic oxyde. But, if what I have stated regarding the lines of force 

 through the bar of magnetized iron be correct, the particles of the 

 oxyde will suddenly set their longest dimensions parallel to the axis of 

 the cylinder, and also in part set themselves end to end when the 

 current is sent round them. More light will be thus enabled to pass ; 

 and now you observe the effect. The moment I establish the circuit 

 the disc upon the screen becomes luminous. I interrupt the current, 

 and gloom supervenes ; I re-establish it, and we have a luminous disc 

 once more. 



