Magnetic Medium in Space. 207 



seemed to illuminate the matter. Such illuminations vanish in 

 nine cases out of ten before the test of subsequent criticism ; 

 but the thought referred to, having thus far withstood the 

 criticism brought to bear upon it, I am emboldened to submit 

 it to you for consideration. 



I shall best explain myself by assuming that a medium of the 

 nature described exists in space, and pursuing this assumption 

 to its necessary consequences. 



Let a cube, formed from the impalpable dust of carbonate of 

 iron*, which has been compressed forcibly in one direction, be 

 placed upon the end of a torsion beam, and first let the line in 

 which the pressure has been exerted be in the direction of the 

 beam. Let a magnet, with its axis at right angles to the beam, 

 and hence also at right angles to the line of pressure, be brought 

 to bear upon the cube. The cube will be attracted, and the 

 amount of this attraction, at any assigned distance, may be 

 accurately measured by the torsion of the wire from which the 

 beam depends. Let this attraction, expressed in degrees of 

 torsion, be called a. Let the cube now be turned round 90°, 

 so that the line of pressure shall coincide with the direction of 

 the axis of the magnet, and let the attraction d in this new 

 position be determined as in the former instance. On compa- 

 rison it will be found that a' exceeds a ; or, in other words, that 

 the attraction of the cube is strongest when the force acts 

 parallel to the line of compression^. 



Instead of carbonate of iron we might choose other substances 

 of a much feebler magnetic capacity, with precisely the same 

 result. Let us now conceive the magnetic capacity of the com- 

 pressed cube to diminish gradually, and thus to approach the 

 capacity of the medium in which, according to our assumption) 

 the carbonate of iron is supposed to be immersed. If it were a 

 perfectly homogeneous cube, and attracted with the same force 

 in all directions, we should at length arrive at a point, when 

 the magnetic weight of the cube, if I may use the term, would 

 be equal to that of the medium, and we should then have a sub- 

 stance which, as regards magnetism, would be in a condition 

 similar to that of a body withdrawn from the action of gravity 

 in Plateau's experiments. Such a body would be neither at- 

 tracted nor repelled by the magnet. In the compressed cube, 

 however, the magnetic weight varies with the direction of the 

 force ; supposing the magnetic weight, when the force acts along 

 the line of compression, to be equal to that of the medium, 

 then if the force acted across the line of compression^ the mag- 



* For an ample supply of this most useful mineral, I am indebted to 

 the kindness or J. Kenyon BlackweN, Esq., F.CJ.S. 

 t Phil. Mag. Sept. 186] ; Pogg. Am. 1851. 



