UPON GASES AND LIQUIDS. 555 



their own degree of rarefaction, they all become alike in their 

 magnetic relation, and apparently equivalent to a perfect vacuum. 

 Bodies which are very marked as diamagnetic substances, imme- 

 diately lose all traces of this character when they become va- 

 porous." 



4. Faraday performed his experiment by taking an open glass 

 tube, which was as indifferent as possible to the magnet, and 

 after the air had been withdrawn from it, hermetically sealing it, 

 allowing it to oscillate, both before and afterwards, between the 

 poles surrounded by air. He found no difference, even when 

 the tubes were allowed to oscillate in various gases, nor when 

 filled with them. Nor did he find any difference on allowing 

 the tube, either containing gas or not so, to oscillate in water, 

 alcohol or oil of turpentine ; nor, lastly, when a solid diamag- 

 netic body, as heavy glass or a bar of bismuth, was made to 

 oscillate in different kinds of, or vai'iously compressed gases. 

 On first perusing these experiments, it was evident to me, on 

 mechanical grounds, that for any effect to be apparent in any of 

 them, the gases must be magnetic or diamagnetic to a perfect]}' 

 enormous extent; for the magnetic or diamagnetic force of a 

 substance must evidently diminish with the rarefaction of the 

 substance. We will for a moment consider, hypothetically 

 only, this diminution and rarefaction as in propoi'tion to each 

 other, as is the case with attractive forces ; and also, which is 

 more to the present point, in the case of the rotation of the plane 

 of polarization in liquids, in which, in a solution of sugar e.g. 

 the amount of rotation is in proportion to the quantity of sugar 

 in solution. A magnetically indifferent tube, when completely 

 filled with water and suspended so as to oscillate horizontally in 

 water, does not assume a definite position between the poles of 

 a magnet, because the same force which urges the w-ater con- 

 tained in the tube to assume an equatorial position, is counter- 

 balanced by the diamagnetic excitation of the surrounding water. 

 If tlie tube, when oscillating horizontally, contains only half the 

 quantity of water which it originally did, and which then for the 

 same length has only half the section, the second force will pre- 

 ponderate. The tube is driven into the axial direction, and is 

 retained in this direction with a force equal to half the diamag- 

 netic force exerted upon the entire mass of the water originally 

 contained in the tube, and which we shall consider as unity. If 

 only the y^'jjjjth of the original water remained in the tube dif- 



2 p2 



