408 THE PRINCIPLES OF SCIENCE. 



glass. But any such envelope is sure to be more or less 

 affected by the magnet, so that it becomes difficult to 

 distinguish between three forces which enter the problem ; 

 namely, the magnetism of the gas in question, that of the 

 envelope, and that of the surrounding atmospheric air. 

 Faraday avoided all difficulties by employing two exactly 

 equal and similar glass tubes connected together, and so 

 suspended from the arm of a torsion balance that the 

 tubes were in similar parts of the magnetic field. One 

 tube being filled with nitrogen and the other with oxygen, 

 it was found that the oxygen seemed to be attracted and 

 the nitrogen repelled. The suspending thread of the 

 balance was then turned until the force of torsion restored 

 the tubes to their original places, where the magnetism of 

 the tubes as well as that of the surrounding air, being 

 exactly the same and in the opposite direction upon the 

 two tubes, could not produce any interference. The force 

 thus required to restore the tubes was measured by the 

 amount of torsion of the thread, and it indicated correctly 

 the comparative attractive powers of oxygen and nitrogen. 

 The oxygen was then withdrawn from one of the tubes, 

 and a second experiment made, so as to compare a vacuum 

 with nitrogen. No force was now required to maintain 

 the tubes in their places, so that nitrogen was found to 

 be, approximately speaking, indifferent to the magnet, 

 that is, neither magnetic nor diamagnetic, while oxygen 

 was proved to be positively magnetic f . It required the 

 highest experimental skill on the part of Faraday and 

 Tyndall, to distinguish between what is apparent and real 

 in magnetic attraction and repulsion. 



Experience alone can absolutely decide when a com- 

 pensating arrangement is conducive to accuracy. As a 

 general rule mechanical compensation is the last resource, 



f Tyndall's 'Faraday/ pp. 114-15. 



