352 THE PRINCIPLES OF SCIENCE. [CHAP. 



one needle being within and another without the coil of 

 wire. As regards the earth's magnetism, the needles are 

 now astatic or indifferent, the tendency of one needle 

 towards the pole being balanced by that of the other. 



An elegant instance of the elimination of a disturbing 

 force by compensation is found in Faraday's researches 

 upon the magnetism of gases. To observe the magnetic 

 attraction or repulsion of a gas seems impossible unless we 

 enclose the gas in an envelope, probably best made of 

 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 into the 

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

 that of the envelope, and that of the surrounding atmo- 

 spheric air. Faraday avoided all difficulties by employing 

 two 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 

 the same and in the opposite directions upon the two tubes, 

 could not produce any interference. The force required 

 to restore the tubes was measured by the amount of 

 torsion of the thread, and it indicated correctly the dif- 

 ference between the 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 The oxygen was then withdrawn from one of 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. 1 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 finally decide when a corn- 



1 Tyudail's Faraday, ]>]>. 114, 115. 



