556 PLUCKER ON THE ACTION OF THE MAGNET 



fused equally throughout its longitudinal direction, the tube 

 would be retained in the axial position with a force amounting 

 to x^^jyj and this force would be equal to unity itself if the tube 

 were entirely free from water. The difference between these two 

 forces, which amounts to only the yjJo^Jth of the magnitude of 

 that diamagnetic force originally acting ujjon the water con- 

 tained in the tube, can never be shown by the rotation of the 

 tube, even independently of the resistance in the surrounding 

 fluid. If the mass of water y^'o o ^'^^ converted into the form 

 of vapour within the tube, no alteration -would be produced, sup- 

 posing that the action of the magnet upon the molecules of its 

 vapour were the same as upon the molecules of water. The dia- 

 magnetism of the vapour of water could never therefore be shown 

 in this way. 



The same occurs in all the other experiments detailed at the 

 commencement of this paragraph. 



5. Faraday says, — " I have imagined an experiment with one 

 of Cagniard de la Tour's asther tubes, but expect to find great 

 difficulty in carrying it into execution, chiefly on account of the 

 strength, and therefore the mass of the tube necessary to resist 

 the expansion of the imprisoned heated aether." — (2435.) If" 

 Faraday's experimental skill should succeed in overcoming the 

 difficulties of this experiment, we should obtain a direct decisive 

 answer to the question, whether diamagnetic fluids lose their 

 diamagnetism on conversion into the state of vapour. Faraday 

 anticipates an affirmative answer ; however, on the ground of 

 the experiment subsequently described, I confidently expect a 

 negative answer. 



6. On my own part, to decide upon the diamagnetism of the 

 gases, I first endeavoured to diminish the mass of the reservoir 

 enclosing them ; and I thus, among other things, arrived at the 

 idea of using a soap-bubble as the envelope. In a preliminary 

 experiment I laid a lamina of mica upoij the two approximated 

 surfaces of the poles of the electro-magnet, and placed a soap- 

 bubble of a hemispherical form upon this ; but I did not find 

 the magnet exert any action upon the form of the soap-bubble ; 

 this was also equally the case when it was filled with air as when 

 it was filled with tobacco-smoke. I then completely gave it up 

 for enclosing the gases ; and the use of the coloured gases imme- 

 diately suggested itself to me as a means of deciding upon the 

 magnetic, diamagnetic or neutral state of the gases. 



