520 Prof. De la Rive on the Action of Magnetism upon 



This globe was filled with air rarefied to 4 millims. ; the dis- 

 charge then took place in the form of a jet, which turned at the 

 rate of sixty revolutions per minute when the ring was positive, 

 and of twenty per minute when it was negative. Under a pres- 

 sure of 6 millims. the velocity of rotation was only forty turns 

 per minute in the former case, and twenty turns in the latter. 

 Lastly, with alcohol-vapour, the pressure being 5 millims., the 

 velocities of rotation were respectively twenty-two and eleven 

 turns per minute. 



These first experiments having put me on the track of re- 

 searches of this, kind, I repeated this investigation, employing 

 the jar X)f 20 centims. by 16 that I have described above. I 

 give, in the first place, the results obtained with dry atmospheric 

 air : — 



Pressure. 



Number of turns per minute. 









Ring positive. 



Ring negative. 



millims. 







16 



55 



36 



12 



83 



55 



9 



99 



63 



6 



.-, , .-. 



100 



3 



... 



128 



2 



... 



... 



At 9 millims., the ring being the positive electrode, the discharge 

 no longer formed a jet, but was spread out into a sector of 30° to 

 45° ; this sector obeyed the movement of rotation as the jet did 

 previously, but it increased in size in proportion as the pressure 

 diminished, and at 6 millims. it formed a complete circular 

 sheet : the rotation, which up to this point had gone on increas* 

 ing in rapidity, was then no longer perceptible. When the ring 

 forms the negative electrode, it becomes covered with a violet 

 sheath, the extent of which likewise increases as the pressure 

 diminishes, but it still occupies only half the circumference 

 of the ring under a pressure of 4 millims. At this pressure 

 it is seen to revolve very rapidly, but under a pressure of % 

 millims. it occupies the whole circumference of the ring, and 

 the rotatory motion is no longer sensible. At the top of the 

 magnetized rod of soft iron there is a rose-coloured halo whence 

 issues a very short point or jet in the shape of a comma, which 

 turns with the violet sheath, and is separated from it by a very- 

 considerable dark space. 



It ought to be mentioned that, under a pressure of 6 or 4 mil- 

 lims. and sometimes under a pressure of 3 millims., it generally 

 happens, when the ring is the positive electrode, that on first 



