273 



As long as the vacuums are at a sufficient distance from the 

 action of the magnet, the luminous discharge is visible in both, and 

 the leaves of the electroscopes partially collapse ; but immediately the 

 discharge in B is placed in the position described in the previous 

 experiment, between the poles of the magnet, the discharges in both 

 vacua instantly disappear, and the leaves of the electroscopes di- 

 verge to their original maximum. 



The actual position of what is termed the magnetical field, around 

 and between the poles of a magnet, has been generally delineated by 

 means of iron filings placed between the poles on a sheet of paper. 

 Assuming the lines in which these particles arrange themselves to 

 represent the direction of the power of the magnet, or the magnetic 

 field, they also explain the actual position through which the vacuum- 

 tube should be placed to obtain the preceding result, and in this 

 manner to show by experiment that a voltaic discharge which has 

 sufficient intensity to pass through a space of upwards of 6 inches in 

 attenuated carbonic acid gas is not only interrupted, but absolutely 

 and entirely arrested by magnetic force. 



Postscript (received Jan. 19, 1860). In repeating the experiments 

 with Dr. Tyndall in my laboratory, the disruption of the luminous 

 discharge in vacuo from 400 cells of the nitric-acid battery was 

 obtained : some most beautiful and striking results were obtained 

 from the same battery on the 16th inst., on repeating the experiment 

 in the Theatre of the Royal Institution, with its large electro- 

 magnet, Dr. Faraday and Dr. Tyndall being present. 



The large receiver (146) was placed between the poles of the 

 electro -magnet, the lines of force going through it; electrodes 

 equatorial. The stratified discharge was extinguished. Subsequently, 

 through the sinking of the battery, or some other cause, the stra- 

 tifications disappeared, and the luminous glow which filled the entire 

 tube remained. On now exciting the magnet with a battery of ten 

 cells, effulgent strata were drawn out from the positive pole, and 

 passing along the upper or under surface of the receiver, according 



