1886.] electric discharge in a uniform electric field. 393 



falls the disc near the negative electrode broadens out, and the 

 handle of the Indian club lengthens, until the appearance is that 

 represented in fig. 4, which represents the discharge at about 

 18 mm. of mercury, the difference in colour between the discharge 

 at the positive and negative electrodes being now very marked. 

 As the pressure diminishes the disc at the negative electrode 

 increases in size, until at about 4 mm. this disc appears to con- 

 stitute the whole of the discharge ; it is clearly separated from 

 the negative electrode. I have not been able to detect with any 

 certainty any discharge at the positive electrode, or any glow 

 throughout the tube, and if they exist at this stage they are certainly 

 exceedingly faint. There is a much greater contrast between 

 the bright disc near the negative electrode and the rest of the 

 discharge than between the glow and dark space in a vacuum 

 tube of the ordinary kind. As the pressure diminishes still further, 

 the disc gradually moves further away from the negative electrode, 

 and a decided glow spreads through the vessel ; the colour of the 

 discharge keeps changing, and when the pressure sinks below a 

 millimetre it is a pale Cambridge blue. Bright specks also appear 

 over the negative electrode. If an air-break be put in the circuit 

 a curious phenomenon is observed. A glow is distinctly visible 

 between the top of the vessel containing the electrodes and the 

 upper electrode, though these are in metallic connection, and if 

 they acquired the same potential simultaneously, could have no 

 electric field between them. This was only observed when the 

 upper electrode was negative, not when it was positive. The 

 lowest pressure reached with this apparatus was about i of a 

 millimetre. At this pressure the disc near the negative, though 

 still observable, was not much brighter than the surrounding 

 glow. 



These experiments were repeated, using coal-gas instead of air: 

 very similar results were obtained, except that at high pressures 

 the discharge jumped about more than it did in air. Coal-gas 

 was used because stratifications are usually produced in it with 

 great facility. I never observed any tendency however in the 

 discharge to become striated where the field was uniform, though 

 some small discharges which started from the edges of the positive 

 plates were beautifully striated ; on one occasion too a spot of 

 dust had got on the positive electrode in the middle of the 

 uniform discharge, a secondary discharge started from this point, 

 which was very plainly striated, though the main discharge 

 shewed no trace of stratification. 



Some experiments were tried with gases which are electrically 

 very weak, such as the vapours of turpentine and alcohol ; with 

 these gases I never was able to limit the discharge to a disc near 

 the negative electrode, as was the case when air or coal-gas was 



