238 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
short junction-tube. As the gas within the double vessel was rarefied, the discharge which, at higher 
pressures, had passed exclusively through the short tube began to divide itself and passed through the 
long circuitous route in proportion increasing with the decrease of pressure until a degree of rarefaction 
was attained, at which it passed exclusively through the long junction tube. 
It has been noticed by ourselves and others that the size of the negative terminal has considerable 
influence upon the general character of the discharge, and that with a very small negative it is difficult 
to produce striae at all. 
With a view of testing this further, Spottiswoode constructed a tube in which the length of one 
terminal could be altered, while the total distance between the extremities of the two terminals remained 
unchanged. With this contrivance it was found that when the negative terminal was lengthened the 
dark space was shortened by a prolongation of the positive column. Thus the tube being about 12 inches 
in length, an increase in the length of the negative terminal from 3 to 6 inches caused a diminution in 
that of the dark space from 3 to U5 inches. 
The importance of size of the negative terminal was also illustrated by Spottiswoode in a striking 
manner with a tube 18 inches in length and 1'75 in diameter, containing from 30 to 50 cubic centimetres 
of mercury. When the tube was laid on its side so as to leave the negative terminal, which was small, 
free from mercury, the tube was filled with an unbroken glow of light ; but as soon as the mercury 
touched the terminal, a fine column of striae was developed. When the mercury spread along any con- 
siderable part of the tube, the column still stretched out towards the negative terminal; but it was 
repelled to the side of the tube opposite to that occupied by the mercury, and the individual striae 
assumed an oblique position, as if under the influence of two forces, one along the other at right angles 
to the axis of the tube. 
