138 
MR. W. CROOKES OK THE ILLUMINATION OF LINES OF 
(positive); the dark space, as already mentioned, was 1*75 millims. from the platinum 
spiral pole; the wires a b were now connected with 3 Grove’s cells through a contact 
key, the induction spark passing at the same time. On pressing the key the spiral 
a b became ignited to bright redness, and the dark space immediately expanded from 
1'75 to 2'5 millims. in thickness. On cutting off the igniting current the dark 
spheroid slowly contracted to its former dimensions. 
494. At a pressure of 1*5 millims. the dark spheroid surrounded the platinum spiral 
to a distance of 4 - 5 millims. Igniting the spiral with the battery caused the dark 
space to expand considerably, but the amount of expansion could not be accurately 
seen, owing to its outline becoming very in distinct after it had expanded to about 
7 millims. semi-diameter. 
495. The poles c and cl were connected with the induction coil, the spiral a b being- 
still attached to the 3 Grove’s cells. The whole tube was filled with light, and I then 
made battery contact with the spiral, igniting it to redness. No space could be seen 
round the hot wire, and no alteration was observed in the illumination of the tube. 
496. Hydrogen gas was now passed into the tube, and after several times exhausting 
and refilling with hydrogen the following experiments were tried : the spiral a b beiug 
always the negative pole of the induction coil, and also being connected with the 3-cell 
Grove’s battery. 
At a pressure of 5 millims. the dark spheroid extended 1'75 millims. from the pole. 
On making the pole red hot the dark space expanded to 2’5 millims. in thickness. 
At a pressure of 3'75 millims. the dark spheroid was 2-25 from the pole, expanding 
to 375 millims. when the pole was ignited. 
At a pressure of 2 75 millims. the expansion of the dark space on heating the pole 
was from 4 millims. to 6 - 5 millims. 
The phenomena in hydrogen are therefore similar to those in air, but the dark space 
round the negative pole is larger for the same degree of exhaustion. 
497. Carbonic acid was then passed into the tube, and after several fillings and 
exhaustions the following experiments were tried :— 
At a pressure considerably less than that at which the dark space would appear in 
am or hydrogen, there was no appearance of it in carbonic acid. 
At a pressure of 4 millims. the dark space was ’75 millim. thick. At a pressure of 
’62 millim. its thickness was 2’5 millims., and at a pressure of "27 millim. its thickness 
was 4 millims. Igniting the negative pole caused expansion of the spheroid, but not 
to so great an extent as if the gas had been air or hydrogen. 
ILLUMINATION OF LINES OF MOLECULAR PRESSURE. 
498. These experiments appear to prove three things :— 
a. Setting up an intense excitement in a disk of metal by electrical means produces 
a molecular disturbance which affects the surface of the disk and the surrounding gas. 
