PHYSICS. 261 



explained by again supposing the size of the molecules to be diminished 

 almost infinitely, so that instead of entering at the rate of 100 millions 

 every second they troop in at the rate of something like 300 trillions a 

 second. I have done the sum, but figures, when they mount so high, 

 cease to have any meaning, and such calculations are as futile as trying 

 to count the drops in the ocean." — {Proc. Roy. Soc, December 5, 1878 ; 

 Fliil. Mag., January, 1879; Am. J. ScL, III, xvii, 218, March, 1879; xviii, 

 241, October, 1879; Nature, xx, 228, 250, 419, 43G, 1879.) 



In a subsequent communication to the Eoyal Society, Crookes saj's that 

 he has examined some of these phenomena more closely, and has con- 

 firmed the molecular-stream hypothesis. If, however, the residual mole- 

 cules, being negatively charged by contact with the negative pole, and so 

 repelled, were solely acted on by the initial impulse from the negative 

 l^ole, they would take a direction accurately normal to the surface re- 

 pelling them, and would start with their full velocity. But the mole- 

 cules, being all negatively electrified, exert mutual repulsion, and there- 

 fore diverge laterally. The negative pole, likewise, not only gives an 

 initial impulse to the molecules, but it also continues to act on them 

 by repidsion, the result being that they move with an accelerating ve- 

 locity the further they get from the pole. AVithin the dark space at a 

 moderate exhaustion, the velocity does not aecumulate to a sufficient 

 extent to i^roduce phosphorescence; but at higher exhaustions the 

 mean free path is long enough to allow the molecules to get up speed 

 sufficient to cause phosphorescence. The luminous boundary to the 

 dark space around the negative pole is probably due to the impact of 

 molecule against molecule, producing ])hospliores('ence of the gas in the 

 same way as the impact of molecules against German glass produces 

 j)hosphorescence of the glass. The effect of deadening produced on 

 glass by long-continued phosphorescence, which was illustrated by 

 stenciling, by means of a discharge from the negative pole, the image 

 of a maltese cross on the end of a large pear-shaped bulb, has been 

 further developed by opening the bulb and heating the wide end in the 

 blowpipe flame until soft and melted out of shape. It was then blown 

 out again into its original shape and re-exhausted ; on connecting it 

 with the coil, the original ghost of the cross was seen to be still there. 

 At Maxwell's suggestion a third, idle, electrode was introduced into a 

 tube. On connecting it with an electroscope, it appeared that when in 

 a direct line between the positive and negative poles, and receiving the 

 full impact of the molecules driven from the negative pole, it had a 

 strong positive charge. The eflect of magnetism on the discharge has 

 been further examined, and this was found to be so sensitive that when 

 the tube was placed parallel to the earth's equator, the earth's magnet- 

 ism was sufficient to cause the spot of light to move five millimeters 

 away from the position it occupied when the tube was parallel to the 

 dipping needle. Complete .and orderlj' rotation of the molecular rays 

 was obtained by placing an electro-magnet beneath the tube in a line 



