220 



SCIENCE. 



gaseous condition, to that extent have the molecules as- 

 sumed the condition of radiant matter." 



Let us now see to what extent Mr. Crookes has reached 

 these conditions in his Radiometer and radiant matter 

 tubes. 



In the Radiometer the blackened vanes become 

 heated by absorbing radiant energy (both heat and light), 

 and they project the contiguous gaseous molecules from 

 their surfaces by communicating to them their molecular 

 motion, much as a vibrating drum-head would project 

 into air grains of sand strewn on it. An increase of 

 pressure is thus produced between the heated lamp- 

 black and the gaseous residue; but this pressure is not 

 transmitted throughout the whole bulb because of the 

 relative infrequency of collisions among the molecules at 

 this degree of exhaustion. Hence the molecules are 

 projected forward against the cooler bulb in right lines, 

 and the blackened vanes retreat because the repulsion 

 between them and the gaseous residue is mutual. [A 

 Radiometer projected on the screen by an oxyhydrogen 

 lantern ; also, a diagram showing the paths of the pro- 

 jected molecules]. 



Mr. Crookes investigated the Radiometer with an in- 

 genuity and a variety of detail that left nothing to be de- 

 sired. Moreover, his visualization of the molecular mo- 

 tions taking place in this little instrument, prepared him 

 for a further research into the motion of the residual gas 

 in Geissler tubes. The dark space around the negative 

 pole, which broadens as the exhaustion proceeds, and the 

 brilliant stratification displayed by many Geissler tubes, 

 were hints in the line of his. special studies. I cannot 

 forbear, at this point, to call your attention to the vibra- 

 tion of the air in a sounding pipe, made visible by very 

 light, precipitated silica powder. The resemblance be- 

 tween the beautiful segmentation of the pipe by the thin, 

 vertical planes of silica powder, and the stratification of 

 a Geissler tube, is most marked and suggestive. [A 

 finely stratified Geissler tube sho Am; and a glass pipe, con- 

 taining silica powder, and vibrated by a whistle pro- 

 jected on the screen]. As the silica planes limit the free 

 swing of the air particles in the pipe, so the bright por- 

 tions of the vacuum tube show when the residual gases 

 of contiguous segments encounter each other — the vibra- 

 tion being set up by the passage of electricity. And it 

 may be observed here that as the transmission of sound 

 is the onward transference of energy by motion, so the 

 passage of electricity is the transference of energy from 

 one point to another by means of another form of mo- 

 tion. 



The exhaustion which answers best for a Geissler tube 

 is comparatively low. I have frequently obtained beauti- 

 ful stratification in a tube six feet long, exhausted by a 

 good air-pump. Mr. Crookes carried the exhaustion much 

 further and obtained entirely new results. The first no- 

 ticeable feature was the gradual broadening of the dark 

 space surrounding the negative pole as the exhaustion ad- 

 vanced [dark space tube, exhibited a transverse sheet of 

 aluminum in the middle constituting the negative pole]. 

 This dark space is regarded as the free path of the mole- 

 cules at this degree of exhaustion. It increases as the 

 exhaustion proceeds, and contracts when the exhaustion 

 diminishes.. The molecules of air are projected normally 

 from the negative pole, and the illumination at the bound- 

 ary of the dark space is due to the collision between the 

 gas projected from the negative pole and the more slowly 

 moving molecules advancing toward it. Here, then the 

 lines of molecular pressure, caused by the excitement of 

 the negative pole, are illuminated by the induction spark. 



With still higher vacua the free path becomes equal to 

 the dimensions of the containing vessel, and the projected 

 molecules impinge directly against its walls. The mole- 

 cules stream from the negative pole with enormous veloci- 

 ties and dart across the tube with comparatively few col- 

 lisions. Their motion is then arrested by the solid matter 

 of the tube. A noteworthy property of this radiant 



matter then appears. Luminosity is produced by the im- 

 pact of the projected molecules against solid matter. 

 Phosphorescence, as this luminosity is called, is thus excited, 

 its color depending upon the kind of matter receiving the 

 impact. [Three phosphorescent tubes shown; also a bulb 

 partly filled with phosphorescing material.] English glass 

 phosphoresces a light blue, uranium glass rather dark 

 green, and soft German glass, light apple green. Rubies 

 always shine with a deep red hue whatever the color of 

 the gem itself. The artificial rubies made in Paris show 

 no variation from the real stones in the color of their 

 phosphorescent light. This phosphorescence takes place 

 better at an exhaustion of about a millionth of an atmos- 

 phere than at any other. A tube designed to show the 

 dependence of the phosphorescence upon exhaustion, is 

 made by connecting to the main tube a small supplemen- 

 tary tube containing caustic potash, which holds captive 

 a certain amount of aqueous vapor [the tube exhibited]. 

 Turning on the coil, the tube now shows green phosphor- 

 escence, but upon heating the potash tube with a small 

 lamp, aqueous vapor is released, the phosphorescence gradu- 

 ally disappears, and is replaced by the stratified discharge 

 of a Geissler tube. Withdrawing the lamp, the vapor is 

 re-absorbed by the caustic potash, the fine stratification 

 widens out slowly and finally retreats towards the potash 

 bulb, while a wave of green light sweeps from the negative 

 pole, driving the last pale stratification into the potash tube. 

 Radiant matter moves in straight lines and absolutely re- 

 fuses to turn a corner. This peculiarity is shown by a 

 V-shaped tube. [The tube exhibited.] The flood of 

 green light proceeds from the negative pole at the top 

 only as far as the bottom, refusing t > turn the angle to- 

 ward the positive in the other branch. Reversing the 

 current, the illuminated branch follows the negative 

 pole. A striking contrast is thus presented between a 

 Geissler and a Crookes tube ; but this contrast is brought 

 out more clearly still by two bulbs exactly alike, except 

 in the degree of their exhaustion. Both are fitted with 

 one negative terminal and three positive ones. With the 

 low vacuum tube the stratification follows the direc- 

 tion of the positive pole, changing its path as that pole is 

 changed. Changing now to the high vacuum or Crookes 

 tube, containing only about a millionth of an atmosphere, 

 and turning on the coil the only light to be seen is the 

 green phosphorescence of the glass. The negative pole 

 is a very shallow cup, and the projected radiant matter 

 crosses its focus and then strikes on the opposite side 

 of the bulb where the impact excites strong phosphor- 

 escence. Changing the positive pole produces no 

 change whatever in the path of the projected molecules 

 or the luminosity of the glass. The positive pole exer- 

 cises no influence whatever upon the direction of dis- 

 charge of radiant matter from the negative pole. The 

 residual gas is here exalted to the fourth or radiant State, 

 and its free path, under the impulsion of the negative 

 discharge, is entirely across the bulb. Moreover, not 

 only is luminosity induced, but the glass bulb rapidly 

 heats where it receives the cannonade of these invisible 

 balls. 



Another peculiarity of radiant matter, depending upon 

 its projection in right lines, is that when intercepted by- 

 solid matter it casts a shadow. This large pear-shaped 

 bulb has the negative pole at the small end. A cross 

 cut out of sheet aluminum is placed across the bulb so 

 as to intercept a part of the gaseous molecules streaming 

 from the negative pole. Connecting with the induction 

 coil, the dark shadow of the cross is seen plainly pro- 

 jected on the large end of the bulb. Here the projected 

 molecules pass by the aluminum cross and bombard the 

 walls of the bulb, producing the usual phosphorescence 

 Nothing could show more distinctly th^n the projection 

 of matter from the negative pole in straight lines. 



This bombarding causes the surface of the glass to 

 lose its sensitiveness to intense phosphorescence. The 

 cross is hinged so as to turn down with a slight shake. 



