June 3, 1880] 



NATURE 



lor 



CONTRIBUTIONS TO MOLECULAR PHYSICS 

 IN HIGH VACUA ' 



THIS paper is a continuation of the Bakerian Lecture 

 '• On the Illumination of Lines of Molecular Pressure 

 and the Trajectory of jNIolecules," read before the Royal 

 Society, December 5, 1S7S. Phenomena there briefly 

 referred to have since been more fully examined ; new 

 facts have been observed, and their theoretical bearings 

 discussed ; and numerous experiments suggested by Prof. 

 Stokes and others have been tried, with the result of 

 acquiring much information which cannot fail to be_ of 

 value in assisting to evolve a theory capable of embracing 

 all the phenomena under discussion. 



Experiments previously described have shown that the 

 molecular stream hypothesis is the correct one. Accord- 

 ing to this, the molecules of the residual gas, coming in 

 contact with the negative pole, acquire a negative charge, 

 and immediately fly oft" by reason of the mutual repulsion 

 exerted by similarly electrified bodies. Were the indi- 

 vidual molecules solely acted on by the initial impulse 

 from the negative pole, they would take a direction 

 accurately normal to the surface repelling them, and 

 would start with their full velocity. But the molecules, 

 being all negatively electrified, exert mutual repulsion, 

 and therefore diverge laterally. The negative pole, like- 

 wise, not only gives an initial impulse to the molecules, 

 but it also continues to act on them by repulsion, the 

 result being that the molecules move with an accelerating 

 velocity the further they get from the pole. The lateral 

 divergence of the molecules, owing to their negative 

 electricity, will naturally increase with the amount of 

 charge they carry ; the greater the number of collisions 



the more the molecules lose negative charge, and the less 

 divergent the stream becomes. This hypothesis is borne 

 out by facts. When the vacuum is just good enough to 

 allow the shadow to be seen, it is very faint (owing to few 

 molecular rays), but is quite sharp (owing to the divergence 

 of the molecules laterally). The variation in mutual 

 repulsion is shown by the fact that the focus projected 

 fi'om a concave pole falls beyond the centre of curvature, 

 and varies in position with the exhaustion, being longer 

 at high than at low exhaustions. 



Assuming that the phosphorescence is due, either 

 directly or indirectly, to the impact of the molecules on 

 the phosphorescent surface, it is reasonable to suppose 

 that a certain velocity is required to produce the effect. 

 Within the dark space, at a moderate exhaustion, the 

 velocity does not accumulate to a sufficient extent to 

 produce phosphorescence ; but at higher exhaustions the 

 mean free path is long enough to allow the molecules to 

 get up speed sufficient to cause phosphorescence. At a 

 very high exhaustion the phosphorescence takes place 

 nearer the negative pole than at lower exhaustions ; this 

 I consider results from the initial velocity of the molecules 

 being sufficient to produce phosphorescence, their greater 

 speed being due to the fewer collisions near the negative 

 pole. 



The luminous boundary to the dark space round the 

 negative pole is probably due to the impact of molecule 

 against molecule, producing phosphorescence of the gas 

 in the same way as the impact of molecules against 

 German glass produces phosphorescence of the glass. 



The following experiments were commenced at the 

 suggestion of Prof Maxw'ell : — 



A tube was made as shown in Fig. i. The terminal a 



is a rectangular phate of aluminium, folded as shown in 

 section Fig. 2 ; the other terminal iJ is a flat disk of 

 aluminium set obliquely to the axis of the tube. In front 

 of the pole b is fixed a screen of mica, with a small hole 

 in it, as shown at c ; this hole is not in the axis of the 

 tube, but a little to one side of it, so that rays starting 

 normally from the centre of the pole b may pass through 

 it and strike the glass at ci, whilst at the same time rays 

 passing direct between the poles a and b can also pass 

 through the hole. 



The questions which this apparatus was to answer are : 

 (i) Will there be molecular projections from the negative 

 pole, a, in two series of plane strata normal to the sides 

 of the individual furrows, or will the projection be perpen- 

 dicular to the electrode as a whole, i.e., along the axis of 

 the tube ? and (21, Will the molecular rays from the pole 

 b, when it is made negative, issue through the aperture of 

 the screen, along the axis of the tube, i.e., direct to tlfe 

 positive pole, or will they leave the pole normal to its 

 surface and strike the glass as shown at </ ? 



The tube was exhausted and connected with an induc- 

 tion coil ; the following results were obtained : — At a 

 moderate exhaustion, the corrugated pole being made 



' " Conlributi jns to MoIecuKir Physics in High Vacu:i. Magnetic Deflec- 

 tion of Molecular Trajectory ; Laws of Magnetic Rotation in High and Lo'.v 

 Vacua ; Phosphorogenic Properties of Molecular Discharge." By William 

 Crookes, F.R.S. (Extracts from a paper in the Phihsophical 1 ransactions 

 ■of the Royal Society, Part 2. 1S79 ) 



negati\-e, the dark space entirely surrounds it, slight 

 indentations being visible opposite each hollow, where 

 there also is a linear concentration of blue light. The 

 appearance is in section as shown in Fig. 2. At higher 

 exhaustions the luminous margin disappears and the rays 

 which previously formed the blue foci are now projected 

 on the inner surface of the tube, where they make them- 

 selves evident in green phosphorescent light as portions 

 of ellipses formed by the intersection of the several sheets 



of molecular rays with the cylindrical tube. Fig. 3 shows 

 this appearance. 

 -. When the other pole was made negative, and the 



