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NATURE 



125 



CONTRIBUTIONS TO MOLECULAR PHYSICS 

 IN HIGH VACUA 1 

 II. 

 TT has been shown that the stream of molecules are 

 ■»• shot off from the negative pole in a negatively charged 

 condition, and their velocity is owing to the mutual 

 repulsion between the similarly electrified pole and 

 molecules. It became of interest to ascertain whether 

 lateral repulsion was exerted between the molecules 

 themselves. If the stream of molecules coming from the 

 negative pole carried an electric current, two parallel 

 rays should exert mutual attraction ; but if nothing of the 

 nature of an electric current was carried by the stream, it 

 ■was likely that the two parallel rays would act simply as 

 negatively electrified bodies and exert lateral repulsion. 

 This was not difficult to put to the test of experiment. 



A tube was made with two fiat aluminium terminals, 

 a b, close together at one end, and one terminal, c, at the 

 other, as shown in Fig. 1 1. Along the centre of the tube, 

 cutting the axis obliquely, is a screen of mica, painted 

 over with a phosphorescent powder, and between the 

 screen and the double poles, a ^, is a disk of mica crossing 

 the axis of the tube, and therefore nearly at right angles 



to the phosphorescent screen. In this mica disk are two 

 slits— one opposite each pole a and b — running in such a 

 direction that the molecular streams emanating from a 

 and /' when made negative shall pass through the slits, 

 forniing two horizontal sheets. These sheets striking 

 against the oblique screen will be made evident as two 

 horizontal lines of light. The poles a and b were some- 

 what bent, so that the lines of light were not quite 

 parallel, but slightly converged. The tube being properly 

 exhausted, the pole a was made negative, and c positive, 

 the lower pole b being left idle. A sharp ray of phosphor- 

 escent light shot across the screen along the line df. The 

 negative wire was now transferred from a to b, when a 

 ray of light shot along the screen from f to/ The two 

 poles a and b were now connected by a wire, and the two 

 together were made the negative pole. Two lines of 

 light now shone on the screen, but their positions, instead 

 of being, as before, (//and cf, were now dg and eh, as 

 shown by the dotted lines. The wire joining the poles 

 ab was removed, and the pole a made negative ; the ray 

 from it followed the line rf/as before. While the coil 

 was working, another wire hanging loose from the pole b 

 was brought up to a, so as to make them both negative. 

 Instantly the xzl^ eh shot across the screen, and simul- 



taneously the ray df shifted its position up to dg. The 

 same phenomena were observed when tfie pole b was 

 connected with the coil, and contact was alternately made 

 and broken with aj as the ray dg shot across, the ray ef 

 dipped to eh. 



These experiments show that two parallel rays of mole- 

 cules issuing from the negative pole exert lateral repulsion, 

 acting like adjacent streams of similarly-electrified bodies. 

 Had they carried an electric current they should have 

 attracted each other, unless, indeed, the attraction in this 

 case was not strong enough to overcome the repulsion. 



Many experiments have been made to ascertain the 

 law of the action of magnets and of wires carrying 

 currents, on the stream of molecules. 



As an indicator, a small tube, as shown in Fig. 12, was 

 employed. The two poles are at a and b, a being the 

 negative. At ^ is a plate of mica with a hole in its centre, 

 and at d is a phosphorescent screen. A sharp image of 

 the hole in the mica is projected on the centre of d, and 

 the approach of a magnet causes this bright spot to move 

 to different parts of the phosphorescent screen. 



A large electro-magnet was used, actuated by two 

 Grove's cells, and the indicator tube was carried round 

 the magnet in different positions and the results noted. 

 The molecular stream when under no magnetic influ- 

 ence passes along the axis of the tube, as shown by 

 the small arrow (Fig. 12). It will be seen that the indi- 

 cator can occupy three different directions in respect to 

 the magnet. The magnet being held horizontally, the 

 direction of the molecular stream mav be parallel to the 

 axis, tangential to it, or at right angles to it. In either of 

 these positions, also, the stream may be directed one way 

 or the other (by turning the tube round endwise). In 

 these different positions various results are obtained 

 which are easily illustrated with a solid model, but are 



■ " Contributims to Molecular Physics in High Vacua. Mag:iietic Deflec- 

 tiou of Molecular Trajectory ; Laws of Magnetic Rotation in High and Low 

 Vacua; Phosphorogenic Properties of Molecular Discharge." By William 

 Crookes, F.R.S. (Extracts from a paper in the Philosophical Transactions 

 of the Royal Society, Part 2, 1879.) Continued from p. 104. 



somewhat complicated to explain by means of fiat 

 drawings. They are fully described in the paper. 



A long tube was made similar to the small indicator 

 shown in Fig. 12, but having a molecular trajectory six 

 inches long. It was only exhausted to the point at which 

 the image of the spot was just seen sharply defined on 

 the screen, as at higher exhaustions the action of mag- 

 netism is less. The phosphorescent screen was divided 

 into squares for convenience of noting the deflection of 

 the spot of light. So sensitive was this to magnetic 

 influence, that when the tube was placed parallel to the 

 earth's equator the earth's magnetism was sufficient to 

 cause the spot to move 5 millims. away from the position 

 it occupied when parallel to the dipping needle (in which 

 position the earth's magnetism did not appear to act). 

 When held ec[uatorially and rotated on its axis, the spot 

 of light, being always driven in one direction independent 

 of the rotation of the tube, appeared to travel round its 

 normal position in a circle of 10 millims. diameter. 



I have long tried to obtain continuous rotation of the 

 molecular rays under magnetic influence, analogous to 

 the well-known rotation obtained at lower exhaustions. 

 Many circumstances had led me to think that such rota- 

 tion could be effected. After many failures an apparatus 

 was constructed as follows, which gave the desired 

 results : — 



A bulb (Fig. 13) was blown of German glass,"and a 

 smaller bulb was connected to each end of the larger 

 bulb by an open, very short neck. .\t each extremity 

 was a long aluminium pole projecting partly into the 

 large bulb and turned conical at the end. After good 

 exhaustion the passage of an induction current through 

 this apparatus fills the centre bulb with a very fine green 

 light, whilst the neck surrounding the pole which happens 

 to be negative is covered with two or three dark and 

 bright patches in constant motion, following each other 

 round first one way and then the other, constantly chang- 

 ing direction and velocity, sometimes dividing into other 

 patches, and at others fusing together into one. After a 



