July 7,, ^^n^ NATU RE 



that the "dark space " has shrunk to small dimensions. It is a 

 natural inference that the dark space is the mean free path of 

 the molecules of the residual gas. 



The radiometer which has just been turning under the influence 

 of the lime-light is not of the ordinary kind. Fig. i will ex- 

 plain its conitraction. 



It is similar to an ordinary radiometer with aluminium disks 

 for vanes, each disk coated on one side with a film of mica. The 

 fly is supported by a hard steel instead of glass cup, and the 

 needle point on which it works is connected by means of a wire 

 with a platinum terminal sealed into the glass. At the top of the 

 radiometer bidb a second terminal is sealed in. The radiometer 

 can therefore be connected with an induction-coil, the movable 

 fly being made the negative pole. 



As soon as the pressure is reduced to a few millims. of mer- 

 cury, a halo of velvety violet light forms on the metallic side 

 of the vanes, the mica side remaining dark. As the pressure 

 diminishes, a dark space is seen to separate the violet halo from 

 the metal. At a pressure of half a millim. this dark space ex- 

 tends to the glass, and positive rotation commences. On con- 

 tinuing the exhaustion the dark space further widens out and 

 appears to flatten itself against the glass, when the rotation 

 becomes very rapid. 



You perceive a dark space behind each vane and moving 

 round with it. In the first experiment, radiation from the lime- 

 light falling on the metallic sides of the vanes, produced a layer 

 of molecular pressure which drove the fly round ; so here the 

 induction-current has produced molecular excitement at the sur- 

 face of the vanes forming the negative pole, extending up to the 

 side of the glass. 



When the negative pole is in rapid rotation it is not easy to 

 see this dark space, so I have arranged a tube in which the dark 

 space will be visible to all present. The tube, as you will see 

 by the diagram (Fig. 2), has a pole in the centre in the form of 

 a metal disk, and other poles at each end. The centre pole is 

 made negative, and the two end poles connected together are 

 made the positive terminal. The dark space will be in the 

 ' centre. When tlie exhaustion is not very great the dark space 

 extends only a little distance on each side of the negative pole 

 in the centre. When the exhaustion is very good, as it is in the 

 tube before you, and I turn on the coil, the dark space is seen 

 to extend for about two inches on each side of the pole. 



Here, then, we see the induction spark actually illuminating 

 the lines of molecular pressure caused by the excitement of the 

 negative pole. The thickness of this dark space— nearly two 

 inches— is the measure of the mean free path between successive 

 collisions of the molecules of the residual gas. The extra 

 velocity with which the negatively electrified molecules rebound 

 from the excited pole keeps back the more slowly moving mole- 

 cules which are advancing towards that pole. The conflict 

 occurs at the boundary of the dark space, where the luminous 

 margin bears witness to the energy of the discharge. 



I will endeavour to throw on the screen an illustration of this 

 dark space. A stream of water falls from a small jet on to a 

 horizontal plate of glass. The water spreads over the plate 

 and forms a thin film. The jet of water in the centre, from the 

 velocity of its fall, drives the film of water before it on all sides, 

 raising it into a ring-shaped heap. As I diminish the force_ of 

 the jet the ring contracts ; this is equivalent to the exhaustion 

 getting less. When I increase the force of water the ring ex- 

 pands in size, the effect being analogous to an increase of ex- 

 haustion in my tubes. The extra velocity of the falling particles 

 of water drives the in-coming particles of water before them, and 

 ■ raises a ridge round the side which exactly represents .the 

 luminous halo to the dark space to be seen in this tube. 



If, instead of a flat disk, a metal cup is used for the negative 

 pole, the successive appearances on exhausting the tube are 

 somewhat difTerent. The velvety violet halo forms over each 

 side of the cup. On increasing the exhaustion the dark space 

 widens out, retaining almost exactly the shape of the cup. The 

 bright margin of the dark space becomes concentrated at the 

 concave side of the cup to a luminous focus, and widens out at 

 the convex side. When the dark space is very much larger than 

 the cup, its outline forms an irregular ellipsoid drawn in towards 

 the focal point. Inside the luminous boundary a dark violet 

 light can be seen converging to a focus, and, as the rays diverge 

 on the other side of the focus, spreading beyond the margin of 

 the dark space ; the whole appearance being strikingly similar to 

 . the rays of the sun reflected from a concave mirror through a 

 foggy atmosphere. This proves a somewhat important point ; it 



229 



shows that the molecules thrown off the excited negative pole 

 leave it in a direction almost normal to the surface. 



I can illustrate this property of the molecular rays by an 

 experiment. This diagram (Fig. 3) is a representation of the 

 tube which is before you. It contains, as a negative pole, a 

 hemi-cyUnder (a) of polished aluminium. This is connected 

 with a fine copper wire, b, ending at the platinum terminal, c. 



Fig. 



At the upper end of the tube is another terminal, d. The induc- 

 tion-coil is connected so that the hemi-cylinder is negative and 

 the upper pole positive, and when exhausted to a sufficient extent, 

 as is the case with this tube, the projection of the molecular rays 

 to a focus is very beautifully shown. The rays are driven from 

 the hemi-cylinder in a direction normal to its surface ; they come 

 to a focus and then diverge, tracing their path in brilliant green 

 phosphorescence on the surface of the glass. 



You will notice that the rays which project from the negative 

 pole and cross in the centre have a bright green appearance ; that 

 colour is entirely due to the phosphorescence of the glass. At a 

 very high exhaustion the phenomena noticed in ordinary vacuum 

 tubes when the induction spark passes through them — an appear- 

 ance of cloudy luminosity and of stratifications — disappears 

 entirely. No cloud or fog whatever is seen in the body of the 

 tube, and with such a vacuum as I am working with in these 

 experiments — about a millionth part of an atmosphere — the inner 

 surface of the glass glows with a rich green phosphorescence, the 

 intensity of colour varying with the perfection of the vacuum. 

 It scarcely begins to show much before the 800,000th of an 

 atmosphere. At about a millionth of an atmosphere the phos- 

 phorescence is very strong, and after that it begins to diminish 



until there are not enough molecules left to allow the spark 

 to pass.^ 



I have here a tube which will serve to illustrate the dependence 

 of the green phosphorescence of the glass on the degree of 

 perfection of the vacuum (Fig. 4). The two poles are at a and 



» I -o millionth of an. itinosphere = 000076 millim. 



i3i5'78o miUionths of an atmosphere = I'o millim. 



1,000,000 = 7600 "'''"ns. 



,, =1 atmosphere. 



