1870.] 



on Molecular Physics in Ilvjh Vacua. 



US 



Fig. 3. 



polislicd aluminium. This is coiiiicctccl with a line copper wire, h, 

 ending at the jdatinum terminal, c. At the upper end of the tube 

 is another terminal, d. The induction-coil is connected so that tlio 

 liemi-cylinder is negative and tlie upper pole ponitive, and when 

 exhausted to a sufficient extent, as is the case with this tube, the pro- 

 jection 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 bril- 

 liant green phosphorescence on the sur- 

 face of the glass. 



You will notice that the rays which 

 project from the negative pole and cross 

 in the centre have a bright green appear- 

 ance ; that colour is entirely due to the 

 phosphorescence of the glass. At a 

 very high exhaustion the iihenomena 

 noticed in ordinary vacuum tubes when 

 the induction spark passes througli 

 them — an appearance of cloudy lumin- 

 osity 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 atmosi^here — the inner sur- 

 face 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 

 phosphorescence 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 phosj)horesccnce of the glass on the degree of perfection 

 of the vacuum (Fig. 4). The two poles arc at a and h, and at the end 

 (c) is a small supplementary tube connected with the other by a 

 narrow aperture, and containing solid caustic potash. The tube has 

 been exhausted to a very high point, and the potash heated so as to 

 drive off moisture and deteriorate the vacuum. Exhaustion has then 



' 1 • millionth of an atmosphere = ■ 00076 millim. 



1315-789 millioutbs of an atmosishere = 1-0 millim. 

 1,000,000 „ „ „ =700-0 miUims. 



„ „ ,, „ =1 atmosphere. 



