1879.] 



Molecular Physics in High Vacua. 



477 



The ratio of tlie wave-lengths of F to G of hydrogen ((2) to (3) in 

 the table above) is nearly identical with the ratio of D 3 to the coronal 

 green line ((2) to (3) in table above). 



This near coincidence in the ratios of certain lines of hydrogen, 

 lithium, and magnesium, substances belonging to the same type, com- 

 bined with a similar ratio in the wave-lengths of the nearly equally 

 persistent lines of the chromosphere, greatly strengthens the probability 

 of the assumption that these lines belong to one substance. 



The fact that the two less refrangible rays have no representative in 

 the Fraunhofer lines, is by no means opposed to their belonging to one 

 substance, since we know that aluminium behaves in a similar way in 

 the atmosphere of the sun ; and in the total eclipse of 1875 the 

 hydrogen line h was not visible in the chromosphere, that is, we 

 suppose, was on the limit between brightness and reversal ; and 

 during the late eclipse the two most refrangible rays of hydrogen 

 were not detected from the same cause. 



Until our knowledge of the order of reversibility of lines belonging 

 to different types of metals has been extended, it would be rash to 

 infer the group of metals to which it belongs, or its probable molecular 

 weight. 



V. " Contributions to Molecular Physics in High Vacua." By 

 William Crookes, F.R.S. Received March 27, 1879. 



(Abstract.) 



This paper is a continuation of one " On the Illumination of Lines 

 of Molecular Pressure, and the Trajectory of Molecules," which was 

 read before the Royal Society on the 5th of December last. The author 

 has further examined the action of the molecular rays electrically pro- 

 jected from the negative pole in very highly exhausted tubes, and 

 finds that the green phosphorescence of the glass (by means of which 

 the presence of the molecular rays is manifested) does not take place 

 close to the negative pole. Within the dark space there is absolutely 

 no phosphorescence ; at very high exhaustions the luminous boundary 

 of the dark space disappears, and now the phosphorescence extends 

 all over the sensitive surface. 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. The author adduces 

 arguments to show that within the dark space, at a moderate ex- 

 haustion, 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 sufficient speed 



VOL. XXVII l. 2 N 



