462 Mr. J. B. B. Burke on the 



desirable to postpone their discussion till their investigation 

 is complete. 



(18) The spectrum of the glow leaves little doubt that, 

 although working with high vacua, we are dealing with such 

 large molecular groups that they may be regarded as separate 

 solid particles. Thus Schuster (Proc. Roy. Soc. vol. 37, 

 p. 322, 1884) has observed that the spectrum of the negative 

 glow in oxygen, on careful examination, consists of three 

 spectra : (1) characteristic bands, (2) spectrum of positive 

 pole, (3) high-temperature spectrum of oxygen. Thereby 

 suggesting the existence of three distinct kinds of molecules. 

 The ones that give the characteristic bands would appear to 

 be the phosphorescent ones. 



The spectroscopic examination of the glow, and the fact 

 that a gas may give out a continuous or band-spectrum, in 

 high vacua combined with the apparent semi-opacity which 

 the glow presents, as in the case of fluorescent bodies like 

 uranium glass when fluorescing (see Phil. Trans. 1898), will 

 be fully discussed in another paper. 



(19) There is one important point which still remains un- 

 touched, and that is the influence of ozone, or the part played 

 by it in the production of the glow. W. Sutherland (Phil. 

 Mag. vol. xliii.) has indicated that at pressures between 15 

 and 0"07 mm. there is a deviation from Boyle's law in the case 

 of oxygen, and he has suggested that this is probably due to 

 the formation of ozone. 1 have myself worked at this point 

 and have obtained considerable variations from the law, but 

 I have not been able to ascertain precisely how far the results 

 have been due to condensation on the surface of the glass. 

 From 760 mm. down to about 2 mm. Boyle's law holds very 

 accurately; then slight variations begin to enter, doe no doubt 

 to the presence of water-vapour, which begins to cause con- 

 siderable retardation in the rate of fall of pressure; this for a 

 while becomes practically constant, but never actually returns 

 to the original relation between pressure and volume, and 

 between 0*5 mm. and 0*01 mm. the proportional rate of 

 fall of pressure is very much lower than what it should be if 

 the gas obeyed Boyle's law. The pressures in the latter case 

 were measured with the McLeod gauge, whereas the greater 

 pressures were measured by means of a cathetometer. The 

 experiment has been made simply as a side issue, and is 

 worth mentioning perhaps, because a record was kept of the 

 number of strokes taken at regular intervals and their cor- 

 responding pressures at one period of the research ; but the 

 results were not conclusive on account of the surface effects. 

 It was found that the subject had already been dealt with very 

 fully by Bohn, Wied. Ann. xxvii., and Baly and Ramsay, 

 Phil. Mag. vol. xxxviii., and also bv Mendeleeff. Prof. Dewar 



