224 Mr Ghree, On some experiments on [June 1, 



general seems so to the eye, and the surface separating it from (C) 

 is usually sharply defined. 



(C) has its brightest side next the cathode, and it is by some 

 writers divided into a brighter portion, the negative glow, and 

 a less bright portion, the negative haze. (C) appears almost as 

 soon as the pressure is sufficiently reduced to allow the dis- 

 charge to pass. It increases in length 1 as the pressure is re- 

 duced within at least certain limits. The transition from (C) 

 to (D) is usually so gradual no exact line of separation can be 

 drawn. (D) is not always visible. Its presence depends to a 

 considerable extent on the strength of the discharge. At low 

 pressures an increase in the strength of discharge tends to make 



(D) contract 2 . At high pressures when the discharge first passes 



(E) consists of a succession of twig-like independent discharges, 

 which, as the pressure is lowered, transform into what seems to 

 the eye a tolerably uniform column whose colour in most gases, 

 especially in air, is bright red. As the exhaustion proceeds the 

 colour becomes less bright, and the apparent uniformity of the 

 column tends to disappear. At moderate pressures such as 1 or 

 2 mm. of mercury with a regular source of current, (E) consists 

 in general of a succession of similar units, striae, each having a 

 sharply defined luminous head on the side next the cathode, and 

 gradually fading on the side next the anode into a seemingly 

 non-luminous portion. At very low pressures the striae are 

 sometimes very faintly defined, if existent, and (E) appears as 

 a hazy luminosity, generally of a blue tint. 



Taking the simplest case, viz. a long, uniform, straight cylin- 

 drical tube with flat metal electrodes, whose planes are per- 

 pendicular to the axis of the tube, and whose diameters are 

 not very small, each stria in the positive column has in ana- 

 tomical language an opisthocoelous form, the convex surface being 

 that of the luminous head. This surface is, however, in general 

 of smaller curvature than a sphere of diameter equal to that 

 of the tube. Whether the positive column be striated or not, 

 the surface separating it from (D) has its convexity on the side 

 next the cathode. This surface is usually sharply defined when 

 (D) appears at all. 



There is another phenomenon whose relation to the discharge 

 is somewhat doubtful, viz. the phosphorescence observed in good 

 vacua. The more brilliant phosphorescent phenomena are 

 beautifully shown by many well-known experiments of Professor 

 Crookes. He believes the " molecular streams " or " radiant 

 matter " — German " Kathodenstrahlen" — whose incidence on the 



1 After completely covering a wire cathode it expands as the current increases. 

 See Hittorf, Wied. Ann. 20, 1883, p. 746. 



2 Hittorf, 1. c. pp. 736—7. 



