MOLECULAR PRESSURE, AND THE TRAJECTORY OF MOLECULES. 
145 
the hemi-cylinder with the platinum terminal c. At the other end of the tube a 
flat aluminium disk forms the terminal. Other aluminium terminals are sealed in at 
e and f In the axis of the tube, resting on the centre of the hemi-cylinder, and at 
right angles to its axis, is a flat glass plate g, on which is etched a millimetre scale. 
515. On exhaustion, the hemi-cylinder a being made the negative pole through the 
wire c, the usual phenomena are observed. The dark space gradually increases in 
size, and as it spreads from the concave surface towards the centre it forms a denser 
and more luminous glow of light. As the exhaustion proceeds this glow becomes 
smaller and brighter, until it forms a narrow brilliant line along the axis of the hemi- 
cylinder. A bright spot forms on the scale at 11 millims. (the centre of curvature 
of the hemi-cylindrical pole), showing that the gaseous molecules from the negative 
pole are projected in a direction normal to the surface. The distance between the 
convex surface of the hemi-cylinder and the luminous boundary of the dark space is 
also about 11 millims. The pressure at this point is 106 M. 
516. As the exhaustion becomes higher the rays projected from the concave surface 
of a are seen on the divided scale to cross each other, and the focus lengthens out 
and becomes ill-defined, as if the molecules from the centre of the pole were projected 
to a greater distance than those from the edges, giving an appearance of spherical 
aberration. It is difficult to fix the exact focal point, but it gradually creeps up 
from 11 millims. to about 19 millims. 
517. When the focus of light has reached 20 millims. it is very faint. The green- 
yellow phosphorescent light has been visible for some time, and now, the pressure being 
19'3 M, a faint concentration of green light is seen to occur at 11 millims. on the scale. 
This shows that the cause to which the green phosphorescence is due is not the same 
to which the luminosity previously noticed in the tube is due ; for the two phenomena 
are now existing simultaneously. 
518. In another tube, similar to the one shown at fig. 12, but without the flat glass 
plate g, the appearance at an exhaustion of one millionth of an atmosphere when the 
induction current passes through, is shown in the coloured lithograph, Plate 14. 
[Having received permission from Mr. De La Hue to test some of these high vacuum 
tubes with his large chloride of silver battery, I gladly availed myself of it, and tried 
some experiments, among others, with this tube. The exhaustion was the millionth 
of an atmosphere. Not knowing what the effect of so large a battery might be, cells 
were put on gradually. Up to 4800 cells scarcely any action was visible. At 6300 
cells a current equal to 0'00048 weber passed through the tube, and the green phos¬ 
phorescence commenced to be visible on the glass. With 7760 cells the current 
passing through was equal to 0'00095 weber, and the appearance of the tube was 
similar to that produced by an induction coil giving about a one-inch spark. When 
9920 cells were used, the current through was =0'00143 weber, and the green phos¬ 
phorescent curves on the tube were about as bright as when an 18-incli spark from an 
inductive coil was passed through. The chromo-lithograph (Plate 14) represents this 
MDCCCLXXIX. 
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