MOLECULAR PRESSURE, AND THE TRAJECTORY OF MOLECULES. 
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violet light by which the focus was rendered visible has become so faint as to be 
difficultly traced, but with care it can be seen converging to a focus beyond the 
focal point noticed at lower exhaustions. At the part of the bulb on which the rays 
impinge, a faint spot of greenish-yellow light is observed, sharp in outline. On 
exhausting to 14 M, and making the cup the negative pole of the coil, the projection 
from the cup is represented by a brilliant green spot of light about 7 millims. diameter, 
and the focus can scarcely be traced. The rest of the bulb is nearly dark, but at 
those parts furthest removed from the negative pole the faintly luminous boundary of 
the dark space can still be seen. A little blue light is seen round the positive pole 
extending somewhat into the bulb. On reversing the poles and making the cup 
positive, the bulb becomes beautifully illuminated with greenish-yellow light. 
511. This phosphorescent light only appears in its full intensity when the dark 
space surrounding the negative pole extends to the surface of the bulb. At lower 
exhaustions, it can be detected when specially sought for outside the luminous 
boundary of the dark space, but it is faint and not easily noticeable. The colour 
depends on the kind of glass used. Most of my apparatus are made of soft German 
glass, and this gives a phosphorescent light of a greenish-yellow colour. English glass 
phosphoresces of a blue colour; uranium glass becomes green; a diamond became 
brilliantly blue. 
512. The greenish-yellow phosphorescence of the soft German glass only takes place 
under the influence of the discharge from the negative pole.* As the exhaustion 
increases the light gets stronger. At 10 M the diffused violet light surrounding the 
positive pole is faint, and the green light on the glass is getting stronger. At 4 M 
* While this paper was passing through the press, my attention has been drawn to two Memoirs by 
H. Etjgen Goldstein, communicated to the Berlin Royal Academy of Sciences, May 4,1876, and November 
23, 1876, in which some of the results announced in this paper have been anticipated. Referring to the 
green phosphorescent light, H. E. Goldstein writes :— 
“ The author has specially examined the green light which appears in tubes of common glass at certain 
degrees of pressure and intensity of the discharge. The luminosity of the sides of the tube is not a 
phenomenon of fluorescence but of phosphorescence, and can change from green to orange. 
“ The negative light which produces this phosphorescence is, as was already assumed by Hittoef, a 
rectilinear radiation, which extends from the negative pole into surrounding space. Still there are 
essential differences between the diffusion of this remarkable motion and the likewise rectilinear movement 
of the light, some of which differences are here brought forward. 
“ Hittorf observed that a body placed between the side of the glass and a point-like cathode, throws a 
shadow in the phosphorescent light of the latter. 
“ Well defined, though not very sharp shadows of small objects may be obtained not merely from a 
point-like or linear negative pole, but also from extended negative surfaces placed at a small distance from 
the opaque object. 
“ A surface which merely radiates light, e.g., an ignited body, under similar conditions throws a scarcely 
visible expanded penumbra. 
“ The negative light is therefore a rectilinear radiation, which is propagated preferably in a manner 
almost normal to the producing surface. 
“ If between the cathode and the green luminous side of the tube there is introduced a solid body, its 
