146 
MR. w. cro6kes ox the illumixatiox of lixes of 
very well. With 11,000 cells the current through was the same as with 9920 cells, and 
the phosphorescence was only a little brighter. The resistance of the tube was found 
to equal 7,642,600 ohms, and the tube potential was 10,600. I have to thank 
Mr. De La Hue for not only letting me use his battery, but for kindly taking these 
measurements for me.—W. C., August 12, 1879.] 
XATURE OF THE OREEX PHOSPHORESCEXT LIGHT. 
519. Other differences are observed. The focus of the green phosphorescent light 
remains at the centre of curvature, appearing independent of the degree of exhaustion, 
provided this is good enough to render it visible ; whilst the bluish focus of light 
lengthens out with the exhaustion. The green focus is not to be seen in the body 
of the tube, but only where the projection causing it strikes the glass ; whilst the 
concentration of light first observed is only visible in the space of the tube, and 
produces no special effect when it touches the sides. 
The bluish focus at low exhaustions, proceeding from the negative hemi-cylinder, is 
only seen when the pole cl is positive, no light on the concave side being seen. When 
the exhaustion is high, and the green phosphorescent focus is bright, it makes little 
difference whether the positive pole is above the concave surface, at d, or below it, 
at e or f; in either case the projection from the concave surface of a is visibly focussed 
on the phosphorescent surface of the glass (526, 527, 549). 
520. Spectrum observations show another difference equally decided. When the 
luminosity observed in the focus of the hemi-cylinder at a low exhaustion (say 100 M) 
is examined in the spectroscope, it shows the lines due to the residual gas, whether 
it be nitrogen, hjMrogen, or carbonic acid ; but when the exhaustion is at a very 
high point (say 1 M) the blue light has disappeared, and no lines are detected in 
the green phosphorescence whatever be the gas whose residue causes it. The spectrum 
of the greenish-yellow light, so beautifully illuminating the tube, is continuous, most 
of the red and the higher blue rays being absent. 
521. The green phosphorescence commences at a lower exhaustion in hydrogen 
than it does in air, and the phenomena known to be due to hydrogen gas also 
disappear sooner than in the corresponding case with air. In all gases, however, 
when a high exhaustion is reached the subsequent phenomena are the same. 
522. The viscosity of a gas is almost as persistent a characteristic of its in¬ 
dividuality as its spectrum. For many years I have been carrying on a research 
on the variations of viscosity of different gases at high exhaustions. In the Pro¬ 
ceedings of the Royal Society for November 16th, 1876,'“ I gave in a preliminary 
note a diagram of the variation of viscosity of air, hydrogen, and other gases, at 
exhaustions varying from 250 M to 0‘1 M. A comparison of the indications on 
that diagram with the data given in the present paper will show that when the 
* Yol. xxv., p. 305. 
