639 
1907-8.] Preparation of a Glass to Conduct Electricity. 
present, the minute trace of lead in the glass prevents the particles vibrating 
in the manner usual with soda glasses. There was not the least suggestion 
of the green tint commonly met with. I am indebted to Mr J. H. Gardiner 
for a series of tests * as to transparency of the glass to ultra-violet light 
and to X-rays. He finds that a plate "5 mm. thick absorbs all radiations 
shorter than X = 3000, so that the glass must be regarded as opaque to ultra- 
violet light. With regard to X-rays it was found to be slightly more 
transparent than ordinary soda glass. 
It would therefore appear that very little energy is required to produce 
this vivid fluorescence, since the photographic effect of the rays after travers- 
ing equal thicknesses of the two glasses is approximately similar in each 
case. The transparency to X-rays might have been predicted from the light- 
ness of the glass, its density being 2'609. There is faint phosphorescence 
under the influence of the rays from radium, and after a three days’ con- 
tinuous exposure the glass is coloured a slate blue. 
Softening Point. 
The softening point was measured by supporting a rod at one end and 
horizontally in a muffle while the temperature was varied and read upon a 
Callender & Griffith’s electric thermometer. A number of experiments 
with rods of diameters varying from one to three millimetres gave a mean 
temperature of 551° C. as the softening point. 
Fine Annealing. 
In order to prepare bars and prisms for some optical work now in 
progress, all the stress in the glass was removed by allowing it to cool very 
slowly. The apparatus used for this purpose is illustrated in fig. 5. 
A brass tube suitably lagged with asbestos, plugged at one end and 
provided with a flue, was arranged so that it could be heated by means of a 
fish-tail bunsen burner. 
The glass to be annealed was placed in this tube and the temperature 
slowly raised. On reaching a dull red heat the automatic regulating device 
was put into action and the flame slowly turned down. The supply of gas 
from the main, after passing through a regulator, enters the vessel A and 
bubbles up through water there on its way to the burner. 
A constant head of water in the tank B feeds a dropper C, which 
supplies water to the vessel A through the funnel. Thus with the tube C 
packed closely with cotton-wool the water may be made to rise so slowly 
* Journal Rontgen Soc ., vol. iv. No. 14, p. 13. 
