Vapour in the Visible and Ultra-violet Regions. 309 
and the temperature gradient takes care of itself in this case. 
The sodium was first melted in a small iron crucible and poured 
into an improvised sodium press ; a piece of apparatus so easily 
made and so useful that a brief description of it may not ‘be 
out of place. A steel cylinder is bored with a hole two 
or three cms. in diameter to within a few millimetres of its 
end. A 3-mm. hole is then drilled through the end at the 
centre of the larger hole. A solid steel piston is turned so 
as to fit easily into the bore of the larger cylinder. The 
molten sodium is poured in, allowed to cool, and the piston 
inserted. The whole is then put into the jaws of a large 
vice, and the sodium wire squeezed out, directly into the 
dispersion-tube. In this way the formation of oxide is pre- 
vented, and the sodium is freed from the hydrocarbon in 
which it iskept. The glass plate is immediately cemented on, 
and the tube exhausted on a mercurial pump. An ordinary 
hand-pump would doubtless answer every purpose, as the 
vacuum does not need to be below a few millimetres. I see 
no reason why a well-made steel tube should not be used year 
after year in class demonstration, though I have not tried 
keeping one so long. With the tube once prepared, the 
apparatus for showing the anomalous dispersion of the vapour, 
which is one of the most beautiful optical experiments that I 
know of, can be set up in five minutes. The former method 
which I described was rather troublesome, as it involved the 
use of a hydrogen generator and drying-tubes, and it was 
often difficult to avoid the formation of smoke in the tube. 
The vacuum-tubes make the experiment as easy to perform 
as the simplest of the laboratory demonstrations. A combus- 
tion furnace is not necessary, two or three Bunsen burners 
turned down low answering every purpose. The amount of 
dispersion can be regulated by turning the flames up or 
down. A full description of the disposition of the apparatus 
will be found in my first paper. In brief, it consists of a 
horizontal slit on which the crater of an arc-lamp is focussed. 
lf a Nernst lamp is available it may advantageously be sub- 
stituted for the illuminated slit, the glower being mounted 
in a horizontal position. The light from the slit is made 
parallel with a lens, and after traversing the sodium tube is 
focussed on the slit of a small spectroscope by means of a 
second lens. It is almost as satisfactory to use a single lens 
passing the convergent beam through the dispersion-tube. 
If it is desired to show the anomalous spectrum, a low- 
power eyepiece is substituted for the spectroscope. The ap- 
pearance of this spectrum is shown by a coloured plate illus- 
trating the earlier paper. In the present work it was desirable 
Phil. Mag. 8. 6. Vol. 8. No. 45. Sept. 1904. ¥ 
