22o 
SCIENCE. 
A form of apparatus which 1 have found to answer very 
well is shown in the accompanying woodcut (Fig. 2). 
A is the tube or retort containing the metal experimented 
on in its lower extremity, and having a platinum wire sealed 
into it at a distance of about two inches from the lower 
end, the other end being drawn out and connected by a 
mercury joint to an ordinary Geissler tube, which is con- 
nected by another mercury joint to the Sprengel pump C. 
Another form of tube which I have used is prepared by 
inserting two platinum poles into a piece of combustion 
tubing sealed at one end, and after inserting the metal to 
be experimented on, drawing out the glass between the 
platinums to a capillary tube. 
I have also tried inserting the platinum pole at the end 
of the retort, so that the spark passes from the surface of 
the metal, but this arrangement did not answer.at all. 
Some other modifications have been tried, but the first 
form I have described is that which I have found to answer 
best, so far as the trials have yet gone. 
D is the spectroscope. 
E is the lens used for focussing the image of the Geissler 
tube on the slit. 
oil can be found by examining the spectrum of this capillary 
tube. 
I now give an account of the phenomena observed when 
we were working with sodium, in order to show the kind 
of phenomena and the changes observed. 
After a vacuum has been obtained the retort is heated 
gradually. The pump almost immediately stops clicking, 
and in a short time becomes nearly full of hydrogen. The 
spectrum of the capillary then shows the hydrogen lines 
intensely bright. After some time the gas comes off far 
less freely, and an approach to a vacuum is again obtained. 
Another phenomenon now begins to show itself : on pass- 
ing the current a yellow glow is seen, which gradually fills 
the whole space between the pole in the retort and the 
metal ; its spectrum consists of the lines of hydrogen and 
the yellow line of sodium, the red and green line being 
both absent until the experiment has gone on for some 
time. 
As the distillation goes on, the yellow glow increases in 
brilliancy, and extends to a greater distance above the pole, 
and the red and green lines presently make their appearance 
as very faint lines. 
Fig. 2 . — Distillation Apparatus. 
F is the spirit lamp for heating the retort. 
H is the batter)'. 
K and L are the wires connected with the coil. 
In the second cut (Fig. 3) the method of observing the 
spectrum of the vapours close to the surface of the metal 
is indicated ; the same letters apply, D' being, however, in 
this case a direct-vision spectroscope, which was sometimes 
employed for convenience. 
For determining the exact positions of the lines in the 
spectrum of the vapor in any part of the retort, a larger 
spectroscope, with its illuminated scale, was used in the 
place of the direct-vision spectroscope. 
The secondary wires of the coil were connected, one with 
the pole in the upper bulb at B. and the other with the plati- 
num at A. 
B is an ordinary Geissler tube with two bulbs separated 
by a capillary tube. The great advantage of this arrange- 
ment is that this capillary portion can be used for ascertain- 
ing what gases or vapors are carried over by the pump 
without any interference with the retort, both wires being 
connected with the Geissler tube. If, for example, we are 
working with sodium which contains an impurity of hydro- 
carbon, the moment at which it begins or ceases to come 
The upper boundary of the yellow is quite sharp, the lines 
and fluted spectrum of hydrogen appearing above it. 
After the yellow glow-giving vapor (which does not at- 
tack the glass) has been visible for some time, the pump is 
stopped and the metal heated more strongly. On passing the 
current a little while afterwards, a very brilliant leaf-green 
vapor is seen underlying the yellow one, and connected 
with it by a sap-green vapor. The spectra then visible in 
the tube at the same time are — 
Leaf-green ... Green and red lines of sodium and C of 
hydrogen ; D absent. 
Sap-green ... Green, red, and yellow sodium lines of 
equal brilliancy and C of hydrogen. 
Yellow D alone and C. 
Bluish-green C and F and hydrogen structure. 
To observe the green sodium line alone it is necessary to 
point the direct-vision spectroscope just above the surface 
of the metal where the green is strongest. It is also neces- 
sary to guard against internal reflections from the glass, as 
this may sometimes cause the D line to be seen by re- 
flection from the surface. 
