AS OBSEEVED IN TOEEICELLIAN AND OTHEE VACUA. 
147 
which has been hitherto observed. To test this opinion, I endeavoured to obtain a 
vacuum with fusible metal instead of mercmy, but I could not succeed in getting it free 
from the presence of air, which was denoted by the colour of the discharge (66). I also 
prepared apparatus for a tin vacuum ; but as similar difficulty would have occurred in this 
as with fusible metal, I laid the apparatus aside, and endeavoured to obtain a vacuum 
by insertmg sodium in a closed glass vessel, previously charging it with pure oxygen ; the 
stratified discharge was very clear, but the sodium as it melted adhering to the sides of 
the glass in irregular masses, conducted the discharge from the different surfaces, and 
the form of the stratifications was rendered so irregular that the results in this respect 
were unsatisfactory ; I however succeeded in obtaining sufficient evidence to convince me 
that with better arrangements, sodium, and probably potassium with oxygen, will pro- 
duce vacua which cannot fail to exhibit the stratified discharge with great brilliancy, and 
possibly by this process even a still more perfect vacuum may be obtained. 
Carionic-acid Vacua*. 
83. The experiments I now proceed to describe were made in glass tubes, many of 
wffiich were of similar form and dimensions to those I had used with mercury, but the 
vacua were obtained by an entirely different process. In the preparation of these I am 
indebted to Dr. Feaxkland, who not only suggested and explained to me the process to 
be adopted, but kindly devoted much of his valuable time to the superintending many 
of the experiments, which were made in his laboratory at St. Bartholomew s Hospital. 
Fig. 8, Plate IX. represents a tube in which the platinum wires a and h are hermetically 
sealed ; in the narrow’ portion A, one or two sticks of caustic potash are inserted, one 
end of the tube being connected by means of gutta-percha tubing to a vessel in wffiich 
pure carbonic acid gas is generated ; the other end passes through a tight-fitting collar 
into a vessel containing a little mercury under a receiver attached to the plate of a good 
au’-pump. A portion of the air is first expelled by the rush of the carbonic acid ; after 
this the passage of the gas is stopped at B, and the tube is exhausted by means of the 
air-pump. The mercury (M) acting as a valve, prevents any risk of air entering into the 
tube ; the process is repeated two or three times, when, if the discharge from the induc- 
tion apparatus is found to be entirely free from any trace of redness, it may be assumed 
that all the air has been expelled, and the tube can be then prepared for sealing. 
When it is full of the gas, the glass is sealed at B. The tube is again exhausted to 
the utmost limit of the capability of the pump, and finally sealed at C. The apparatus 
is now ready for experiment. 
84. In carbonic-acid vacua obtained by this process, the discharge from an induction 
coil is at first in a wave line, strongly affected by the magnet or by the hand when 
placed on the tube ; in this state the discharge does not generally present the stratified 
appearance, or if present the stratifications are only near the positive terminal ; some- 
times in the course of a few minutes, but often not until several days have elapsed, the 
* Dr. Andbews of Belfast was, I believe, the first to describe a method of obtaining carbonic acid vacua 
in the receiver of an air-pump (Philosophical Magazine, Eebruary 1852, p. lOl). 
X 2 
