EEPULSION RESULTING EEOM EADIATION. 
517 
where a tubeful could be collected from time to time (as shown in the figure) to test 
with potash. It was found necessary to keep the evolution going on all the time 
pretty briskly, to prevent air diffusing in. The joints were made of double caoutchouc 
tubing, the smaller one tightly wired on and coated with glycerine before the larger 
tube was slipped over it. The whole was then tightly bound with wire. To prevent 
air creeping down between the mercury and the glass, glycerine was poured over all 
the mercury joints, except the one at the top of the mercury fall-tube, which was kept 
for oil of vitriol, with which the pump was lubricated from time to time. 
52. The apparatus being exhausted of air, the balance was adjusted by heating the ends 
so as to slightly char the one which happened to be the lower, f and g are two collars 
of silver foil encircling the tube where the heat is to be applied, and connected with 
earth by wire. At a very high rarefaction the flame of a spirit-lamp excites so much 
electrical disturbance in the balance, that its adjustment becomes well nigh impossible. 
This arrangement was adopted in the endeavour to carry off the electricity ; it is, how- 
ever, only partially successful, and the electrification of the balance at the highest rare- 
factions is still very troublesome. 
The air having been removed from the apparatus as perfectly as the Sprengel pump 
would effect it, carbonic acid was let into the tube by cautiously opening the tap h. 
Exhaustion was again effected, and carbonic acid passed in a second time. This was 
then pumped out, and the apparatus was filled a third time. This alternate filling with 
carbonic acid and exhaustion was continued until the gas collected at the bottom of the 
mercury fall-tube of the pump was entirely absorbed by potash. When this was found 
to be the case, the exhaustion was allowed to proceed to the highest possible point *. 
The pump was then stopped ; an induction-current now being passed between the wires 
at the end b show r ed the usual white light of a carbonic-acid vacuum (a trace of red 
shows atmospheric nitrogen). 
The sticks of potash in the copper boat in c were then heated to incipient fusion, 
and the whole was allowed to cool for some hours. The tube was then sealed off by 
applying a spirit-flame to the contracted part a ; the potash was then heated again, and 
the whole was set aside to give the potash time to absorb the residual carbonic acid. 
53. By testing from time to time with an induction-coil, the progress of the absorption 
could be traced ; and when the current ceased to pass through the tube, but preferred 
to strike across in air the full length of the spark, the vacuum was considered nearly 
perfect. Warming the potash with a spirit-flame, at any time, will cause it to give off 
sufficient aqueous vapour to allow the spark to pass as a cloud-like luminosity. This 
will be gradually absorbed until, in the course of from a few 7 days to a few weeks, the 
vacuum will again cease to conduct. 
54. It is very difficult to get the vacuum in large tubes so nearly absolute as not to 
* When the pump is working in a very good vacuum, the friction of the falling mercury produces a very 
beautiful effect in the dark. Brilliant points .of light flash about wherever the mercury-drops are splashed 
from side to side, and the pump is frequently illuminated with a phosphorescent glow filling all the tubes. 
MDCCCLXXIV. 3 Z 
