AS OBSEEVED IN TOEEICELLIAN AND OTHEE VACUA. 
149 
tion to these, in one tube, No. 66, small crystals of Sulphite were placed, and in the other. 
No. 67, pieces of Selenium. The tubes were then charged with carbonic acid, and 
exhausted as before. No. 66, on the 21st of June, showed clear narrow stratifications, 
the potash was then heated; on the 26th the stratifications had increased, assuming the 
conical form, clear and defined ; some of the sulphur was melted, condensing at each end 
of the tube ; as it cooled the stratifications which had been destroyed reassumed their 
conical form. The stratifications improved very slowly, and on the 14th of July they 
assumed the large cloud-like appearance, similar to those of the mercury vacuum (63). 
The caustic potash was then again heated, and the stratifications became very narrow ; 
from this time they gradually improved, and on the 28th, on trying the discharge, I 
found that it no longer passed. The potash was then heated and the discharges con- 
tinued, the tube shortly became illuminated ; this was followed by large cloud-like strati- 
fications, then by the conical and narrow; on allowing the potash to cool, the tube 
resumed its insulating state. 
87. Selenium, No. 67. — From the 12th to the 19th of June the discharge was a wave 
line ; on the 20th the potash was heated; on the 22nd stratifications appeared of a greenish 
tinge, inclined to the conical form; from the 23rd to the 28th they assumed the cloud- 
like character; on the 28th the selenium was melted ; the stratifications were then of a 
reddish tinge, but they remained nearly of the same form, the vapour of the selenium con- 
densing very quickly in the cooler portions of the tube. On the 6th of July the end of 
the tube was placed in a freezing mixture, and the tube was heated in order to free the 
sides thereof from a quantity of selenium which had deposited and thus obscured the 
view of the discharge. On the following day (7th July) the discharge illuminated the 
tube without stratification, and on grasping the tube with the hand both wires were 
surrounded with the intense blue discharge (84), which, under ordinary conditions, only 
appears at the negative. On the 11th of July, on heating the caustic potash, the blue 
phosphorescence at the positive wire was destroyed, and the cloud-like stratifications 
reappeared. This tube was subsequently placed aside, after having been heated two or 
three times, for a month, when, on trying the discharge on the 4th of October, I ascer- 
tained that the discharge no longer passed, the vapour which had evolved by heating the 
caustic potash having been again absorbed. 
88. When a vacuum-tube is in a state to show the stratified discharge, it is so good a 
conductor that sparks from the outer terminal of the induction coil will pass to one of 
its wires (the other being attached to the inner terminal) one inch in length through air ; 
but when no signs of a luminous discharge can be observed through the tube, or merely 
faint luminosity without stratifications, the spark is scarcely visible, although it will pass 
from the outer terminal of the coil through air to a wire attached to a glass rod, or to 
one insulated by shell-lac, showing that a vacuum-tube in this state is a more perfect 
insulator of the electrical discharge than air. The following experiment will perhaps be 
considered as conclusive as to vacua under this condition insulating the discharge. 
Fig. 9 represents two tubes and a galvanometer attached to the induction coil C. 
