289 
while in vacuo. Then applying (still under the mercury) 
the disc, the bell rises by its flotation, till this is balanced 
by the weight of mercury which is raised in it. If, however, 
it be placed under a receiver, on exhaustion the bell rises 
about four inches, leaving a Torricellian vacuum within, 
through which, by bringing the sliding rod of the receiver in 
contact with the platinum wire, discharges can be made. A 
hole 0'2 diameter, and 1'-0 long, is drilled in the apex of the 
cast-iron core, by inserting in which a miniature jar of quill- 
tube, filled with a known bulk of any gas, before applying 
the covering disc, this will escape when the bell-glass rises, 
and thus enable one to experiment in a vacuum of that gas at 
any required attenuation. 
‘* With respect to the next in interest of these facts, the 
existence of the luminous menisci, I am unable to form an 
opinion as to whether the differences which I have mentioned 
arise from specific qualities of the gases, or merely from the 
degree of density. The decided manifestation of them in 
hydrogen would seem to imply the latter. Ifso, air at 0.06 
should show the same as I have described for hydrogen at 
0°85 and at 0:005, as the other at 0°07. The latter exhaus- 
tion will require a better pump than mine to try it: but in the 
first the hydrogen shows the phenomena far more distinct 
than the air, and the same thing is true of carbonic acid, 
notwithstanding its high specific gravity. 
‘¢ My present notion of these menisci and their divisions 
is, that they are surfaces of interference. The fact of their 
being produced by a single discharge shows that they do not 
depend on the discontinuity of the current (unless, indeed, 
that single discharges may be a succession of waves); and 
the absence of the negative blue light seems equally to show 
that they do not result from zones of alternating electric con- 
dition in the medium. 
‘‘ The colour is related to the nature of the medium; but 
the rotation of the positive portion of the light, the quies- 
cence of the negative, and its invariable division into two 
