l, rammed
pretty tightly into the tube through which the air passed, was finally
found competent to hold back the motes. They appeared from time to
time afterwards, and gave me much trouble; but they were invariably
traced in the end to some defect in the purifying apparatus--to some
crack or flaw in the sealing-wax employed to render the tubes
air-tight. Thus through proper care, but not without a great deal of
searching out of disturbances, the experimental tube, even when filled
with air or vapour, contains nothing competent to scatter the light.
The space within it has the aspect of an absolute vacuum.

An experimental tube in this condition I call _optically empty_.

The simple apparatus employed in these experiments will be at once
understood by reference to a figure printed in the last article (Fig.
3.) s s' is the glass experimental tube, which has varied in length
from 1 to 5 feet, and which may be from 2 to 3 inches in diameter.
From the end s, the pipe pp' passes to an air-pump. Connected with
the other end s' we have the flask F, containing the liquid whose
vapour is to be examined; then follows a U-tube, T, filled with
fragments of clean glass, wetted with sulphuric acid; then a second
U-tube, T, containing fragments of marble, wetted with caustic potash;
and finally a narrow straight tube t t', containing a tolerably
tightly fitting plug of cotton-wool. To save the air-pump gauge from
the attack of such vapours as act on mercury, as also to facilitate
observation, a separate barometer tube was employed.

Through the cork which stops the flask F two glass tubes, a and b,
pass air-tight. The tube a ends immediately under the cork; the tube
b, on the contrary, descends to the bottom of the flask and dips into
the liquid. The end of the tube b is drawn out so as to render very
small the orifice through which the air escapes into the liquid.

The experimental tube s s' being exhausted, a cock at the end s' is
turned carefully on. The air passes slowly through the cotton-wool,
the caustic potash, and the sulphuric acid in succession. Thus
purified, it enters the flask F and bubbles through the liquid.
Charged with vapour, it finally passes into the experimental tube,
where it is submitted to examination. The electric lamp L placed at
the end of the experimental tube furnishes the necessary beam.

*****

The facts here forced upon my attention had a bearing too evident to
be overlooked. The inab