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306 FRAGMENTS OF SCIENCE.

been carried mechanically into the experimental tube. Precautions were
therefore taken to prevent any such transfer. They produced little or no
mitigation. I ’did not imagine at the time that the dust of the external
air could find such free passage through the caustic potash and the sul-
phuric-acid tubes. But the mot-es really came from without. They also
passed with freedom through a variety of ethers and alcohols. In fact,
it requires long-continued action on the part of an acid first to wet the
motes and afterward to destroy them. By carefully passing the air
through the flame of a spirit-lamp or through a platinum tube heated to
bright redness, the floating matter was sensibly destroyed. It was there-
fore combustible, in other words, organic matter. I tried to intercept it
by a large reSpirator of cotton-wool. Close pressure was necessary to
render the wool effective. A plug of the wool 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 afterward 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 pr0per
care, but not without a great deal of searching out of disturbances, the
experimental tube, even when filled with pure air or vapor, contains
nothing competent to scatter the light. The Space within it has the as-
pect 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 the figure on page 307. S S’ is the glass ex-
perimental 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, con-
taining the liquid Whose vapor 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, tt’, containing a tolerably
tightly-fitting plug of cotton-wool. To save the air-pump gauge from the
attack of such vapors as act on mercury, as also to facilitate observa-
tion, a separate barometer tube was employed.

Through the cork which st0ps the flask F two glass tubes, a and 6, pass
air-tight. The tube a ends immediately under the cork ; the tube 6, on
the contrary, descends to the bottom of the flask and dips into the liquid.
The end of the tube 5 is drawn out so as to render very small the orifice
through Which the air escapes into the liquid.

The experimental tube S 8' being exhausted, a cock at the end S’ is

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