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The large horizontal tube that you see before you is called
an “ experimental tube;” it is connected with our small
flask; between them, however, a stop-cock intervenes, by'
means of which the passage between the flask and the ex-
perimental tube can be Opened or closed at pleasure. The
other tube, passing through the cork of the flask and de-
scending into the liquid, is connected with a U-shaped ves-
sel, filled with fragments of clean glass, covered with sul-
phuric acid. In front of the U-shaped vessel is a narrow
tube stuffed with cotton-wool. At one end of the experi-
mental tube is our electric lamp; and here, finally, is an air-
pump, by means of which the tube has been exhausted.
We are now ready for experiment.

Opening the cock cautiously, the air of the room passes,
in the first place, through the cotton-wool, which holds
back the numberless organic germs and inorganic dust-
particles floating in the atmosphere. The air, thus cleansed,
passes into the U-shaped vessel, where it is dried by the
sulphuric acid. It then descends through the narrow tube
to the bottom of the little flask, and escapes there through
a small orifice into the liquid. Through this it bubbles,
loading itself to some extent with the nitrite-of-amyl vapor,
and then the air and vapor enter the experimental tube

The closest scrutiny would now fail to discover any
thing Within this tube; it is, to all appearance, absolutely
empty. The air and the vapor are both invisible. We
will permit the electric beam to play upon this mixture.
The lens of the lamp is so situated as to render the beam
slightly convergent, the3focus being formed in the vapor
at about the middle of the tube. You will notice that the
tube remains dark for a moment after the turning on of the
beam; but the chemical action will be so rapid that atten-
tion is requisite to mark this interval of darkness. .I ignite
the lamp; the tube for a moment seems empty; but sud-

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