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Hence the cross-section of the sheath surrounds the dark
band as a darker ring.

Oxygen, hydrogen, nitrogen, carbonic acid, so prepared
as to exclude all floating particles, produce the darkness
when poured or blown into the beam. Coal-gas does the
same. An ordinary glass shade placed in the air with its
mouth downward permits the track of the beam to be seen
crossing it. Let coal-gas or hydrogen enter the shade by a
tube reaching to its top, the gas gradually fills the shade
from the top downward. As soon as it occupies the space
crossed by the beam, the luminous track is instantly abol-
ished. Lifting the shade so as to bring the common bound-
ary of gas and air above the beam, the track flashes forth.
After the shade is full, if it be inverted, the gas passes up-
ward like a black smoke among the illuminated particles.

The air of our London rooms is loaded with this organic
dust, nor is the country air free from its presence. However
ordinary daylight may permit it to disguise itself, a suffi-
ciently powerful beam causes dust suspended in air to ap~
pear almost as a semi-solid. Nobody could, in the first
instance, without repugnance, place the mouth at the illu-
minated focus of the electric beam and inhale the thickly-
massed dust revealed there. Nor is the repugnance abol-
ished by the reflection that, although we do not see the
floating particles, we are taking them into our lungs every
hour and minute of our lives.

The Germ-Theory 0f Contagious Disease.

There is no respite to this contact with the floating mat-
ter of the air 3 and the wonder is, not that we should suffer
occasionally from its presence, but that so small a portion
of it, and even that but rarely diffused over large areas,
should appear to be deadly to man. And what is this por-
tion? It was sometime ago the Current belief that epidemic

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