forced through the cotton-wool. When the current was interrupted,
and the air within the shade cooled, the returning air did not carry
motes along with it, being filtered by the wool. At the beginning of
this experiments the shade was charged with floating matter; at the
end of half an hour it was optically empty.

On the wooden base of a cubical glass shade, a cubic foot in volume,
upright supports were fixed, and from one support to the other 38
inches of platinum wire were stretched in four parallel lines. The
ends of the platinum wire were soldered to two stout copper wires
which passed through the base of the shade and could be connected with
a battery. As in the last experiments the shade rested upon
cotton-wool. A beam sent through the shade revealed the suspended
matter. The platinum wire was then raised to whiteness. In five
minutes there was a sensible diminution of the matter, and in ten
minutes it was totally consumed.

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

The Germ Theory of Contagious Disease.

There is no respite to our contact with the floating matter of the
air; 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 portion? It was some time ago the current belief
that epidemic diseases generally were propagated by a kind of malaria,
which consisted of organic matter in a state of motor-decay; that when
such matter was taken into the body through the lungs, skin, or
stomach, it had the power of spreading there the destroying process by
which itself had been assailed. Such a power was visibly exerte