with
one of the poles of a dry battery, the other pole of which is connected
to the metal case of the clock for the purpose of actuating an ordinary
magnet alarm bell. In the centre of each of the holes in the plate a
metal rod is fixed, which then passes through an insulating ring and
projects inside the clock face, where it makes contact with the hour
hand. The clock is mounted on a heavy base, with a key-board containing
20 numbered plugs. If one of the plugs is inserted in a hole in the
plate it makes contact with the rod, and when the hour hand of the clock
touches the other end the circuit is completed and the bell starts
ringing. The period of this friction contact is approximately 20
seconds. The clock can therefore be used for electrically noting the
periods of time from one minute by multiples of one minute up to one
hour.

[Illustration: FIG. 31.--Electric signal timing clock.]

~Filtration.~--(a) _Cotton-wool Filter._--Practically the only method in
use in the laboratory for the sterilisation of air or of a gas is by
filtration through dry cotton-wool or glass-wool, the fibres of which
entangle the micro-organisms and prevent their passage.

Perhaps the best example of such a filter is the cotton-wool plug which
closes the mouth of a culture tube. Not only does ordinary diffusion
take place through it, but if a tube plugged in the usual manner with
cotton-wool is removed from the hot incubator, the temperature of the
contained air rapidly falls to that of the laboratory, and a partial
vacuum is formed; air passes into the tube, through the cotton-wool
plug, to restore the equilibrium, and, so long as the plug remains dry,
in a germ-free condition. If, however, the plug becomes moist, either by
absorption from the atmosphere, or from liquids coming into contact with
it, micro-organisms (especially the mould fungi) commence to multiply,
and the long thread forms rapidly penetrate the substance of the plug,
and gain access to and contaminate the interior of the tube.

[Illustration: FIG. 32.--Cotton-wool air filter.]

METHOD.--

If it is desired to sterilise gases before admission to a vessel
containing a pure cultivation of a micro-organism, as, for instance,
when forcing a current of oxygen over or through a broth cultivation of
the diphtheria bacillus, this can be readily effected as follows:

1. Take a length of glass tubing of, say, 1.5 cm. diameter, in the
centre of which a bulb has been blown,