Untitled

CHA PTER Vii.
3CO.
JETS may be divided into three broad divisions, the oxycalcium,
the blow-through, and the mixed, to which we have
already referred (pp. 12 and 13, Figs. 7, 8, and 9). The
various features which these jets possess in common have
been considered ; it only remains to discuss those points in
which the types differ. The oxy-calcium, as being the
simplest and the one which yields the least amount of light,
will be first dealt with.
The oxy-calcium jet, as it is generally styled (Fig. 7),
should be called more properly the oxygen-alcohol jet, since
all limelight jets would reasonably conic under the title oxycalcium,
meaning one employing oxygen and lime. This form
of the limelight does not give anything like so much light as the
blow-through jet, or still more as the mixed jet, but is nevertheless
a simple and convenient illuminant, and one much
more brilliant than any oil lamp. It consists of a cylindrical
reservoir, from which a pipe leads to the front of the jet and
terminates in an upright tube containing a cotton wick.
Just behind this is the lime pin. The oxygen is supplied by
means of a tube which is fixed under the reservoir, and
which passes along close to the other tube and bends round
in front of it, terminating in a nozzle. The reservoir is filled
with methylated spirit, the wick-holder being filled with
straight lengths of the loose cotton wick supplied for the
purpose, which should not be packed too closely together, and
the ends of which should stand out about a quarter of an
4
inch above the top of the wick holder. A soft lime having beer
put upon the pin and the lamp lighted, when it is seen thal
the flame is burning properly, the oxygen can be turned slowl)
on until the best light is produced. With this, as indeed with
all jets, it will be found that on turning on the oxygen there is
a point beyond which the light, instead of getting more brilliant,
positively decreases with the increase of oxygen. This is due
to the rush of cold gas, more than is required for the purposes
of combustion, which cools the flame. When, therefore, a jet
is burning badly, it should be seen that it is not getting too
much oxygen, nor, for the matter of that, too little either.
The form of the oxy-alcohol light, shown in Fig. 7, was
designed by Mr. E. G. Wood, and is more elaborate than
many oxy-calcium jets, but the principle in all is the same.
In this case the lime is provided with a shield, which
protects it from the currents of cold air in the lantern, and
the wick chamber is annular or ring-shaped, the oxygen
passing up the centre and being blown into the middle of
the flame. This lime shield can be applied to any other
form of limelight, and its use is advocated by some, who
maintain that more light is obtained with than without it.
The blow-through jet is often spoken of as the "safety,"
and sometimes as the "separate," jet, because, as originally
designed (shown diagrammatically in Fig. 29), the two gases
being kept in separate tubes throughout and only mixed in
the flame itself, by no mischance could either tube contain
Fig. 29. EARLY FORM
OF BLOW-THROUGH Jr.T.
JETS. 39
Fig. 30.
BLOW-THROUGH JET.
a mixture of the two gases. Blow-through jets at the
present time are usually constructed in the manner shown in
Fig. 30, where it will be seen that the jet of oxygen is blown
right into the middle of the coal-gas flame. Such a jet is as
little likely to allow of any mixture in it as Fig. 29, but when