educe the annular space round the
tube, and will have a choking effect on the flame--much the same as
referred to above, when there is too much gas and not enough air. A
simple method of lining the chimney is to cut a block of wood to the
inside dimensions of the chimney, less 1/4 in. in width and thickness,
then soften the asbestos cardboard by immersing in water, and bend it
round the wood, cutting off to the required size, _i.e._, till the two
edges form a neat butt joint. It can be allowed to remain on the mould
until dry--when it will retain its shape--or can be put into the chimney
straight away, if it is wanted for use immediately. In the latter case,
however, it will be some fifteen minutes or so before the tube will
attain its working temperature. Asbestos linings gradually become worn
and ragged, and small flakes are apt to detach themselves and fall down
into the burner, which, of course, prevents the flame playing as it
should around the tube. In such cases it is not always necessary to fit
a new lining; if the chimney is removed, the loose flakes shaken out and
the asbestos well damped and patted down with a wooden or steel
foot-rule or other suitably shaped tool, it will be fit for another long
spell of work.

The nickel or hecknum tubes are treated in the same manner as the iron,
but, as we mentioned before, are more durable, but require more heating
to get them up to a workable temperature. Their greater first cost is
compensated to some extent by makers in some cases guaranteeing them for
six months.

Of the porcelain ignition devices, we will deal with the double-ended
tube first, it being the more commonly used of the two in this country.
This form of tube is usually about 3 in. long, 1/2 in. diameter, and
open at both ends. It may be mounted in a metal casting, in form not
unlike the small gas stoves for heating soldering irons. It is heated
the greater part of its length by a couple of rows of gas jets, and is
frequently surrounded by an asbestos lining. The whole arrangement is
in reality a tiny furnace. When in position for working, one end of the
tube is open to the ignition passage leading and communicating with the
combustion chamber, while the other end is sealed, through butting up
against a metal cap or plate. An asbestos washer is interposed between
the tube at each end and the metal it bears against, thus making a more
or less flexible joint. A thumb screw is arranged at the outside end o