with the blue flame of gas-stoves where heat, and
not light, is required, the blue flame in both cases being caused by the
admixture of air with the gas.

Thus, in order that gas may give the best illumination, we must increase
the yellow or white space of carbon particles at a white heat, and a
burner that will do this, and at the same time hold the balance so that
unconsumed particles of carbon shall not escape in the way of smoke, will
give the most successful illuminating results. With this end in view the
addition of albo-carbon to a bulb in the gas-pipe has proved very
successful, and the incandescent gas-jet is constructed on exactly the
same chemical principle. The invention of burners which brought about
this desirable end has doubtless not been without effect in acting as a
powerful obstacle to the widespread introduction of the electric light.

Without entering into details of the manufacture of gas, it will be as
well just to glance at the principal parts of the apparatus used.

The gasometer, as it has erroneously been called, is a familiar object to
most people, not only to sight but unfortunately also to the organs of
smell. It is in reality of course only the gas-holder, in which the final
product of distillation of the coal is stored, and from which the gas
immediately passes into the distributing mains.

The first, and perhaps, most important portion of the apparatus used in
gas-making is the series of _retorts_ into which the coal is placed, and
from which, by the application of heat, the various volatile products
distil over. These retorts are huge cast-iron vessels, encased in strong
brick-work, usually five in a group, and beneath which a large furnace is
kept going until the process is complete. Each retort has an iron exit
pipe affixed to it, through which the gases generated by the furnace are
carried off. The exit pipes all empty themselves into what is known as
the _hydraulic main_, a long horizontal cylinder, and in this the gas
begins to deposit a portion of its impurities. The immediate products of
distillation are, after steam and air, gas, tar, ammoniacal liquor,
sulphur in various forms, and coke, the last being left behind in the
retort. In the hydraulic main some of the tar and ammoniacal liquor
already begin to be deposited. The gas passes on to the _condenser_,
which consists of a number of U-shaped pipes. Here the impurities are
still further condensed out, and are collected in t