bourhood of gas-works is still
allowed to become contaminated by the escape of impure compounds from the
various portions of the gas-making apparatus. Go where one may, the
presence of these compounds is at once apparent to the nostrils within a
none too limited area around them, and yet their deleterious effects can
be almost reduced to a minimum by the use of proper purifying agents, and
by a scientific oversight of the whole apparatus. It certainly behoves
all sanitary authorities to look well after any gas-works situated within
their districts.

Now let us see what these first five products of distillation actually
are.

Firstly, house-gas. Everybody knows what house-gas is. It cannot,
however, be stated to be any one gas in particular, since it is a
mechanical mixture of at least three different gases, and often contains
small quantities of others.

A very large proportion consists of what is known as marsh-gas, or light
carburetted hydrogen. This occurs occluded or locked up in the pores of
the coal, and often oozes out into the galleries of coal-mines, where it
is known as firedamp (German _dampf_, vapour). It is disengaged wherever
vegetable matter has fallen and has become decayed. If it were thence
collected, together with an admixture of ten times its volume of air, a
miniature coal-mine explosion could be produced by the introduction of a
match into the mixture. Alone, however, it burns with a feebly luminous
flame, although to its presence our house-gas owes a great portion of its
heating power. Marsh-gas is the first of the series of hydro-carbons
known chemically as the _paraffins_, and is an extremely light substance,
being little more than half the weight of an equal bulk of air. It is
composed of four atoms of hydrogen to one of carbon (CH_{4}).

Marsh-gas, together with hydrogen and the monoxide of carbon, the last of
which burns with the dull blue flame often seen at the surface of fires,
particularly coke and charcoal fires, form about 87 per cent. of the
whole volume of house-gas, and are none of them anything but poor
illuminants.

The illuminating power of house-gas depends on the presence therein of
olefiant gas (_ethylene_), or, as it is sometimes termed, heavy
carburetted hydrogen. This is the first of the series of hydro-carbons
known as the _olefines_, and is composed of two atoms of carbon to every
four atoms of hydrogen (C_{2}H_{4}). Others of the olefines are present
in minute quant