In addition to coal gas made in the way just described, there is
another form of illuminating gas, in the manufacture of which coal is
indirectly employed. This gas, known as water gas, because it is formed
by the decomposition of water, is produced by passing steam over red
hot carbon, in the form of hard coal or coke. When this is done, the
hydrogen in the steam is set free and the oxygen combines chemically
with the carbon, to form the carbon monoxide, that was mentioned as
being present, in small proportions, in ordinary coal gas. This carbon
monoxide is poisonous, if much of it is breathed, and as it has no
odor it is difficult to detect when escaping. A number of deaths have
resulted from water gas for this reason, and in some states the laws
forbid its use for lighting purposes.
When water gas is used it must be enriched with some other substances
before it will yield much light. You have already learned that neither
hydrogen nor carbon monoxide burns with a bright flame, and you will
see that water gas must have something added to it to fit it for
lighting purposes. The substance usually added is the vapor of some
light, volatile oil, like gasoline. This vapor is composed of compounds
of carbon and hydrogen, and when it is mixed with the water gas it
forms a gas that yields a very satisfactory light; and that may be
produced more cheaply than common coal gas.
There remains one more form of illuminating gas which has been the
subject of much discussion in recent years, namely, acetylene. This is
a compound of carbon and hydrogen, in which there is twelve times as
much carbon as hydrogen. It has not been discovered recently, for it
was known early in the nineteenth century, but its possible use for
lighting purposes was not considered then.
Attention was directed to it a few years ago by the discovery of a
substance called calcium carbide. This is a compound of carbon and the
metal calcium, formed by heating to a very high temperature a mixture
of coal and lime. It has the peculiar property of decomposing, when
treated with water. The calcium present combines with the oxygen and
half the hydrogen of the water, to form common slacked lime or calcium
hydrate, while the carbon and the remainder of the hydrogen combine to
form acetylene gas.
The gas formed in this way needs no purifications before burning; it
can be produced in small generators, and the production can be checked
at any time. When burned in the proper form of burner it yields the
brightest of all gas flames. For these reasons it is adapted for use in
small villages and for lighting single houses. It is also frequently
used in magic lanterns, where a strong and steady light is necessary.
But the cost of producing acetylene in large quantities is greater than
that of coal gas, and it seems extremely unlikely that it will ever be
much used for lighting large cities and towns.
How the Light Gets Into the Electric Light Bulb.
The incandescent lamp was invented in 1879 and the patents were granted
to Thomas A. Edison. There were, however, a number of electrical men
who were working on the idea at this time who deserve a great deal of
credit for developing the lamp.
The incandescent lamp, which is used chiefly for house lighting,
consists of a glass bulb from which the air has been exhausted by
pumps and chemical processes--in which there is a thin filament of
tungsten metal wound on what is called an arbor (as shown in Fig. 4).
This filament opposes high resistance to the passage of the current
of electricity, and, consequently, is heated to incandescence when
a current passes through it. The removal of the air from the bulb
prevents the tungsten metal from burning up, as it would do if oxygen
were present.
The filaments of the first lamps were made of vegetable fibre. The next
development was the cellulose process, which is still used in carbon
and metallized lamps, although a number of processes are used now which
improve the filament considerably.
The discovery that tungsten metal could be used in incandescent lamps
was made in 1906. The first tungsten lamp manufactured in America was
made in 1907.
[Illustration: THE DEVELOPMENT OF INCANDESCENT LAMPS