CHAPTER LV. ILLUMINATING GAS.

304. Source.--The three main elements in combustion are O, H, C.
Air supplies O, the supporter; C and H are usually united, as
hydro-carbons, in luminants and combustibles. H gives little
light in burning; C gives much. The fibers of plants contain
hydro-carbons, and by destructive distillation these are
separated, as gases, from wood and coal, and used for
illuminating purposes. Mineral coal is fossilized vegetable
matter; anthracite has had most of the volatile hydro-carbons
removed by distillation in the earth; bituminous and cannel coals
retain them. These latter coals are distilled, and furnish us
illuminating gas.

Experiment 129.--Put into a t.t. 20 g. of cannel coal in fine
pieces. Heat, and collect the gas over H2O. Test its
combustibility. Notice any impurities, such as tar, adhering to
the sides of the t.t., or of the receiver after combustion. Try
to ignite a piece of cannel coal by holding it in a Bunsen flame.
Is it the C which burns, or the hydrocarbons? Distil some wood
shavings in a small ignition-tube, and light the escaping gas.

305. Preparation and Purification.--To make illuminating gas,
fire-clay retorts filled with coal are heated to 1100 degrees or
more, over a fire of coke or coal. Tubes lead the distilled gas
into a horizontal pipe, called the hydraulic main, partly filled
with water, into which the ends of the gas-pipe dip. The gas then
passes through condensers consisting of several hundred feet of
vertical pipe, through high towers, called washers, in which a
fine spray Fig. 60. Gas Works.

A, furnace; C, retorts containing coal; T, gas-tubes leading to
B, the hydraulic main; D, condensers; O, washers, with a spray of
water, and sometimes coke; M, purifiers-ferric oxide or lime; G,
gas-holder. In C remain the coke and gas carbon. At B, D, E, and
O, coal tar, H2O, NH3, CO2, and SO2 are removed. At M are taken
out H2S and CO2.of water falls, into chambers with shelves
containing the purifiers CaO or hydrated Fe2O3, and finally into
a gas-holder, whence it is distributed. At the hydraulic main,
condensers, washers, and purifiers, certain impurities are
removed froth the gas. Coke is the solid C residue after
distillation. Gas-carbon, also a solid, is formed by the
separation of the heavier hydro-carbons at high temperature, and
is deposited on the sides of the retort.

Coal gas, as it leaves the retort, has many impurities. It is
accompanied with about 3 its weight of coal tar, 1/2 its weight
of H2O vapor, 1/50 NH3, 1/20 CO2, 1/20 to 1/50 H2S, 1/300 to
1/600 S in other forms. The tar is mostly taken out at the
hydraulic main, which also withdraws some H2O with other
impurities in solution. The condensers remove the rest of the
tar, and the H2O, except what is necessary to saturate the gas.
At the main, the condensers, and the washers, NH3 is abstracted,
CO2 and H2S are much reduced, and the other S compounds are
diminished. Lime purification removes CO2 and H2S, and, to some
extent, other S compounds. Iron purification removes H2S. Fe2O3 +
3 H2S = 2 FeS + S + 3 H2O.

The FeS is revivified by exposure to the air. 2 FeS + O3 = Fe2O3
+ 2S. It can then be used again. H2S, if not separated, burns
with the gas, forming H2S03, which oxidizes in the air to H2SO4;
hence the need of removing it. CO2 diminishes the illuminating
power.

306. Composition.--Even when freed from its impurities coal-gas
is a very complex mixture, the chief components being nearly as
follows:--


Percent Diluents, having little C, give
H 45) very little light. Notice the small
CH, 41) diluents. percentage of luminants, or light-
CO 5 ) giving compounds, also the proportion
C,HB 1.3) of C to H in them.
C,H6 1.2)luminants.
CZH4 2.5) Cannel coal contains more of
C02 2) impurities. the heavy bydro-carbons, CnH2n,
N, etc. 2) etc., than the ordinary bituminous
100 coal. Ten per cent of the coal should be
cannel; naphtha is, however, often employed to subserve the same
purpose, one ton of ordinary bituminous coal requiring four gallons
of oil.

In Boston, 7,000,000 cubic feet of gas have been burned in one
day, consuming 500 tons of coal; the average is not more than
half that quantity. Of the other products, coke is employed for
heating purposes, gas carbon is used to some extent in electrical
work, and coal-tar is the source of very many artificial products
that were formerly only of natural origin. NH3, is the main
source of ammonium salts, and S is made into H2SO4.

307. Natural Gas occurs near Pittsburg, Pa., and in many other
places, in immense quantities. It is not only employed to light
the streets and houses, but is used for fires and in iron and
glass manufactories. It is estimated that 600,000,000 cubic feet
are burned, saving 10,000 tons of coal daily in Pittsburg, Only
half a dozen factories now use coal. More than half the gas is
wasted through safety valves, on account of the great pressure on
the pipes as it issues from the earth.

These reservoirs of natural gas very frequently occur in
sandstone, usually in the vicinity of coal-beds, but sometimes
remote from them. In all cases the origin of the gas is thought
to be in the destructive distillation, extending through long
geological periods, of coal or of other vegetable or animal
matter in the earth's interior.

Natural gas varies in composition, and even in the same well,
from day to day; it consists chiefly of CH4, with some other