ies and only in limited quantity.

148. Acetylene. In 1892 it was discovered that lime and coal fused
together in the intense heat of the electric furnace formed a
crystalline, metallic-looking substance called calcium carbide. As a
result of that discovery, this substance was soon made on a large
scale and sold at a moderate price. The cheapness of calcium carbide
has made it possible for the isolated farmhouse to discard oil lamps
and to have a private gas system. When the hard, gray crystals of
calcium carbide are put in water, they give off acetylene, a colorless
gas which burns with a brilliant white flame. If bits of calcium
carbide are dropped into a test tube containing water, bubbles of gas
will be seen to form and escape into the air, and the escaping gas may
be ignited by a burning match held near the mouth of the test tube.
When chemical action between the water and carbide has ceased, and gas
bubbles have stopped forming, slaked lime is all that is left of the
dark gray crystals which were put into the water.

When calcium carbide is used as a source of illumination, the crystals
are mechanically dropped into a tank containing water, and the gas
generated is automatically collected in a small sliding tank, whence
it passes through pipes to the various rooms. The slaked lime, formed
while the gas was generated, collects at the bottom of the tanks and
is removed from time to time.

The cost of an acetylene generator is about $50 for a small house, and
the cost of maintenance is not more than that of lamps. The generator
does not require filling oftener than once a week, and the labor is
less than that required for oil lamps. In a house in which there were
twenty burners, the tanks were filled with water and carbide but once
a fortnight. Acetylene is seldom used in large cities, but it is very
widely used in small communities and is particularly convenient in
more or less remote summer residences.

Electric Lights. The most recent and the most convenient lighting is
that obtained by electricity. A fine, hairlike filament within a glass
bulb is raised to incandescence by the heat of an electric current.
This form of illumination will be considered in connection with
electricity.

CHAPTER XVI

MAN'S WAY OF HELPING HIMSELF

149. Labor-saving Devices. To primitive man belonged more especially
the arduous tasks of the out-of-door life: the clearing of paths
through the wilderness; the hauling of ma