It has not been thought necessary to include in the tabular
statement electric arc-lamps, as they are only suitable for the lighting
of large spaces, where the steadiness and uniformity of the illumination
are of secondary importance. Under such conditions, it may be stated
parenthetically, the electric arc-light is much less costly than
acetylene lighting would be, but it is now in many places being
superseded by high-pressure gas or oil incandescent lights, which are
steady and generally more economical than the arc light.
The illuminant which acetylene is best fitted to supersede on the score
of convenience, cleanliness, and hygienic advantages is oil. By oil is
meant, in this connection, the ordinary burning petroleum, kerosene, or
paraffin oil, obtained by distilling and refining various natural oils
and shales, found in many countries, of which the United States
(principally Pennsylvania), Russia (the Caucasus chiefly), and Scotland
are practically the only ones which supply considerable quantities for
use in Great Britain. Attempts are often made to claim superiority for
particular grades of these oils, but it may be at once stated that so for
as actual yield of light is concerned, the same weight of any of the
commercial oils will give practically the same result.
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