distillation of
petroleum or other mineral oil in retorts heated externally. The product
consists chiefly of methane and heavy hydrocarbons with a small amount
of hydrogen. In the early days of railways, some trains were not run
after dark and those which were operated were not always lighted. At
first attempts were made at lighting railway cars with compressed
coal-gas, but the disadvantage of this was the large tank required.
Obviously, a gas of higher illuminating-value per volume was desired
where limited storage space was available, and Pintsch turned his
attention to oil-gas. Gas suffers in illuminating-value upon being
compressed, but oil-gas suffers only about half the loss that coal-gas
does. In about 1880 Pintsch developed a method of welding cylinders and
buoys which satisfied lighthouse authorities and he was enabled to
furnish these filled with compressed gas. Thus the buoy was its own
gas-tank. He devised lanterns which would remain lighted regardless of
wind and waves and thus gained a start with his compressed-gas systems.
He compressed the gas to a pressure of about one hundred and fifty
pounds per square inch and was obliged to devise a reducer which would
deliver the gas to the burner at about one pound per square inch. This
regulator served well throughout many years of exacting service. The
system began to be adopted on ships and railroads in 1880 and for many
years it has served well.

Although gas-lighting has affected the activities of mankind
considerably by intensifying commerce and industry and by advancing
social progress, the illuminants which eventually took the lead have
extended the possibilities and influences of artificial light. In the
brief span of a century civilized man is almost totally independent of
natural light in those fields over which he has control. What another
century will bring can be predicted only from the accomplishments of the
past. These indicate possibilities beyond the powers of imagination.

IX

THE ELECTRIC ARCS

Early in 1800 Volta wrote a letter to the President of the Royal Society
of London announcing the epochal discovery of a device now known as the
voltaic pile. This letter was published in the Transactions and it
created great excitement among scientific men, who immediately began
active investigations of certain electrical phenomena. Volta showed that
all metals could be arranged in a series so that each one would indicate
a positive electr