The supply of compressed gas in metal cylinders has now assumed the
proportions of an important industry, more especially since it was
found possible, by the Brin process, to obtain oxygen direct from the
atmosphere. The industry is not exactly a new one, for carbon dioxide
and nitrous oxide (the latter for the use of dentists) have been
supplied in a compressed state for many years. Now, with the creation
of the modern amateur photographer, who can make lantern slides, and
the more general adoption of the optical lantern for the purposes of
demonstration and amusement, there has arisen a demand for the
limelight such as was never experienced before, and as the limelight
is dependent upon the two gases, hydrogen and oxygen, for its support,
these gases are now supplied in large quantities commercially. At
first the gas cylinders were made of wrought iron; they were cumbrous
and heavy, and the pressure of the inclosed gas was so low that a
receptacle to hold only ten feet was a most unwieldy concern. But
times have changed, and a cylinder of about the same size, but half
the weight, is now made to hold four times the quantity of gas at the
enormous initial pressure of 1,800 pounds on every square inch.
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