Goodyear found that he could vary this "vulcanized rubber" at will. By
adding a little more sulfur he got a hard substance which, however,
could be softened by heat so as to be molded into any form wanted. Out
of this "hard rubber" "vulcanite" or "ebonite" were made combs,
hairpins, penholders and the like, and it has not yet been superseded
for some purposes by any of its recent rivals, the synthetic resins.

The new form of rubber made by the Germans, methyl rubber, is said to be
a superior substitute for the hard variety but not satisfactory for the
soft. The electrical resistance of the synthetic product is 20 per cent,
higher than the natural, so it is excellent for insulation, but it is
inferior in elasticity. In the latter part of the war the methyl rubber
was manufactured at the rate of 165 tons a month.

The first pneumatic tires, known then as "patent aerial wheels," were
invented by Robert William Thomson of London in 1846. On the following
year a carriage equipped with them was seen in the streets of New York
City. But the pneumatic tire did not come into use until after 1888,
when an Irish horse-doctor, John Boyd Dunlop, of Belfast, tied a rubber
tube around the wheels of his little son's velocipede. Within seven
years after that a $25,000,000 corporation was manufacturing Dunlop
tires. Later America took the lead in this business. In 1913 the United
States exported $3,000,000 worth of tires and tubes. In 1917 the
American exports rose to $13,000,000, not counting what went to the
Allies. The number of pneumatic tires sold in 1917 is estimated at
18,000,000, which at an average cost of $25 would amount to
$450,000,000.

No matter how much synthetic rubber may be manufactured or how many
rubber trees are set out there is no danger of glutting the market, for