It is not too late. [Footnote: See "The Coal Resources of the
World," International Geological Congress, 1913.]
_Petroleum._--Its probable life may be lengthened beyond ninety years by
its restriction to lubricating and illuminating uses only and by the
prevention of its exportation. [Footnote: Van Hise, pp. 50-55.]
_Natural Gas._--Its flame is ephemeral at best, but its light may be kept
burning a little longer if the prodigious waste is prevented. During 1907
four hundred billion feet were consumed and almost as great an amount
wasted through uncontrolled wells, leaky pipes, etc. [Footnote: Van Hise,
p. 58.]
_Iron (and, in less measure, gold, silver, and other metals)_, whose life
does not, as coal and oil and gas, perish with the using, but some of
whose value is lost in the transformation from one state of use to
another, needs only to be more economically mined and used. [Footnote: Van
Hise, p. 68.] Non-metallic, inexhaustible materials, as stone, clay,
cement, should be employed in their stead when possible.
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