Many physical treatises still assert that moist air conducts electricity,
though Silberman and others have proved the contrary. An interesting
experiment bearing on this has been described lately by Prof. Marangoni.
Over a flame is heated some water in a glass jar, through the stopper of
which passes a bent tube to bell-jar (held obliquely), which thus gets
filled with aqueous vapor. The upper half of a thin Leyden jar charged is
brought into the bell-jar, and held there four or five seconds; it is
then found entirely discharged. That the real cause of this, however, is
condensation of the vapor on the part of the glass that is not coated with
tin foil (the liquid layer acting by conduction) can be proved; for if that
part of the jar be passed several times rapidly through the flame, so as
to heat it to near 100° C., before inserting in the bell-jar, a different
effect will be had; the Leyden jar will give out long sparks after
withdrawal. This is because the glass being heated no longer condenses the
vapor on its surface, and there is no superficial conduction, as in the
previous case.
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