he electrodes of a battery,
the other electrode being placed in the acid. When the mercury is made
positive it becomes dull and spreads itself out; when it is made
negative it gathers itself together and becomes bright again. G.
Lippmann, who has made a careful investigation of the subject, finds
that exceedingly small variations of the electromotive force produce
sensible changes in the surface-tension. The effect of one of a
Daniell's cell is to increase the tension from 30.4 to 40.6. He has
constructed a capillary electrometer by which differences of electric
potential less than 0.01 of that of a Daniell's cell can be detected by
the difference of the pressure required to force the mercury to a given
point of a fine capillary tube. He has also constructed an apparatus in
which this variation in the surface-tension is made to do work and drive
a machine. He has also found that this action is reversible, for when
the area of the surface of contact of the acid and mercury is made to
increase, an electric current passes from the mercury to the acid, the
amount of electricity which passes while the surface increases by one
square centimetre being sufficient to decompose .000013 gramme of water.

[The movements of camphor scrapings referred to above afford a useful
test of the condition of a water surface. If the contamination exceed a
certain limit, the scrapings remain quite dead. In a striking form of
the experiment, the water is contained, to the depth of perhaps one
inch, in a large flat dish, and the operative part of the surface is
limited by a flexible hoop of thin sheet brass lying in the dish and
rising above the water-level. If the hoop enclose an area of (say)
one-third of the maximum, and if the water be clean, camphor fragments
floating on the interior enter with vigorous movements. A touch of the
finger will then often reduce them to quiet; but if the hoop be
expanded, the included grease is so far attenuated as to lose its
effect. Another method of removing grease is to immerse and remove
strips of paper by which the surface available for the contamination is
in effect increased.

The thickness of the film of oil adequate to check the camphor movements
can be determined with fair accuracy by depositing a weighed amount of
oil (such as .8 mg.) upon the surface of water in a large bath.
Calculated as if the density were the same as in a normal state, the
thickness of the film is found to be about two million