388. Upon warming the tin case and removing the piece of ice, it was found
that a cork having slipped, one of the edges of the platina had been all
but in contact with the inner surface of the tin vessel; yet,
notwithstanding the extreme thinness of the interfering ice in this place,
no sensible portion of electricity had passed.

389. These experiments were repeated many times with the same results. At
last a battery of fifteen troughs, or one hundred and fifty pairs of
four-inch plates, powerfully charged, was used; yet even here no sensible
quantity of electricity passed the thin barrier of ice.

390. It seemed at first as if occasional departures from these effects
occurred; but they could always be traced to some interfering
circumstances. The water should in every instance be well-frozen; for
though it is not necessary that the ice should reach from pole to pole,
since a barrier of it about one pole would be quite sufficient to prevent
conduction, yet, if part remain fluid, the mere necessary exposure of the
apparatus to the air or the approximation of the hands, is sufficient to
produce, at the _upper surface_ of the water and ice, a film of fluid,
extending from the platina to the tin; and then conduction occurs. Again,
if the corks used to block the platina in its place are damp or wet within,
it is necessary that the cold be sufficiently well applied to freeze the
water in them, or else when the surfaces of their contact with the tin
become slightly warm by handling, that part will conduct, and the interior
being ready to conduct also, the current will pass. The water should be
pure, not only that unembarrassed results may be obtained, but also that,
as the freezing proceeds, a minute portion of concentrated saline solution
may not be formed, which remaining fluid, and being interposed in the ice,
or passing into cracks resulting from contraction, may exhibit conducting