r plates of large surface,
so as to make the density exceedingly small. Otherwise, when it is
practicable, an oil condenser should be used in preference. In oil or
other liquid dielectrics there are seemingly no such losses as in
gaseous media. It being impossible to exclude entirely the gas in
condensers with solid dielectrics, such condensers should be immersed
in oil, for economical reasons if nothing else; they can then be
strained to the utmost and will remain cool. In Leyden jars the loss
due to air is comparatively small, as the tinfoil coatings are large,
close together, and the charged surfaces not directly exposed; but
when the potentials are very high, the loss may be more or less
considerable at, or near, the upper edge of the foil, where the air is
principally acted upon. If the jar be immersed in boiled-out oil, it
will be capable of performing four times the amount of work which it
can for any length of time when used in the ordinary way, and the loss
will be inappreciable.

It should not be thought that the loss in heat in an air condenser is
necessarily associated with the formation of _visible_ streams or
brushes. If a small electrode, inclosed in an unexhausted bulb, is
connected to one of the terminals of the coil, streams can be seen to
issue from the electrode and the air in the bulb is heated; if,
instead of a small electrode, a large sphere is inclosed in the bulb,
no streams are observed, still the air is heated.

Nor should it be thought that the temperature of an air condenser
would give even an approximate idea of the loss in heat incurred, as
in such case heat must be given off much more quickly, since there is,
in addition to the ordinary radiation, a very active carrying away of
heat by independent carriers going on, and since not only the
apparatus, but the air at some distance from it is heated in
consequence of the collisions which must occur.

Owing to this, in experiments with such a coil, a rise of temperature
can be distinctly observed only when the body connected to the coil is
very small. But with apparatus on a larger scale, even a body of
considerable bulk would be heated, as, for instance, the body of a
person; and I think that skilled physicians might make observations of
utility in such experiments, which, if the apparatus were judiciously
designed, would not present the slightest danger.

A question of some interest, principally to meteorologists, presents
itself here.