solid, liquid, and gaseous states of matter on the one hand, and
degrees of pressure and of heat on the other. Almost all, even the
most refractory, solids have been vaporised by the intense heat of the
electric arc; and the most refractory gases have been forced to assume
the liquid, and even the solid, forms by the combination of high
pressure with intense cold. It has further been shown that there is no
discontinuity between these states--that a gas passes into the liquid
state through a condition which is neither one nor the other, and that
a liquid body becomes solid, or a solid liquid, by the intermediation
of a condition in which it is neither truly solid nor truly liquid.

Theoretical and experimental investigations have concurred in the
establishment of the view that a gas is a body, the particles of which
are in incessant rectilinear motion at high velocities, colliding with
one another and bounding back when they strike the walls of the
containing vessel; and, on this theory, the already ascertained
relations of gaseous bodies to heat and pressure have been shown to be
deducible from mechanical principles. Immense improvements have been
effected, in the means of exhausting a given space of its gaseous
contents; and experimentation on the phenomena which attend the
electric discharge and the action of radiant heat, within the
extremely rarefied media thus produced, has yielded a great number of
remarkable results, some of which have been made familiar to the
public by the Gieseler tubes and the radiometer. Already, these
investigations have afforded an unexpected insight into the
constitution of matter and its relations with thermal and electric
energy, and they open up a vast field for future inquiry into some of
the deepest problems of physics. Other important steps, in the same
direction, have been effected by investigations into the absorption of
radiant heat proceeding from different sources by solid, fluid, and