Cohesion.

_Liquid._--When matter is in a liquid condition, as, for example, water
and oil, the condition of its molecules are not so fixed and stable as
they are in the solid state. The molecules can move freely about one
another, and their freedom is increased compared with their condition
when in the solid state.

As already indicated, the reduction of a solid body to a liquid or
molten state may be effected by heat. When heat is applied to a solid
body, several results follow, each of which is the outcome of the other.

1. There is an increase of temperature which is due to the increased
energy of the molecules, through the added heat.

2. There is an enlargement of the volume or size of the body, and if the
addition of heat be continued, the molecular forces which hold the
molecules together are broken down, and then the molecules, loosened
from those forces which in the solid state have bound them together,
begin to move about with greater freedom, and thus give rise to the
molten condition of metals, or liquid condition of water. Thus, it is
the heat which has set the atoms which compose the molecules in motion.
The atoms of the solid have absorbed the heat, and the heat which has
thus been absorbed has imparted vibratory energy to the atoms, which
they did not possess before. Now when a substance is in the liquid
state, the atoms of that substance have not only a vibratory motion, but
have also a translatory motion, so that they can move in and out among
one another. This is proved by the phenomenon of diffusion, where we
have the case of two different-coloured liquids, for example,
intermingling with each other, which is conclusive evidence of the
translatory motion of the atoms in liquids.

_Gaseous._--The third state in which matter is found is the gaseous
state. In this condition, the particles of matter which form the gas
have the greatest possible freedom of movement, and are able to move
about with inconceivable velocity. There is abundant evidence to prove
that gases consist of particles of matter which are perfectly free, and
are able to fly about in all directions. The simplest proof is obtained
by mixing two gases together, as, for example, when any gaseous
substance is allowed to mix with the air of a room, when we find that
the particular gas soon mixes itself thoroughly with all the air in the
room. This process of mixing is known as Diffusion, and the lighter a
gas is, the more quickly does