Heat and the Various States of Aggregation
In all the states of aggregation there are attracting forces between neighbouring molecules and atoms. These are opposed by forces which are the results of the thermal agitations of the molecules. In solid substances there is a continuous exchange between the two, just as in a vibrating body there is a continuous exchange between its inertia and the elastic forces. As the distance from the zero position increases, however, the force of attraction between the molecules which is also the force tending to bring them back to their zero position must diminish in intensity. When the amplitude of vibration has reached a certain value, in other words, when the temperature of the body is sufficiently high, the above forces are not capable of arresting the motion of a particle and causing it to return to its zero position. When the liquid state has been reached the molecules are no longer constrained by forces tending to hold them in zero position.