An apparatus has been devised to measure the velocity of ultrasonic vibrations through liquids at temperatures up to 1000° C by a phase-interference method. Velocities of sound through molten salts have been measured up to a maximum temperature of 1000° C for LiCl, NaCI, KCl, CsCl, CdCl
2
, LiBr, KBr, CsBr, NaI, KI, LiNO
3
, NaNO
3
and KNO
3
. Traces of water do not affect the velocities, which increases linearly with
T
½
The isothermal compressibility (
β
T
) increases approximately linearly with the square of the mean radius of the ions of the salts,
β
T
increases with increase of temperature. Increase of the ratio of the specific heats,
C
P
/
C
V
, and decrease of
C
V
occur upon increase of the size of
X
for a given
M
in a series
MX
and vice versa. The free volumes (
V
f
) increase with the increasing particle size and increase of temperature. From
β
T
the internal pressures are calculated and these enable a satisfactory equation of state to be evaluated. Approximate values of the repulsive exponents in the equation for the energy of the liquid lattice vary from 9·9 (LiBr) to 12·5 (KBr). Comparison of the free volumes with the changes in volume on melting, ∆
V
, indicate the major role played by holes in the structure of these liquids. This is confirmed by the decrease of the co-ordination number observed on melting. A detailed model for the ionic liquid is proposed and gives excellent agreement with experiment in respect of calculations of compressibility and expansivity. A modification of the sound-velocity method for the calculation of free volumes is suggested, and the values of
V
f
so calculated are compared with those calculated from a method involving the equation of state and the internal pressure values. A relation between the hole volume and free volume is adduced and lends support to the model for molten salts developed. Consideration of the temperature coefficient of free volume gives conclusive evidence concerning the rate-determining step in viscous flow. The cell properties of the model are utilized together with the free volumes to calculate partition functions and evaluations using these of heat capacities and entropy changes are in agreement with experiment.
