I am most grateful to the Syndics of the Cambridge University Press for inviting me to prepare a third edition of Gases, Liquids and Solids. The basic structure is unchanged. The main theme is that the three primary states of matter are the result of a competition between thermal energy and intermolecular forces. The second motif is that a whole range of properties e.g. the specific heat capacity of solids, the thermal conductivity of nonmetals, the elastic modulus of rubber, thermal expansion, surface tension, the viscosity of gases and liquids, osmotic pressure, the adiabaticity of sound waves in air, the dielectric properties of gases, liquids and solids, van der Waals forces between solid bodies, the hardness of metals, may be understood in terms of simple models and unsophisticated mathematics.

Few changes have been made in the early chapters on the properties of gases. In dealing with solids I have added short sections on the structure of surfaces and the phenomenon of surface melting and have extended the treatment of the elastic properties of crystalline solids and of rubber. In liquids there are further elaborations of the theory of viscosity and its application to the behaviour of lubricating oils.

But the main change in the book is the recognition that there are other states of matter which are of great scientific interest and which impinge on many aspects of everyday life.