I do not know where you are reading this, but I expect that you are close to some solids with extraordinary properties. This is true for me as I write this on my computer. Beneath my fingers is a keyboard made of plastics, some transparent and some opaque. In front of me is a screen made of glass, the reverse of which is coated with solids that glow under bombardment by electrons. Nearby are paper, cloth and wood. I can see metals, some at temperatures in excess of 2000 °C. And 50 centimetres away is a chip of patterned silicon which enables my computer to function. There is also the semi-solid matter I call my body, which possesses the capacity for abstract thought. This diversity of properties makes comparison with the simple models outlined in Chapter 6 particularly difficult. There are so many ‘special cases’ and ‘exceptions’ that it is unusual to find strict numerical agreement between theory and experimental data. However, we will find that often we can understand the order of magnitude of results, and the trends of the results from one material to another. With these limitations on our ambition in mind, let us see what Chapter 7 holds.

§7.2, §7.3, and §7.4 Density, Compressibility, Thermal expansivity: We will find that the density of solids reflects the type of bonding present; the compressibility tells us about the pair-potential between atoms; and thermal expansivity tells us about the asymmetry of the pair-potential .

§7.5 Speed of sound: Sound waves in solids are considerably more complex than in gases. However we relegate much of this complexity to Appendix A2 and focus on the trends in the data, allowing us to get a good feel for the factors involved.

§7.6 Heat capacity: The heat capacity of most solid elements at around room temperature can be understood using an exceptionally simple model of a solid. However, the temperature-dependence of the data, and the data which cannot be explained by this model, will lead us to a more realistic model, and a surprising connection to §7.5 on the speed of sound.

§7.7 Electrical properties: The difference in conductivity between metals and insulators is an amazing 20 orders of magnitude. Not surprisingly, we will use quite different models to understand the two categories of solids. More surprising is the simplicity of each of the models that we will use. For metals we will use the free electron model of a solid we discussed in §6.5, and for insulators we will ignore the bonding between the atoms almost entirely!

§7.8 Thermal conductivity: Although simple in principle, the data and the theory are surprisingly complex, and we attempt to understand only the temperature-dependence of the data.

§7.9 Optical properties: In Chapter 2 we said that the refractive index of a material is related to its dielectric constant. Using this result and Newton’s laws of motion we will be able to explain the values, and the variation with wavelength, of the refractive index of transparent solids. Amazing.