The physics of the solid state is nothing new. In 1900 it was as well realized as now that mechanics, heat, electricity, magnetism, optics, all have their solid‐state aspects. In mechanics we have elasticity, plasticity, elastic vibrations or sound. In heat we have specific heat, thermal expansion, heat conductivity. In electricity we have conductivity, dielectric constant. In magnetism we have paramagnetism, diamagnetism, ferromagnetism, the Hall effect. Optics involves the refractive index, optical absorption, double refraction, and so on. There are interrelations between different effects: Magnetostriction, the relation between magnetism and elasticity; pyroelectricity, the relation between heat and dielectric behavior; the Faraday effect, the relation between magnetism and optical double refraction; and a host of others. All these were known in 1900, there were tentative theories of many of them, and very elaborate studies of the interrelations of them with crystal symmetry, leading up to the proper mathematical description of many of the properties in terms of tensors, and such mathematical devices.