In everyday life one encounters many different systems where it is apparent that various processes take place at the line of contact between a liquid phase and a solid phase. The molecules in the liquid phase can move larger distances than those in the solid phase. Therefore, since the molecules in a solid phase are well fixed, this gives rise to the fact that it is in general not possible to study the surface forces of the solid phase in the same way we have described for the liquid phase elsewhere in this book. In spite of this, there exist many examples where we have to attribute a definite value of surface tension to the solid surface, in order to be able to describe the different reactions taking place at the solid surface in contact with liquid phase.