Introduction

Everybody is familiar with the three classical aggregation states of matter: gas, liquid, and solid, and everybody knows some of the characteristics that differentiate each other. Gases are easily compressible and do not have a fixed shape nor volume, so that they tend to occupy the whole volume of the container; liquids are hardly compressible and do not have a fixed shape, so that they are easily deformable, but have a well-defined volume which only varies slightly with temperature and pressure; solids are hardly compressible and deformable and, similarly to liquids, their volume is nearly constant with temperature and pressure. A fourth aggregation state of matter, less commonly known, that can be added to the three former is the plasma, in which the particles are strongly ionized.