Materials used in industry are generally defined and described according to their physical and chemical properties or characteristics. For most purposes the chemical properties are not so important as the physical, although in some cases the composition of the materials must be taken into account. The chief properties of materials are cohesion, adhesion, inertia, elasticity, ductility, brittleness, toughness, malleability, compressibility, porosity, durability, infusibility, hardness. Some of these characteristics, such as inertia, porosity, cohesion, and adhesion, are common to all forms of matter and may be considered as general characteristics. Others, such as brit-tleness and ductility, which are found only in certain kinds of matter, are called specific characteristics.