Porosity is a term which is used to describe an important physical property of most materials. The porosity of a material is determined by measuring the amount of void space inside, and determining what percentage of the total volume of the material is made up of void space. Porosity measurements can vary considerably, depending on the material, and high or low porosity will impact the way in which the material performs.
The property of porosity is actually slightly more complex than the simple percentage of void space inside a material. Another important consideration is the shape and size of the void spaces in the material. Swiss cheese, for example, is famous for having fairly large and distinct bubbles, while Alpine lace cheese has much smaller bubbles, and these two cheeses behave differently because their void spaces are differently shaped.
Another issue is the level of interconnection between void spaces. If all of the spaces are discrete and isolated from each other, a material will not behave in the same way as a material with the same percentage of void space in the form of connected void spaces. When people study the rate of flow through materials, this property becomes especially important. Obviously, if the spaces are connected, the flow is going to be higher, because liquids can move freely.
This property can be measured in a wide range of materials including wood, stone, paper, cement, and ceramic. There are a number of ways to measure porosity, including soaking materials to see how much water they absorb. In the case of stone, people may also be interested in whether the porosity is primary or secondary. Primary porosity is the result of processes which occurred during deposition, while secondary porosity happens due to processes which occur after rock is formed. For example, deposits of a more soluble mineral inside a hard mineral may be gradually dissolved over time, making the rock more porous.
Porosity plays an important role in a wide range of processes. Porous membranes are routinely used for filtration of all sorts of materials, with the effectiveness of the filter being limited by the size of the pores, and porous deposits of rock are commonly exploited to access deposits of oil which may be embedded in the rock. People even take advantage of this property in the kitchen; sponge cakes, for example, are designed to be highly porous so that they can soak up syrups used for flavoring.