With increasing temperature, all materials are successively
transformed from the solid to the liquid and then to the gaseous state.
If the temperature is further increased, one gets plasma, the "fourth
aggregate state of matter": The atoms of the gas decompose into their
constituents, electrons and nuclei.
Everyday examples are the plasma column in a neon
tube, an electric spark or the plasma streak of lightning.
The properties
of a plasma are very different from those of ordinary gases. Plasma is,
for example, electrically conductive. Its motion can therefore be influenced
by electric and magnetic fields. This property is exploited in fusion
devices by confining the hot plasma in a "magnetic filed cage",
thereby keeping it away from material walls.