​AGGREGATION (from the Lat. ad, to, gregare, to collect together), in physics, a collective term for the forms or states in which matter exists. Three primary “states of aggregation” are recognized—gaseous, liquid and solid. Generally, if a solid be heated to a certain temperature, it melts or fuses, assuming the liquid condition (see Fusion); if the heating be continued the liquid boils and becomes a vapour (see Vaporization.) On the other hand, if a gas be sufficiently cooled and compressed, it liquefies; this transition is treated theoretically in the article Condensation of Gases, and experimentally in the article Liquid Gases.