AGGREGATION, States of, an expres sion sometimes used to signify, collectively. the various physical states in which matter can exist. For ordinary purposes it is suffi cient to distinguish two fundamentally differ ent states of aggregation, the solid and fluid; fluids being further subdivided into liquids and gases. A solid body may be defined as one that is capable of resisting a considerable shear ing-stress. It is important to note, however, that a true solid does not yield continuously to a small deforming force; it resists deformation, and its resistance increases as the deformation increases. A fluid, on the contrary, is a body having almost no shearing-strength and offer ing very little resistance to forces that tend to change its shape. A fluid yields continuously to a deforming force, and a force that will de form it at all will deform it indefinitely, so long as it is allowed to act. Considering the subdivision of fluids into gases and liquids, it may be said that a gas is a fluid that presses continuously and in every direction on the walls of the vessel containing it, and which follows them indefinitely if they retreat. A gas, if left to itself, tends to expand infinitely in every direction. A liquid may be defined as a fluid which does not follow the walls of the contain ing vessel if they retreat, and which has no tendency to sudden and indefinite expansion when freed from all restraint.
These distinctions between the various states of aggregation in which matter occurs are to a certain extent arbitrary, elastic, indefinite and inexact. For example, certain kinds of pitch resist the action of deforming forces that are applied for a short time only, and are brittle enough to fracture, like glass, under the influ ence of a sudden stress; yet they yield slowly but continuously to very small deforming forces, when those forces act for a long time. A body of this sort, strictly speaking, is neither a solid nor a liquid, and to include it in a gen eral classification we should have to have a °semi-solid° division. The distinction between liquids and gases is even more artificial than that between solids and liquids; for a liquid may be made to pass into its vapor in such a manner that it is impossible to state at what moment it ceases to be a liquid. Thus, if water is heated under a sufficiently great pressure up to F., and is then allowed to expand by a sufficient amount at this temperature and is finally cooled at constant volume, we shall find, at the end of this operation, that it has been entirely transformed into steam, although we cannot say at what stage the transformation took place. See CRITICAL POINT; EQUILIBRIUM, Chemical; MOLECULAR THEORY ; THERMODY NAMICS ; MATTER.