FLUIDITY is that state of a substance in which its constituent particles are so slightly cohesive that they yield to the ' smallest impressions. The term is usual ly confined to express the condition of the nonelastic fluids ; and hence it de notes one of the three states in which matter exists; namely, the solid, the fluid or liquid, and the gaseous. The state of fluidity is best defined as that in which bodies tend to form drops, as this disposition does not belong either to bodies in h gaseous form, or to solid bodies reduced to fine powder. The for mation of drops arises from this, that the molecules of fluid bodies adhere to each other with a certain force, at the same time that they glide over one another without any sensible resistance. It is in correct to say that the molecules of bodies in a state of fluidity offer no resistance to separation ; for, on bringing a flat disc of glass or metal into contact with the sur thee of a liquid, a very sensible degree of force is required to separate them. That adhesion exists among the molecules of fluid bodies is by various other phenomena. Water or mercury on a flatplate of metal collects in glo= bides, when slowly poured into a wine glass will remain heaped up, as it were, above the level of the edge.
Various hypotheses have been framed by philosophers to explain the different states in which matter is found to exist.
Confining ourselves to the most general views, we may regard all bodies as assem blages of particles constantly maintained in equilibrium between two forces, an attractive force which tends to unite the particles, and a repulsive force which tends to increase the distance between them. The solid state results from the preponderance of the attractive force. Conceive the repulsive force _to receive an augmentation until it becomes equal to, or forms an equilibrium with, the at tractive force. When the two forces are thus balanced, the particles exert on each other neither attraction nor repulsion, and the body is in the fluid state. Last ly, if the repulsive energy be still increas ed, the particles will be separated from each other to such distances that their mutual attractions will cease altogether to be sensible, and then the body passes Into the gaseous state. Hence we may pronounce that there is no natural state of body ; and that fluidity, solidity, the state of vapor, and the ariform state are only accidental, and determined by the temperature of the medium in which the body is placed.