Note:  Do not rely on this information. It is very old.


Fluid is a substance that allows indefinitely great deformation under the action of small stresses. In other words, the particles of a fluid are able to move about with more or less perfect freedom without being constrained to remain in the neighbourhood of other particles. This is the distinction between a fluid and a solid, and is expressed mathematically by saying that a fluid has no rigidity; the particles of a fluid can slide or shear past each other under the action of extremely small forces. [Elasticity.] The question of the time taken to effect this shearing does not occur in the definition, which, therefore, includes such substances as sealing-wax and pitch under the head of fluids rather than regarding them as solids; for experiment shows that these substances do actually undergo continuous change of shape under the action of such small forces as their own weights. Nevertheless, any attempt to effect similar changes in short time by the more sudden action of greater forces renders them solid-like in behaviour; they fracture rather than submit to the rapid deformation. Distinction must be made between a hard fluid such as sealing wax and a soft solid such as a tallow candle. The latter is the more readily deformed by small forces, but the deformation reaches its limit almost immediately after the forces are applied. A tallow candle will not bend to an indefinite extent under the action of its own weight when supported in a horizontal position at its extremities, however long it be left in that position. A stick of sealing-wax will so behave in the course of weeks, and will ultimately fall down from its supports. Under the action of great shearing stresses there are various metals and other substances that behave like fluids in that they "flow" in the direction of the lines of force. In fact, most substances do so to some extent, though the amount of flow may be very slight before fracture and a dissociation of particles occur. Copper wire may be forced through holes of smaller diameter and come out the other side of greater length and smaller sectional area. If the metal be occasionally annealed, the process may be repeated again and again. Fluids proper are divided into two classes, liquids and gases. The distinction between these is that the former are practically incompressible under the action of small forces, whereas the latter are very readily compressible. Liquids have, therefore, a very great modulus of elasticity of bulk, though small modulus of rigidity or elasticity of form, whereas both moduli are small for gases. [Gas, Liquid, Elasticity.]