FLUID

, or Fluid Body, according to Newton, is that whose parts yield to the smallest force impressed, and by yielding are easily moved among each other; in which sense it stands opposed to a solid. This is the definition of a perfect fluid: if the Fluid require some sensible force to move its parts, it is imperfect in proportion to that force; such as perhaps all the fluids we know of in nature.

That Fluids have vacuities in their substance is evident, because certain bodies may be dissolved in them without increasing their bulk. Thus, water will dissolve a certain quantity of salt; after which it will receive a little sugar, and after that a little alum; and all this without incerasing its first dimensions. Which shews that the particles of these solids are so far separated as to become smaller than those of the Fluid, and to be received and contained in the interstices between them.

Fluids are either elastic, such as air; or non-elastic, | as water, mercury, &c. These latter occupy the same space, or are of the same bulk, under all pressures or forces; but the former dilate and expand themselves continually by taking off the external pressure from them; for which reason it is that the density and elasticity of such fluids, are proportional to the force or weight that compresses them. The doctrine and laws of Fluids are of the greatest extent in philosophy: the properties of elastic Fluids constituting the doctrine of Pneumatics; those of the non-elastic ones, that of Hydrostatics; and their motions, Hydraulics. For which see these respective articles. Also,

For the laws of the pressure and gravitation in Fluids specifically heavier or lighter than the bodies immerged, see Specific Gravity.

For the laws of the resistance of Fluids, or the retardation of solid bodies moving in Fluids, see RESISTANCE. And

For the ascent of Fluids in capillary tubes, or between glass planes, &c, see Ascent, and Capillary Tubes.