Fluid
2 of the 6 encyclopedias on this shelf carry an entry for Fluid. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
having the parts easily separable; consisting of particles which move and change their relative lative positions very readily; capable of flowing; liquid; gaseous. The fundamental property of fluids, viewed as forces, in physics, is their equality of pressure in all directions. The term includes both liquids and gases. Elastic fluids: In physics, gases. Electric or electrical fluid: In electricity, a fluid composed, in the opinion of Symmer, now generally accepted, of two fluids, the positive and the negative. Imponderable fluids: In physics, a name sometimes given to heat, light, magnetism, and electricity. They are mobile and yet, if consisting of matter, are in such a state of tenuity that they possess no perceptible weight. Magnetic fluids: In magnetism, two fluids assumed to exist. They are called respectively the N. or boreal fluid and the S. or austral fluid, the former predominating at the N. and the latter at the S. pole of the magnet. Sometimes the N. fluid is called the positive, and the S. fluid the negative one. Ponderable fluids: In physics, those possessed of weight; as water and carbonic acid gas.
The Encyclopedia of Founding (1892)
A body whose parts yield to the slightest force when impressed, and by yielding are easily moved against each other. Fluids may be divided into elastic and non-elastic. Elastic fluids are those which may be compressed into a very small compass, but resume their former dimensions on removing the compressing force. These are distinguished as airs or gases. Non-elastic fluids are those which occupy the same bulk under all pressures, or, if compressible, it is only in a slight degree--as water, oils, etc. These are denominated liquids, except in the case of metals when melted. The physical nature, laws, and effects of non-elastic fluids at rest constitute the science of hydrostatics, and when in motion, of the science of hydraulics; those that relate to elastic fluids appertain to pneumatics. See SOLID.
What Has Changed in the Last Century
The basic definition still holds: a fluid is matter that yields continuously to shear, and hydrostatics, hydraulics and pneumatics are still taught much as the 1892 Encyclopedia of Founding lays them out. What has gone is the fluid theory of everything else. Collier's still spoke in 1921 of "imponderable fluids" — heat, light, magnetism and electricity — and of two electric fluids after Symmer, but J. J. Thomson's discovery of the electron in 1897 had already replaced electric fluid with a flow of charged particles, heat had been understood as molecular motion since the mid-1800s, and no separate "boreal" and "austral" magnetic fluids exist: magnetism comes from moving charge and electron spin, and no isolated magnetic pole has ever been found. Modern usage also treats fluids through continuum mechanics and the Navier-Stokes equations, and adds states the old entries had no room for, such as plasmas and superfluid helium, discovered in 1937.
Written for this edition, 2026.