Fluid

(floo'id.) [From L. fluo, I flow.] (Phys.) Theoretically, a collection of material particles which can be moved among each other by an indefinitely small force. No fluid in nature, however, strictly fulfils this definition, though very many do so sufficiently nearly to make the conclusions founded on the definition practically correct. The pre ure of a fluid on any surface is equal to the surface multiplied by the depth of its centre of gravity below the surface of the fluid. Fluids are divided into elastic and non-elastic; these are, however, relative terms, all fluids being more or le elastic. That which is termed an elastic fluid, is in the form of an air or vapor; it is permanently elastic if it retains its aëriform nature at the ordinary temperature of the atmosphere; but non-permanently elastic, if, like steam, it becomes a liquid under the same circumstances. The elasticity of non-elastic fluids is so trifling that for a long time it was not certain that they had any. Whether a given body shall be a solid, liquid, or gas, depends on heat, or heat and pre ure. Fluke, (flook.) [From Ger. flügel, a wing.] (Naut.) That part of an anchor which takes hold of the ground. -(Mining.) An instrument employed in cleansing the charge preparatory to blasting. Fluor Albus, (floo'or al'būs.) [L., white flower.] (Med.) The WHITES, q. v. Fluorescence, (-es's ins.) [From fluor.] (Opt.) The property which some substances po e of presenting a self-luminous appearance, when viewed by reflected light. FLUORESCENT RAYS are those rays which have the greatest actinic influence, and exist beyond the violet end of the spectrum, invisible under ordinary conditions. Fluorine, (loo'o-rin.) (Chem.) An element supposed by most chemists to belong to the chlorine group. It is a gas, but its properties in the free state are almost unknown, owing to its intense affinities which cause it to unite with almost every substance with which it comes in contact; the most succe ful attempt at isolating it having been performed in ve els of fluor-spar. The most important compounds of fluorine are the hydrogen compound and fluor-hydric acid, or hydro-fluoric acid. At. weight, 19; symbol, F. Fluorotype, (floo-oro -tip.) (Photog.) A proce in which salts of hydro-fluoric acid are employed for the purpose of producing pictures by the agency of light. Fluor-spar. (Min.) A fluoride of calcium which abounds in nature. It occurs sometimes ma ive, but almost always crystallized. The variously colored spec i mens called Derby shire spar are, by means of the turning-lathe, formed into vases and other ornaments. [s. 371]