flux

Flux is a late-fourteenth-century medical borrowing from Latin that has since opened a whole vocabulary of motion in English.‌​‌​‌​‍​‍​‌​‌​‍​‍​‌​‍​‌​‌​‌​‍​‍​‌​‌​‍​‍​‍​‍​‌​‍​‍​‌​‌​‌​‌​‌​‌​‌ Its source is Latin fluxus, the past participle of fluere ("to flow"), used as a noun to mean "a flowing." The Latin verb fluere is itself of uncertain Proto-Indo-European origin — possibly from a root *bhleu- ("to swell, overflow"), though the derivation is not secure — and it sits at the head of one of the most productive word-families in the European learned vocabulary: influence, influenza, affluent, fluent, fluid, confluence, superfluous, mellifluous, effluent, reflux, fluctuate, fluvial. Each of these is a compound or derivative of fluere with a different prefix or suffix, and together they form a conceptual map of how Latin thought about motion: flowing in, flowing out, flowing together, flowing away, flowing abundantly, flowing sweetly. Flux, the bare past-participle form, is the simplest member of the family — "a flowing," in the abstract.

Flux, influence, influenza, affluent, fluent, fluid, confluence, superfluous, mellifluous — all of these English words come from a single Latin verb: fluere, "to flow." Few roots in the language have been so generous.

Fluere gave Latin a whole vocabulary of motion. Flumen was a river. Fluctus was a wave. Fluidus was anything that flowed. Add a prefix and the meaning sharpens: influere, to flow in; effluere, to flow out; confluere, to flow together; affluere, to flow toward; refluere, to flow back; defluere, to flow down; superfluere, to flow over, to be in excess. Flux itself is fluxus, the past participle used as a noun — literally "a flowing." Latin already knew the noun fluxus in classical prose, where Cicero uses it for the flux of time and Pliny for pathological discharge; the two senses travelled together into the Romance languages and into medieval Latin.

The word entered English around 1374 with a grim medical sense. A flux was a pathological discharge, and "the bloody flux" (fluxus sanguinis) was the standard name for dysentery well into the seventeenth century. Chaucer's Parson's Tale, around 1390, uses the word in this sense; so does the Wycliffite Bible, translating fluxum sanguinis in the episode of the woman healed by touching Christ's garment. Shakespeare knows the word: Othello's "she was false as water, false as water, false as flux" is one of the clearer poetic uses, and in Timon of Athens the word appears in medical context. "Bloody flux" remained a standard term in English medical prose from the sixteenth through the early nineteenth century and is documented extensively in plague and famine records.

By the 1500s, metallurgists had borrowed the term for substances that make metals flow together during soldering and smelting — a chemical helper named for what it causes. A flux in metalworking is a material added to a weld or an ore to lower the melting point, clean the surfaces, and allow the metals to flow and bond. The sense is a technical specialisation of the general "flowing" meaning, and it is the sense that survives most robustly in modern English. "Flux" as a physics term (magnetic flux, luminous flux, the flow of a field through a surface) is a nineteenth-century extension of the same idea, formalised by Maxwell and others into the vocabulary of electromagnetism.

The strangest descendant of fluere is influenza. In medieval Italian, influenza meant the flowing-in of an astrological force — the stars were thought to exert a fluid influence on human bodies. The word appears in the thirteenth century in Italian astrological texts, translating Latin influentia. When a mysterious epidemic swept Europe in 1743, Italians called it influenza di freddo ("influence of the cold") or influenza di stelle ("influence of the stars"), and the shortened name stuck. The disease carried the astrological name across Europe; French and English borrowed the Italian form, and by the nineteenth century influenza was the standard medical term. Every flu season carries a fossil of medieval astrology: a disease named for celestial flowing.

Across the Romance languages, fluere and fluxus produced a uniform family. Italian flusso, Spanish flujo, Portuguese fluxo, French flux — all mean "a flow" and all share the Latin past-participle stem. The English verb "to influence" is a back-formation from "influence" (noun), itself from Medieval Latin influentia (a flowing-in, originally of astrological power). French influence, Italian influenza, Spanish influencia and Portuguese influencia are cognate forms. The vocabulary of social and political influence ("she has great influence in the party") thus still carries the metaphor of fluid flow — a useful reminder that metaphors outlive their original meanings by centuries.

And affluence? Literally a flowing-toward. The wealthy are named for rivers of money running in their direction. Latin affluentia was already used in the abstract sense of "abundance" by Cicero; English borrowed it in the fifteenth century, and the modern association with wealth consolidated in the eighteenth century with the rise of commercial society. "The affluent society" (J. K. Galbraith, 1958) keeps the old river-metaphor doing economic work. Superfluous ("flowing over, in excess"), confluent ("flowing together"), and effluent ("flowing out," now chiefly applied to wastewater discharge) are the other high-traffic English members of the family.

Heraclitus said panta rhei — "everything flows." Latin agreed, and English inherited the whole vocabulary of the river. The Heraclitean fragment is preserved by Plato and by Simplicius, and its Greek root (rhe-, from PIE *sreu-, "to flow") is a separate family — English stream, rheum, rheostat, catarrh all come from *sreu-. The two river-metaphors (Greek *sreu-, Latin fluere) thus run in parallel through the European learned vocabulary: everywhere one looks, the mind uses flowing water to think about change. Flux is the English word that stands closest to the abstract Latin original, unprefixed and unspecialised — the bare idea of continuous change, which is why Heraclitean philosophy is still taught under the English heading "the doctrine of flux."