"buoyancy," 1806, from buoyant + -ance. The more usual word is buoyancy.
Entries linking to buoyance
buoyant adj.
"having the quality of rising or floating in a liquid," 1570s, perhaps from Spanish boyante, present participle of boyar "to float," from boya "buoy," from Dutch boei (see buoy (n.)). Of personalities, etc., from c. 1748. Related: Buoyantly.
-ance
word-forming element attached to verbs to form abstract nouns of process or fact (convergence from converge), or of state or quality (absence from absent); ultimately from Latin -antia and -entia, which depended on the vowel in the stem word, from PIE *-nt-, adjectival suffix.
Latin present-participle endings for verbs stems in -a- were distinguished from those in -i- and -e-. Hence Modern English protestant, opponent, obedient from Latin protestare, opponere, obedire.
As Old French evolved from Latin, these were leveled to -ance, but later French borrowings from Latin (some of them subsequently passed to English) used the appropriate Latin form of the ending, as did words borrowed by English directly from Latin (diligence,absence).
English thus inherited a confused mass of words from French (crescent/croissant), and further confused it since c. 1500 by restoring -ence selectively in some forms of these words to conform with Latin. Thus dependant, but independence, etc.
buoyancy n.
1713, "relative lightness, quality of floating on water or other liquid," from buoyant + -cy. The figurative sense "cheerfulness, hopefulness" (of spirits, etc.) is from 1819. The meaning "power of supporting a body so that it floats" is from 1831.