Etymology
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marcasite (n.)

crystallized pyrite, early 15c., originally in medicine and alchemy, from Medieval Latin marchasita (c. 1200 in translations from Arabic), from Arabic marqashīthā "iron sulfide" (though OED doubts this), attested from 9c.; perhaps ultimately from Persian marquashisha [Klein]. "This name has been used for a number of substances but mainly for iron pyrites and especially for the crystalline forms used in the 18th century for ornaments." [Flood]

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rotation (n.)

1550s, "act of rotating or turning, action of moving round a center," from Latin rotationem (nominative rotatio) "a turning about in a circle," noun of action from past-participle stem of rotare "turn round, revolve, whirl about, roll," from the same source as rota "wheel" (see rotary).

Sense of "a recurring series or period" is by 1610s. Used earlier in alchemy, "transmutation of the four elements into one another" (late 15c.).

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circulation (n.)

mid-15c., circulacioun, in alchemy, "process of changing something from one element into another," from Latin circulationem (nominative circulatio), noun of action from past-participle stem of circulare "to form a circle," from circulus "small ring" (see circle (n.)).

Of blood, "act of moving so that it returns and begins again," first by William Harvey, 1620s. Meaning "act or state of being distributed" is from 1680s; that of "extent to which a thing circulates" (of periodical publications) is from 1847.

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aqua (n.)

"water," late 14c., from Latin aqua "water; the sea; rain," from PIE root *akwa- "water." Used in late Middle English in combinations from old chemistry and alchemy to mean "decoction, solution" (as in aqua regia, a mix of concentrated acids, literally "royal water," so called for its power to dissolve gold and other "noble" metals). As the name of a light greenish-blue color, 1936.

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quintessence (n.)

early 15c., quint-essence, in ancient philosophy and medieval alchemy, "a pure essence latent in all things, and the substance of which the heavenly bodies are composed," literally "fifth essence," from Old French quinte essence (14c.) and directly from Medieval Latin quinta essentia, from Latin quinta, fem. of quintus "fifth" (from PIE root *penkwe- "five") + essentia "being, essence," abstract noun formed (to translate Greek ousia "being, essence") from essent-, present-participle stem of esse "to be" (from PIE root *es- "to be").

The Latin term is a loan-translation of Greek pempte ousia, the "ether" that was added by Aristotle (perhaps following the Pythagoreans) to the four known elements (water, earth, fire, air) and said to permeate all things. It was naturally bright, incorruptible, and endowed with circular motion. Its extraction was one of the chief goals of alchemy.

The transferred or figurative sense of "purest essence" (of a situation, character, etc.), "an extract from anything containing in a small quantity its virtues or most essential part" is by 1560s.

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tertium quid (n.)
something indeterminate between two other things, 1724, Latin, literally "third something," from tertius "third, a third," from the root of tres "three" (see three). A loan-translation of Greek triton ti (Plato), used in alchemy for "unidentified element present in a combination of two known ones." The Latin word also figures in phrases tertium non datur "no third possibility exists," and tertius gaudens "a third party that benefits from conflict between the other two."
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projection (n.)

late 15c., projeccioun, in alchemy, "transmutation by casting a powder on molten metal," from Old French projeccion and directly from Latin proiectionem (nominative proiectio) "a throwing forward, a stretching out," noun of action from past-participle stem of proicere "stretch out, throw forth" (see project (v.)).

From 1560s in the cartographical sense of "system of continuous correspondence between the points of a spherical surface and those of a plane." From 1590s as "action of projecting." From 1756 as "that which projects or juts out."

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occult (adj.)

1530s, "secret, not divulged," from French occulte and directly from Latin occultus "hidden, concealed, secret," past participle of occulere "cover over, conceal," from assimilated form of ob "over" (see ob-) + a verb related to celare "to hide" (from PIE root *kel- (1) "to cover, conceal, save"). Meaning "not apprehended by the mind, beyond the range of understanding" is from 1540s. The association with the supernatural sciences (magic, alchemy, astrology, etc.) dates from 1630s. A verb occult "to keep secret, conceal" (c.1500, from Latin occultare) is obsolete.

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pseudo-science (n.)

also pseudoscience, "a pretended or mistaken science," 1796 (the earliest reference is to alchemy), from pseudo- + science.

The term pseudo-science is hybrid, and therefore objectionable. Pseudognosy would be better etymology, but the unlearned might be apt to association with it the idea of a dog's nose, and thus, instead of taking "the eel of science by the tail," take the cur of science by the snout; so that all things considered we had better adopt the current term pseudo-sciences ["The Pseudo-Sciences," in The St. James's Magazine, January 1842]
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spermaceti (n.)

"waxy, fatty stuff in the head of certain whales," late 15c., from Medieval Latin sperma ceti "sperm of a whale" (it has when fresh something of the appearance of sperm), from Latin sperma "seed, semen" (see sperm) + ceti, genitive of cetus "whale, large sea animal" (see Cetacea). The substance in olden times was credited with medicinal properties, as well as being used for candle oil.

Use ... Sperma Cete ana with redd Wyne when ye wax old. [Sir George Ripley, "The Compound of Alchemy," 1471]

Scientists still are not sure exactly what it does. Sperm whale, short for spermaceti whale, is from 1830.

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