Magnesium, element number 12, was first isolated by British chemist Humphry Davy in 1808. But the name magnesium is older and was earlier used to refer to the mineral we now call manganese. The word, referring to manganese, appears in a 1781 letter by mineralogist John Black:
The Swedish Chemists [...] have got a Metal from it, they call it Magnesium.1
This sense is utterly obsolete and no one now refers to manganese as magnesium except in error.
Davy had originally proposed to call the new element magnium. From the 1808 Philosophical Transactions of the Royal Society:
I shall venture to denominate the metals from the alkaline earths barium, strontium, calcium, and magnium: the last of these words is undoubtedly objectionable, but magnesium has been already applied to metallic manganese.2
But by 1812, Davy was calling element number twelve magnesium, from his Elements of Chemical Philosophy:
That magnesia consists of magnesium and oxygene, is proved both by analysis and synthesis.1
In the same work he also writes his reasons for switching from magnium to magnesium:
In my first paper on the decomposition of the earths [...] I called the metal from magnesia, magnium, fearing lest, if called magnesium, it should be confounded with the name formerly applied to manganese. The candid criticisms of some philosophical friends have induced me to apply the termination in the usual manner.2
The name magnesium, whether referring to manganese or element number twelve, comes from the region of Thessaly in Greece known as Magnesia. The name magnesia has been applied to various metal ores, presumably found in that region, since classical times. The word magnet is from the same root, although that word has a very different etymological path. English use of magnesia dates to at least Chaucer, who uses it (probably to refer to magnesium oxide) in the Canon Yeoman’s Tale, lines 1452–56:
And Plato answerde unto hym anoon,
“Take the stoon that men call Titanos.”
“Which is that?” quod he. “Magnasia is the same,”
(And Plato answered him immediately,
“Take the stone that men call Titanos.”
“Which is that?” said he. “Magnasia is the same,”
Titanos is probably gypsum.3
The chemical symbol for magnesium is Mg, taken from two of the first three letters of its name.
3Oxford English Dictionary, magnesia, 3rd Edition, June 2009, Oxford University Press, accessed 28 August 2009, http://dictionary.oed.com/cgi/entry/00299504; and The Riverside Chaucer, Third Edition, Larry D. Benson, ed., Boston: Houghton Mifflin, 1987.
Copyright 1997-2016, by David Wilton