Magnesium, an element important in biology and lightweight alloys, was discovered through a multi-stage process spanning centuries. This historical journey began with the recognition of its compounds and culminated in the isolation of the pure metal. The discovery involved ancient medicinal knowledge, the chemical revolution of the 18th century, and the early application of electricity to chemistry.
Early Recognition of Magnesium Compounds
Compounds containing magnesium were known and used long before chemists understood the concept of an element. A famous early example is Epsom salt, which is hydrated magnesium sulfate (\(\text{MgSO}_4 \cdot 7\text{H}_2\text{O}\)). It gained recognition in 1618 when a farmer near Epsom, England, noticed the bitter taste and healing properties of water from a local well. The residue left after evaporating this water became known as Epsom salts.
Another important compound was “magnesia alba,” or “white magnesia,” primarily magnesium carbonate (\(\text{MgCO}_3\)). This white powder was used medicinally as an antacid and laxative, produced by mixing Epsom salt with a carbonate like potash. While these compounds were highly valued, their fundamental composition remained a mystery until the mid-18th century, as scientists considered them forms of “earth.”
The Identification and Naming of the Element
The identification of magnesium as a unique substance occurred in the 1750s through the work of Scottish chemist Joseph Black. In 1755, Black chemically distinguished magnesia alba (magnesium carbonate) from lime (calcium oxide). Previously, chemists had incorrectly grouped magnesia and lime, believing them to be variations of the same substance.
Black demonstrated that when magnesia alba was heated, it lost weight by releasing a gas he called “fixed air,” which we now know as carbon dioxide (\(\text{CO}_2\)). The resulting calcined magnesia (magnesium oxide, MgO) reacted differently with acids than the residue from heated limestone. This distinction proved that magnesia was the oxide of a new metallic element, even though Black did not isolate the pure metal. The name “magnesium” is believed to originate from the ancient Greek district of Magnesia in Thessaly, associated with the mineral magnes.
Final Isolation and Confirmation
The final stage of discovery, the isolation of the pure metallic element, was achieved in the early 19th century. In 1808, English chemist Sir Humphry Davy successfully produced an amalgam of magnesium. Davy used electrolysis, applying an electric current to a mixture of magnesia (magnesium oxide) and mercuric oxide.
The resulting product was an alloy with mercury, which Davy initially named ‘magnium’, not pure magnesium metal. His work confirmed the element’s existence by separating it from its compound, a major achievement in electrochemistry. Obtaining the metal in a pure, coherent state proved challenging due to the element’s high reactivity. French chemist Antoine-Alexandre Bussy achieved this final step in 1831, preparing pure, metallic magnesium by heating magnesium chloride with potassium.