thumb|Image of Alchimia, the embodiment of alchemy. Woodcut published by Leonhard Thurneysser in 1574. Thurneysser was a student of Paracelsus.
In Renaissance alchemy, alkahest was the theorized "universal solvent". It was supposed to be capable of dissolving any composite substance, including gold (then not considered an element), without altering or destroying its fundamental components. By extracting from composite substances their fundamental virtues and properties, alchemists hoped to gain control of invaluable medical healing properties (see also azoth). For this reason the alkahest was earnestly sought. At the same time, its very existence was debated among alchemists and philosophers.
The first, or one of the first, to mention the alkahest was the Swiss physician and alchemist Paracelsus.
Etymology
There is no consensus on the origin and etymology of the word alkahest, as Paracelsus left no trace or history of the word. One such view is that the term is of Arabic origin or pseudo-Arabic modeled on similar names of other chemical entities. George Starkey argued it came from the German word al-gehest (all spirit). Johann Rudolph Glauber posed that it could have come from the words alhali est, the German word al gar heis, or Al zu hees, meaning "very hot". He also used the term "maccabean fire" because of its similarities to the "thick water" in the deuterocanonical Book of Maccabees in the Old Testament. Paracelsus's own recipe for alkahest was made of caustic lime, alcohol, and carbonate of potash; however, his recipe was not intended to be a "universal solvent".
17th century
Alkahest became very popular in the 17th and 18th centuries through Jan Baptist van Helmont. Its prevalence in the 17th and 18th centuries, despite its otherwise absurd and extreme qualities, was likely due to the popularity of alchemy at the time and the lack of an adequate alternative theory of chemistry.
George Starkey
Colonial American alchemist George Starkey (1628–1665) described alkahest as a circulated salt that is neither acid nor alkali. According to Helmont and Anglo-Irish scientist Robert Boyle (1627–1691), the alkahest had a "microstructure", meaning it was composed of extremely small, homogeneous corpuscles. hence, a hypothetical container made of a pure element (say, lead) would not be dissolved by alkahest.
Herman Boerhaave – alkahest and the philosopher's stone
Paracelsus believed that alkahest was, in fact, the philosopher's stone. Dutch chemist and physician Herman Boerhaave (1668–1738), in his textbook Elementa Chymiae (1732), did not think alkahest was the philosopher's stone, but of greater importance and value than the stone. This is due to its chemical polarity and amphoterism.
The old remark "spit is the universal solvent" satirizes the idea.
Alkahest, Inc. is a biopharmaceutical subsidiary of Grifols which is developing products derived from blood plasma to reverse and inhibit aging.
"Alkahestry" is the term used for a type of alchemy in the manga series Fullmetal Alchemist.