What Is Alkahest? The Universal Solvent of Alchemy

A liquid that dissolves any other substance, that reduces everything it touches to its prima materia, that does not change itself in the process. Colorless, odorless, tasteless. A universal solvent, an alchemical miracle, a metaphysical embarrassment. The alkahest is the hypothetical substance that Paracelsus named in the early sixteenth century, and the name is one of the more interesting pieces of Paracelsian invention (Principe, The Secrets of Alchemy, 2013, pp. 105-110; Newman, Promethean Ambitions, 2004, pp. 180-195; Holmyard, Alchemy, 1957, pp. 145-150; Newman & Principe, Alchemy Tried in the Fire, 2002, pp. 65-85). The alkahest is closely related to the azoth of the earlier tradition, and it plays an important role in the spagyric art and in the Paracelsian medical system. The debate about whether the alkahest could exist was one of the most consequential debates of the seventeenth century, and the debate is more interesting than the question it pretended to be about.

The coinage

The term “alkahest” was coined by Paracelsus in the early sixteenth century, and the etymology of the word is one of the more obscure corners of the Paracelsian corpus. The most plausible theory derives the word from the Arabic al-kāhal (the corrosive). A competing theory holds that Paracelsus invented the word as a pseudo-Arabic formation, modeled on the genuine Arabic definite article al- and a made-up second element, and the invention would be characteristic of Paracelsus’s habit of manufacturing technical vocabularies out of plausible-looking materials. The truth is probably unrecoverable, and the uncertainty is itself part of the Paracelsian mystique.

Paracelsus described the alkahest as a powerful liquid, capable of dissolving metals, minerals, plants, and animals. The alkahest, properly prepared, could be used to extract the active principles of any substance, to purify the three principles of the tria prima in any substance, and to prepare medicines of extraordinary potency (Principe, 2013, pp. 105-110). Paracelsus did not reveal the recipe. He described its powers in detail, but he did not say how it was prepared, and the mystery was part of the Paracelsian tradition. The apprentice was expected to discover the recipe by long study of the Paracelsian texts and by long practice in the laboratory. The Paracelsian literature is full of these deliberately unfulfilled promises, and the deliberate unfulfillment is part of how the tradition kept its initiates engaged.

The van Helmont-Boyle debate

The most important debate about the alkahest in the seventeenth century was the debate between Johann Baptist van Helmont and his followers, who defended the reality of the alkahest, and Robert Boyle and other critics, who denied it (Newman & Principe, 2002, pp. 65-85). The debate is one of the more interesting episodes in the history of the philosophy of chemistry, and the debate is worth being clear about.

Van Helmont (1580-1644) was a Flemish chemist and physician, a follower of the Paracelsian tradition, and one of the most important figures of the early modern chemical tradition. Van Helmont claimed to have prepared the alkahest, and he described the preparation in his chemical writings. According to van Helmont, the alkahest is a powerful liquid that can dissolve any substance without losing its own power. The alkahest, properly prepared, is colorless, odorless, and tasteless, and it has no chemical action on any substance except to dissolve it.

Van Helmont’s son, Franciscus Mercurius van Helmont (1614-1698), continued the defense of the alkahest in the later seventeenth century, and the alkahest became a major topic of debate in the alchemical and chemical literature. Franciscus Mercurius’s Alphabeti vere naturalis Hebraici brevissima Delineatio (1667) defends the alkahest in cabalistic and alchemical terms, and the defense is one of the more interesting documents of the seventeenth-century chemical-philosophical debate.

Robert Boyle (1627-1691), the founder of modern chemistry and a serious alchemist himself, was skeptical of the alkahest. Boyle argued in the Sceptical Chymist (1661) and in later writings that the alkahest is an impossible substance, that no liquid can dissolve all other substances, and that the claims of the Paracelsians and the van Helmonts are not supported by the evidence. Boyle’s critique was influential, and the debate about the alkahest gradually subsided in the eighteenth century as the modern chemical tradition was established (Principe, 2013, pp. 105-110; Newman & Principe, 2002).

The honest reading of the debate, in my view, is that Boyle was right about the impossibility of a universal solvent. The honest reading is also that the debate was about more than chemistry. The van Helmonts were defending a Paracelsian picture of the cosmos in which the alchemist’s power over matter was a real and important feature, and Boyle was defending a mechanical-philosophy picture in which the powers of the alchemist were sharply limited. The chemical question was a real chemical question. The chemical question was also a stand-in for a much larger philosophical disagreement, and the philosophical disagreement is what made the debate matter to its contemporaries.

The alkahest in the spagyric tradition

Despite the criticism of the alkahest by Boyle and others, the concept remained an important part of the spagyric tradition, and the persistence tells you something about how the spagyric tradition handles its failures. The spagyric alchemists continued to seek the alkahest, or at least a powerful solvent that could approach the properties of the alkahest. The spagyric alchemists distinguished between the alkahest proper, the universal solvent that can dissolve any substance, and the “philosophical mercury,” the solvent that can dissolve the metals but not all other substances. The alkahest proper was considered to be the most difficult of the alchemical preparations, and most spagyric alchemists settled for the philosophical mercury or for one of the more specific solvents.

The most successful of the specific solvents developed by the spagyric alchemists are the mineral acids — sulfuric acid, nitric acid, hydrochloric acid, aqua regia. These acids can dissolve many substances, including the metals, and they are powerful laboratory reagents. The mineral acids are not the alkahest, but they approach some of its properties, and they have been among the most important of the laboratory reagents since the alchemical period. The development of the mineral acids is, in fact, one of the most consequential practical achievements of the Arabic and Latin alchemical tradition, and the consequence is something Boyle would have been the first to acknowledge.

The alkahest in the modern esoteric tradition

The alkahest remains a central concept in the modern esoteric tradition, and the persistence is partly because the symbol does more useful work than the substance. The Theosophical Society, the Hermetic Order of the Golden Dawn, the Anthroposophical Society, and the various modern Rosicrucian orders all use the alkahest as a central symbol in their alchemical vocabulary. The alkahest is, in these traditions, a symbol of the transformative power of the spiritual life, a symbol of the divine spirit that can dissolve all the impurities of the soul and reduce the soul to its prima materia. The symbol does the work the substance could not do, and the substitution is a real feature of the modern esoteric inheritance.

The alkahest is also a symbol in modern popular culture. The concept of a universal solvent has appeared in science fiction, fantasy, and popular chemistry writing, and the alkahest is one of the more evocative of the alchemical symbols. The alchemical symbol has migrated into the modern imagination, and the migration is part of the legacy of the alkahest.

The alkahest and the modern concept of the solvent

The alkahest is closely related to the modern concept of the solvent, and the relationship is worth being clear about. Modern chemistry has developed a sophisticated understanding of the solvent properties of different substances, and modern chemistry knows that no single solvent can dissolve all other substances. The universal solvent is, from the modern perspective, an impossibility. The modern chemist has also vindicated some of the alchemical intuitions, however. The modern chemist knows that the solvent properties of a substance depend on the chemical properties of the substance and the solvent, and that some solvents are more powerful than others. Water, the “universal solvent” of the modern chemist, can dissolve more substances than any other common solvent. The modern chemist has also developed the concept of the supercritical fluid, a state of matter that combines the properties of liquids and gases and that can be a powerful solvent. Supercritical carbon dioxide is used in a wide range of industrial applications, including the extraction of essential oils, the decaffeination of coffee, and dry-cleaning.

The alchemists’ search for the alkahest led to the development of a wide range of powerful solvents, including the mineral acids, the alkalis, and the organic solvents. The search also led to a sophisticated understanding of the solvent properties of different substances, and the search was one of the sources of the modern concept of the solvent. The alchemists did not find the alkahest. The alchemists did find a great deal else, and what they found is part of the modern chemical inheritance.

The alkahest in the history of chemistry

The alkahest is an important part of the history of chemistry, both as a theoretical concept and as a practical goal, and the history is more useful than the symbol. The alchemists’ search for the alkahest led to the development of a wide range of powerful solvents. The search also led to a sophisticated understanding of the solvent properties of different substances, and the search was one of the sources of the modern concept of the solvent. The alkahest is also an important part of the history of the philosophy of chemistry, and the van Helmont-Boyle debate is one of the more interesting episodes in the early history of the discipline. The debate raised important questions about the nature of the solvent, the nature of the solution, and the nature of chemical action, and the questions are still being asked in the modern philosophy of chemistry. The alkahest remains a useful case study, and the case is more useful than the alchemists themselves understood.