[The alkahest, universal dissolvent or when theory turns an impossible practice into the imaginable].

Using again a term coined by Paracelsus, Van Helmont calls Alkahest the universal solvent able to reduce any body in its prime matter. The search for Alkahest takes up a great part of the chemical works of the second half of the seventeenth century. Simple corrosive product for some, "Philosophers' Mercury" for others, Alkahest withstands objections in spite of its absurd properties, by means of the persistence of alchemical theories in the field of chemistry.