Iatrochemistry alchemy: medical

A typical alchemical treatise (this one from the early eighteenth century, but a revised version of a work from 1698) by a Dutch pharmacist and physician notes that iatrochemistry (medical alchemy) is divided into theory and praxis. The author acknowledges that Theory holds a higher place in his own writing. Yet Theory alone is useless unless married with Praxis. 

The term valence, of which ambivalence is not merely a variation, but a decidedly new and separate concept, derives from chemistry and atomic physics. Valence can refer to an extract or tincture, usually from an herb. In this connotation, it has obvious ties with the field of medical alchemy, or iatrochemistry. In the mid-i8oos, valence theory began to be used to signify the normal number of bonds that a given atom can form with other atoms—a register that links valence with philosophical materialism, matter, and Epicurianism. In recent scientific work, valence refers specifically to the number of electrons in the outermost shell of atoms. It is not provisional or occasional in its relation to the atom. Valence is atomicity. It defines a given chemical element, perhaps not in its essence, but in its capacity to combine with other elements—its potentiality. Valence is denoted by a simple number, and elements are said to be monovalent, bivalent, trivalent, quadrivalent, and so on. About one-fifth of all elements have a fixed valence (sodium is always i, or monovalent calcium is always 2, or bivalent and so on). Many elements have valences that are variable, depending on the other elements with which they are combined. 

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