In chemistry, affinity reactions were verbal or diagrammatic representations of attachment rearrangements between chemical species based on affinity preferences. Affinity reactions were prototypes for the modern “chemical reactions”. The stimulation or origin of verbal descriptions of affinity reactions and their diagrams stems from Query 31 of English physicist Isaac Newton’s 1718 edition of his Opticks, in which he verbally describes various gradients of “affinity” relationships between alchemical species. This statement later served the basis for the design of "affinity tables" and later the first affinity reactions diagrams made in the 1757 lectures of Scottish physician and chemist William Cullen. [1] Affinity reactions went by various names such as single elective affinity or double elective affinity, etc.

The origin of affinity reactions, and hence the transition from alchemy to chemistry, arose in the works and theories of English physicist Isaac Newton. This is evidenced in his 1678/79 letter to Irish chemist Robert Boyle, which was preoccupied with the phenomenon of elective affinity among chemicals; he states, for example: [4]

“There is a certain secret principle in nature by which liquors are sociable to some things and unsociable to others. Thus water will not mix with oil but readily with spirit of wine or with salts.”

Newton elaborates on this further:

“Just as water ‘elects’ to mix with ethyl alcohol or with salts, so it ‘chooses’ not to mix with oil, Similarly, water will sink into wood while quicksilver will not, but quicksilver will penetrate and amalgamate with metals, which water will not. Likewise aqua fortis (nitric acid) will dissolve silver and not gold, while aqua regis (mixed nitric and hydrochloric acid) will dissolve gold and not silver. Nonetheless these rules are not written in stone: ‘but a liquor which is of itself unsociable to a body may by a mixture of a convenient mediator be made sociable. So molten lead which alone will not mix with copper or with Regulus of Mars, by the addition of tin is made to mix with either.”

Newton goes on, in his writing of this period, to speak about how “particles of spirits floating in the water, will strike on metal, and will by their sociableness enter into its pores, and gather around the outside of its particles.” The finalized version of Netwon's search, research, and experiments for this "secret principle", however, was presented some three decades later, following his successes in gravitational theory and optics and light, specifically in his famous appended Query 31 of the 1718 edition of his Opticks.

Is it not for want of an attractive virtue between the parts of water () and oil, of quick-silver ()(Hg) and antimony ()(Sb), of lead ()(Pb) and iron ()(Fe), that these substances do not mix; and by a weak attraction, that quick-silver ()(Hg) and copper ()(Cu) mix difficultly; and from a strong one, that quicksilver ()(Hg) and tin ()(Sn), antimony ()(Sb) and iron ()(Fe), water () and salts, mix readily?
— Isaac Newton (1718), “Query 31” of Optics

That year, French chemist Étienne Geoffroy, while doing a translation into French of Newton's Opticks, famously took these "verbal descriptions" of affinity reaction preferences and powers of combination and decombination, as found in Query 31, and made the world's first "affinity table" (see: Geoffroy's affinity table), a tabulated listing of reaction bonding powers, so to speak, the first of many to follow over the next century.

Single elective affinity reaction
The basic example is the "single elective affinity" reaction. In 1718, during a translation into French of Newton's Opticks, French physician and chemist Étienne Geoffroy made the world's first "affinity table" a sixteen-column, eight-row table, containing twenty-four reacting species, showing specifically what affinity reactions would occur between various combinations of reactants. Geoffroy's law of affinity was that:

“Whenever two substances are united that have a disposition to combine and a third is added that has a greater affinity with one of them, these two will unite, and drive out the other.”

To expound on this law, using data from the 1718 edition of Newton's Opticks (query 31), Geoffroy made a sixteen-column, eight-row, affinity containing twenty-four reacting species, showing specifically what affinity reactions would occur between various combinations of reactants.

Cullen diagrams
In 1757, using Geoffroy's table, Scottish physician and chemist William Cullen utilized bonding brackets "{" and affinity preference darts "" to pictorially discuss affinity preferences. Shown below, for instance, using Geoffroy's first law, if chemical species A and B are attached in a weakly bonded chemical union, signified by the bonding bracket “{“, ordered such that if species C were introduced into the system, the greater affinity preference of A for C would cause A to displace B and to thus form a new union with C:

Cullen's reaction diagram (modern view)

This equates to the following in modern terms:

AB + C AC + B

Soon other chemist began to use Cullen's diagrams in publication. In 1775, Swedish chemist Torbern Bergman, in his textbook A Dissertation on Elective Attractions, made sixty-four affinity reaction diagrams. [2] In 1809, using Bergman's textbook and diagrams, of single and double elective affinity reactions, as a basis of universal chemical logic, German polymath Johann Goethe published his famed book Elective Affinities, the founding book of the science of human chemistry, in which he presented the view that people react according to the laws of affinity, just as do smaller chemical species. [3]

See also
Affinity of reaction
Elective affinity
Chemical affinity
Human elective affinity

1. (a) Crosland, M. P. (1959). “The use of diagrams as chemical ‘equations’ in the lecture notes of William Cullen and Joseph Black.” Annals of Science, Vol 15, Num 2, June.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (pgs. 387, 392, 430, 534, 612, 655), (preview), (Google books). Morrisville, NC: LuLu.
2. Bergman, Torbern. (1775). A Dissertation on Elective Attractions. London: Frank Cass & Co.
3. Tantillo, Astrida O. (2001). Goethe's Elective Affinities and the Critics. New York: Camden House.
Newman, William R. (2003). Gehennical Fire: the Lives of George Starkey, and American Alchemist in the Scientific Revolution (Elective affinity, pgs. 231-34). University of Chicago Press.

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