Powder of sympathy
Powder of sympathy was a form of sympathetic magic, current in 17th century in Europe, whereby a remedy was applied to the weapon that had caused a wound in the hope of healing the injury it had made.
The method was first proposed by Rudolf Goclenius, Jr. and was later expanded upon by Sir Kenelm Digby. An abstract of Digby's theory is found in an address given before an assembly of learned men in Montpellier, France, and which is discussed in Thomas Joseph Pettigrew's Superstitions Connected with Medicine and Surgery. The recipe for the powder is: "take Roman vitriol [copper sulphate] six or eight ounces, beat it very small in a mortar, shift it through a fine sieve when the sun enters Leo; keep it in the heat of the sun and dry by night."
The powder was also applied to solve the longitude problem in the suggestion of an anonymous pamphlet of 1687 entitled "Curious Enquiries." The pamphlet theorised that a wounded dog could be put aboard a ship, with the animal's discarded bandage left in the trust of a timekeeper on shore, who would then dip the bandage into the powder at a predetermined time and cause the creature to yelp, thus giving the captain of the ship an accurate knowledge of the time. There are no records of the effectiveness of this procedure. It is also uncertain if it had ever been tried, and it is possible that the pamphlet was a form of satire.
The powder of sympathy was termed weaponsalve ("A salve which was supposed to cure the wound, being applied to the weapon that made it.") by Dr. Johnson in his Dictionary of the English Language (1755).
In literature and media 
Learned he was in med'c'nal lore,
For by his side a pouch he wore,
Replete with strange hermetic powder,
That wounds nine miles point-blank would solder;
By skilful chymist, with great cost,
Extracted from a rotten post;
The concept of the powder of sympathy plays a significant role in the plot of Umberto Eco's novel The Island of the Day Before. In the novel, set in the 17th century, the protagonist learns of the powder, and gives a lecture on it in a salon. He is then ordered by Cardinal Mazarin to spy on a secret English Pacific voyage to test an unknown application of the powder to solve the longitude problem. The method attempted in the novel involved a dog wounded with a weapon which would then be heated every day at noon in London. The men on the ship would interpret the dog's suffering as a sympathetic response, and thus would try to calculate the difference between local time and London time.
In 2010, BBC series James May's Man Lab attempted to navigate the English Channel using the Powder of Sympathy, among other traditional methods. Rather than injure the dog with a knife, they played "I Dreamed a Dream" to it, dipping the CD in the powder at the appropriate time. It is unclear if this had the necessary effect, as the dog barked incessantly during the voyage regardless.
- Lewis Spense, Encyclopedia of Occultism and Parapsychology 1920, vol. 2, p. 725.
- Sir Kenelm Digby, A late discourse made in solemne assembly of nobles and learned men at Montpellier in France, touching the cure of wounds by the powder of sympathy. London, R. Lowdes 1658, 2nd edition.
- Sir William Osler, Sir Kenelm Digby's Powder of Sympathy. An unfinished essay, Plantin Press 1972.
- Umberto Eco, The Island of the Day Before. Milan, R.C.S. Libri & Grandi Opere SpA-Milano 1994