Golpalott's Third Law

Golpalott's Third Law is a law for making antidotes, which appears to say that the antidote for a blended poison - that is, a poison created by mixing several other poisons together - cannot simply be created by finding the antidotes to each separate poison in the blended whole and mixing them together. Instead, as Professor Slughorn explained, the potionmaker must find that single ingredient which, when added to the blended antidotes, transforms them near-alchemically into a combined whole which will counteract the entire blended poison. Simply put, according to this law, a true antidote to a blended poison is more than the sum of its parts. The book Advanced Potion-Making contains this law, but does not explain it.
 * "The antidote for a blended poison will be equal to more than the sum of the antidotes for each of the separate components."
 * —Golpalott's Third Law. [src]

Creating an antidote following this law may be quite difficult; Hermione Granger made a half-finished antidote that included fifty-two ingredients, one of them being a chunk of her own hair. The law dictates the protocol for making antidotes for all poisons. In the Potions class during which Professor Slughorn asked the class to make antidotes according to Golpalott's Third Law, Harry (under the directions of the Half-Blood Prince) used a Bezoar. While Slughorn was thrilled with Harry's "cheekiness," he explained that "the Bezoar would act as an antidote for most of the poisons in this room."

Classroom work
At Hogwarts, the Sixth years learn to produce antidotes referring to this law, under the instructions of their Potions master, Horace Slughorn. In the 1996–1997 school year, Hermione Granger "recited this law at top speed," upon being asked about it by the Potions master, Professor Slughorn. While the entire class had trouble with concocting the antidote for the practical portion, Harry Potter daringly used a bezoar as his work, gaining praise from Slughorn.

Appearances

 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Deathly Hallows (Mentioned only)

Behind the scenes
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 * The existence of this law relates to the concept of synergy, or the idea that a whole product is greater than the sum of its parts. In particular, it brings to mind drug synergy; the combined effect of multiple drugs taken simultaneously is often greater than the effects of taking those same drugs separately.
 * The name of the law suggests that there is probably a Golpalott's First Law and Second Law.
 * This may be derived from Newton's Three Laws of Motion, the most famous being Newton's Third Law.