Q: What did Britain's "Act Against Multipliers" prohibit, and why did the chemist Robert Boyle fight (successfully) to have it repealed?
A: It prohibited making gold and silver by means of alchemy. Boyle fought to have it repealed because he was trying to do just this!
The term "multiplication" refers to the fact that alchemists believed that under certain conditions substances could "multiply", allowing one to make more. The Act Against Multipliers was passed under King Henry IV in 1404, and said "that none from henceforth should use to multiply gold or silver, or use the craft of multiplication; and if any the same do, they incur the pain of felony". It was repealed in 1689, and according to Isaac Newton this repeal was initiated by Robert Boyle. In fact both Newton and Boyle worked very hard at alchemy in their later years; it had not acquired the taint of disrepute it now carries, and it included a lot of what we would now call chemistry.
Source: Lawrence M. Principe, The Aspiring Adept: Robert Boyle and his Alchemical Quest, Princeton U. Press, Princeton, NJ, 1998.
For more fun try these: