The Act Against Multipliers was signed into law on January 13, 1404, the work of the British parliament under Henry IV. "Chemistry" then didn't involve a whole lot, but one facet was a looming danger in the eyes of the state: multiplication.
Multiplication was a term that in crude old-school chemistry—alchemy—just meant making more of a certain material. Synthesis, really. The powers that be were very concerned that such multiplication might involve gold or silver, leading to the sudden enrichment of sinister forces, or really any force that was not the one currently in power.
The Act declared, "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."
The law probably offered more protection to wealthy suckers than the state itself
The law probably offered more protection to wealthy suckers than the state itself. Prior to the act, a popular scam was for alchemists or "alchemists" to be entrusted with the wealth of some goofball, perhaps a rich merchant, looking to have their savings cloned. Lore had it that the Pope himself practiced alchemy, using the technique to enrich the church.
Chemistry wasn't all scams and witchery in the Middle Ages. Just a century before the Act Against Multipliers, Francis Bacon was at work turning alchemy into a science. He had described for the first time the role of air in combustion, for one, and went on to experiment with saltpeter (potassium nitrate, a component of gunpowder among other things), devising a purification method using dilution and crystallization.
The Act survived until 1689, when Isaac Newton and Robert Boyle, considered to be the father of modern chemistry, both became preoccupied with alchemical experimentation. Boyle in particular petitioned to have this "secret science" legitimized once again. The "chymist" Boyle would in his lifetime clarify the idea of chemical elements, describe the differences between mixtures and solutions, study the phenomena of combustion and respiration, and hypothesize that the nature of reality was of indivisible particles.
Boyle died just two years after the Act was repealed, publishing in his lifetime over 40 books. Nearly all of his pioneering work in chemistry would have occurred while the Act was still in place.