I did think that "Beast no purple light" might be "Beast no. purple light" referring to the stained glass puzzle's blue as purple for some reason. I don't think you ever actually examined the trapdoor in the lab, but that's secondary. Aqua Regia is a volatile mixture of acids that is notable for being one of the few things capable of dissolving gold (as well as being pretty nasty in general). Probably just as well you didn't explore what happens if you try drinking it :)
