The alchemists of legend and yore sought to transmute elements, among other things -- the classical "lead into gold", for one. Nowadays, we've figured out the nuclear physics required for transmutation well enough that a motivated amateur can pull it off in their garage (fusor + target, or a small particle accelerator for that matter) and synthetic elements are a part of daily life (Am-241 in ionization smoke detectors).
Jill is faced with an intelligent alchemist transported from the past (or another world) who:
- has no prior knowledge of nuclear physics (i.e. may have heard of the concept of atoms, but nothing more)
- has sought the transmutation of elements through other means (looking for the ol' Philsopher's Stone et al)
- and has a basic understanding of chemical elements (i.e. they understand the difference between a pure element such as a single metal and a compound or an alloy)
, and has the knowledge and parts access to build particle-physics laboratory gear at the level of a physics graduate student, as well as a quite beefy wall socket to run it off of. Given this, how can our transmuter of elements convincingly demonstrate to the alchemist that she can perform such a feat at will? It doesn't have to be a specific transmutation -- anything vivid enough to prove to the alchemist that the target matter's been transmuted will do.