Alchemy can be done. Gold has been synthesized on the nuclear level by way of neutron bombardment of other elements. It's just not economically practical since the process costs more in equipment and energy than the value of the gold.
"Alchemy" predates meaningful definition of the word "chemical"! If you had presented them the modern solution, they would not have been so inclined to split hairs. They were looking for any way, regardless of whether it was "chemical" or "nuclear" or magic for all they cared.
Right, and finding another way is how the "can do" person wins over the "can't" naysayers. It's like saying Jules Verne's cannon to the moon is impossible... which is true, but not really the point, since rocket travel to the moon has been proven.
My point is that you can make any prediction 'come true' if you redefine enough words in it. This is a dumb parlor trick and tells us nothing about how people of the past actually thought.
http://en.wikipedia.org/wiki/Synthesis_of_noble_metals#Gold