Alchemy is the 3rd expansion Dominion.  It adds new Kingdom cards to Dominion and as it is an expansion, you will need Dominion, or Dominion: Intrigue to play the game.

There are strange things going on in your basement laboratories. They keep calling up for more barrels of quicksilver, or bits of your hair. Well it’s all in the name of progress. They’re looking for a way to turn lead into gold, or at least into something better than lead. That lead had just been too good of a bargain to pass up; you didn’t think, where will I put all this lead, what am I going to do with this lead anyway. Well that will all be sorted out. They’re also looking for a universal solvent. If they manage that one, you will take whatever they use to hold it in and build a castle out of it. A castle that can’t be dissolved! Now that’s progress.