William Connolley is still hard at work banishing junk science and nonsense from Wikipedia. This edit is very funny. But then I saw the claim in that article about Thomas Aquinas being an alchemist*. This is an idea perpetuated throughout the murkier corners of the internet, but I can see little evidence for it.
Searching the complete works of Thomas for 'alchimicum' or 'alchimicae' yields only two results. The first here, where he says "the sale of a thing does not seem to be rendered unlawful through a fault in its substance: for instance, if a man sell instead of the real metal, silver or gold produced by some chemical (alchimicum) process, which is adapted to all the human uses for which silver and gold are necessary, for instance in the making of vessels and the like". As usual there is the problem of whether he means 'chemical' or 'alchemical' - there is a similar problem with the word 'astrology' as I noted here. But in any case, he is clearly mentioning this as an example - the actual subject of the section is fraud, and whether it is legal or not to sell something that has been transformed into gold by some chemical process.
The only other section I could find was here where he says "Water may cease to be pure or plain water in two ways: first, by being mixed with another body; secondly, by alteration" (aqua suam puritatem et simplicitatem potest amittere dupliciter, uno modo, per mixtionem alterius corporis; alio modo, per alterationem). But again it is merely an example. Of course what he is talking about here is a bit weirder (the transformation of plain water into the blood of Christ). But even there, he is not claiming there is any chemical process going on.
*Now sadly removed by Connolley after I mentioned it, but I am sure someone will put it back, since it says so on the Internet.