What It Actually Is

Powder of Algaroth is identified as antimony oxychloride, SbOCl, an identification well confirmed and essentially uncontested, its formation from butter of antimony explained cleanly by modern chemistry as a hydrolysis reaction: antimony trichloride reacts with water, replacing some of its chlorine with oxygen from the water molecules and releasing hydrochloric acid, precipitating the far less soluble oxychloride as a solid white powder in the process.

The compound's misleading historical name, mercurius vitae, is a documented naming error worth flagging explicitly rather than repeating uncritically, since it attributes the preparation's properties to mercury when the underlying chemistry is entirely a matter of antimony reacting with water, a clear example of how an alchemical name could mislead about a substance's actual elemental composition even when the preparation method itself, as recorded, is perfectly clear and reproducible. The seventeenth century antimony controversy that surrounded preparations like this one, real physicians on both sides arguing in earnest over whether antimony compounds belonged in medicine at all, is itself well documented and offers useful context for why a compound this hazardous was nonetheless in genuine, sustained therapeutic use.