The study of chemicals and their interactions in an environment. With this skill, you know how to create dangerous and useful chemical compounds. You also know the tools used to observe and create such concoctions. You're good at separating ingredients and recognizing ill-fated combinations.
In ancient settings, this is alchemy. You know some useful compounds, but hardly have the accuracy or knowledge of today. Combined with Botany, this is about as good as ancient medicine can get.