Recently I've been reading Locke's biography by Roger Woolhouse. On p. 94 the discussion is on his manuscript de Arte medica:

"He that thinks he came to be skilled in diseases by studying the doctrine of the humours, that the notions of obstructions and putrefaction assist him in the cure of fevers, or that by the acquaintance he has with sulphur and mercury he was led into this useful discovery that what medicines and regimen as certainly kill in the latter end of some fevers as they cure in others, may as rationally believe that his cook owes his skill in roasting and boiling to his study of the elements, and that his speculations about fire and water have taught him that the same seething liquor that boils the egg hard makes the hen tender." - Locke 1669: 223, 225