(Sarah of the Clue Crew gives the clue.) We've rubbed the pigment of a radish on these cards; lemon juice turns the pigment orange, indicating that it's an acid; milk of magnesia turns it blue, indicating that it's this
[DOC] [TLE] Full text of "A treatise on medical jurisprudence : based on lectures ... [PAR] 138 CHAPTER X PHOSPHORUS Historical  Forms of Phosphorus  Its ... of 
Acute Arsenical Poisoning Chronic Arseni- cal Poisoning  Arsenical 
Pigments .....