alum vs alumni

alum

verb
  • To steep in, or otherwise impregnate with, a solution of alum; to treat with alum. 

noun
  • A past attendee or graduate (of either gender) of a college, university or other educational institution. 

  • Any similar double sulphate in which either or both of the potassium and aluminium is wholly or partly replaced by other univalent or tervalent cations. 

  • An astringent salt, usually occurring in the form of pale crystals, much used in the dyeing and tanning trade and in certain medicines, and now understood to be a double sulphate of potassium and aluminium (K₂SO₄·Al₂(SO₄)₃·24H₂O). 

alumni

noun
  • An individual alumnus or alumna. 

How often have the words alum and alumni occurred in a corpus of books? (source: Google Ngram Viewer )