Sentence:  In structure, the <WSD>alums</WSD> consist of simple ions, being not complexes, but double salts. Potash <WSD>alum</WSD> or potassium <WSD>alum</WSD> is the common <WSD>alum</WSD>, with the formula KAl(SO<sub>4</sub>)<sub>2</sub>.12H<sub>2</sub>O) which, for convenience, may be written K<sub>2</sub>SO<sub>4</sub>.Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>.24H<sub>2</sub>O 
Meaning of the word marked with <WSD> tags:  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. (from 17th c.)