CHALK

Syn. Soft carbonate of lime , or Carbonate of calcium , Earthy c. of l. ; Cre′ta , L. Chalk is largely used in the arts and manufactures, and in medicine. The natural varieties are remarkable for the fo ils which they contain. The COLOURED CHALKS which are used as pigments and for crayons generally contain both clay and magnesia, as well as oxide of iron, and are minerals quite distinct from WHITE CHALK , or CHALK properly so called. The latter is an AMORPHOUS CARBONATE OF LIME . Exposed for some time to a red heat, it is converted into QUICK-LIME ; ground in mills and elutriated, it forms WHITING ; the same proce performed more carefully and on a smaller scale produces the PREPARED CHALK used in medicine. When prepared artificially (by precipitation), it is the PRECIPITATED CHALK of modern pharmacy. (See below .)