January 25, 1683—4. A letter from Mr. Aston gave an account, of ye specific difference between ye Christalls of Sea-salts and common salt, or Inland salt, which consists in this: The angles of ye Chrystalls of common salt are entire (as likewise are those of Sal Gemmœ and ye lixiviated marine salts of Dr. Grew) but ye angles of true sea-salt are cut off, into triangular planes, at least in one of ye sides. Dr. Plot shewed us some Chrystalls of sea-salt, in which we observed ye triangular plane mentioned in Mr. Aston's letter. The severity of ye frost led us then to discourse of cold, particularly of making holes in ye Ice with salt, which eats through ye ice, which means ye common-people make use of, in opening their frozen pumps.