NUCLEATION AND GROWTH OF ICE CRYSTALS 227 
TaBLE 1—Substances active as ice nuclei 
Natural nuclei Artificial nuclei 
———_ — 2s ae 
| | j 
Threshold 
Substance | Crystal symmetry areca Substance Crystal symmetry fem: 
| cc | | IP Se 
Covellite | Hexagonal | —5 | Silver iodide Hexagonal | —4 
Vaterite | Hexagonal —7 | Lead iodide Hexagonal | —6 
B-Tridymite Hexagonal | —7 | Cuprie sulphide Hexagonal —6 
Magnetite | —8 | Mercurie iodide Tetragonal —8 
Kaolinite | Triclinic | —9 | Silver sulphide Monoclinic —8 
Glacial debris —10 | Silver oxide | Cubie —11 
Hematite | Hexagonal } —10 | Ammonium fluoride | Hexagonal -9 
Brucite | Hexagonal —11 | Cadmium iodide | Hexagonal | —12 
Gibbsite | | —11 | Vanadium pentoxide | Orthorhombic —14 
Halloysite | Monoclinic —13 | Iodine Orthorhombic —14 
Voleanic ash | —13 | 
Dolomite Hexagonal } Sane | 
Biotite —14 
Vermiculite Monoclinic | —15 | 
Phlogopite | ls 
Fic. 1—A deposit of oriented, hexagonal ice crystals growing on special impurity sites on the surface 
of a single crystal of silver iodide; whole field = 300u 
