Further Tests on the Ice Nucleation Potential of Meteoritic Material
SCHAEFER1 was unable to detect any trace of ice-crystal formation when he vaporized a sample of iron–nickel meteorite by electric spark in a supercooled cloud—even in clouds as cold as −25° C. Mason and Maybank2, who ground up and vaporized three samples of stony meteorites, and Mason3 with three further samples also reported that they were unable to detect any production of ice-crystals at temperatures above −17° C This communication records the results of tests on the nucleating properties of a further thirteen samples of meteoritic materials. These comprised iron–nickel types (siderites), stony type (aerolites), plus two samples of the intermediate (siderolite) variety (all obtained from the American Meteorite Laboratory, Denver).
Ice‐nucleating properties of clay minerals and stony meteorites
The ice-nucleating properties of clay minerals have been investigated in relation to their crystalline and surface structures. The kaolin minerals, the illites and halloysite are the most effective and important, their threshold temperatures for ice nucleation lying between −9°Cand −12°C. Minerals of the montmorillonite group and related forms are active only below −15°C. The relative contributions which the various clay minerals are likely to make to the ice-nucleus content of the atmosphere are discussed in relation to their natural abundance, wind-borne dispersion and particle size. Kaolinite is considered to be the most important source of highly efficient nuclei in confirmation of results obtained earlier by Mason and Maybank (1958). 
 
 
 
Tests on three new specimens of stony meteorites showed them to be inactive at −17°C.
Electron-microscope study of snow-crystal nuclei
Experimental work was conducted in an igloo, half way up Mt. Taisetsu (1050 m), Mt. Tokachi (1030 m) and Mt. Okuteine (820 m) in Hokkaido. One solid nucleus (center nucleus) was observed almost always in the central portion of a snow crystal. From their electron micro-diffraction patterns, the materials of the nuclei were classified into three kinds; a single crystal, polycrystalline or amorphous substances. The patterns of the single crystal gave a hexagonal cross grating, the atomic arrangement in their basal plane had some similarity to that of an ice crystal. Some polycrystalline center nuclei were small sea salt, particles. Their shape was similar to that of the nuclei of sea sprays, and their patterns coincide with that of the sea salt. Some amorphous center nuclei were observed which looked like a liquid droplet under the operating condition of the electron-microscope. It seemed that they are chemical components in the sea salt such as KCl or MgCl particles.