With the object of examining whether or not Marshall and Langleben’s theory on the snow-crystal habit is in agreement with experimental fact, we carried out a series of experiments on artificial snow crystals by means of a diffusion cloud chamber where the cloud droplets had teen previously removed. The ambient vapour density was measured by a “drop hygrometer” method.
 It was found (1) that the snow-crystal habit is primarily determined not by the excess of the ambient vapour density tut by the ambient temperature; (2) that snow crystals produced in aerosol-free air show tendencies to grow either in sheath form or in dendritic form with sector-like structure; (3) that the trace of foreign vapours e. g. silicone or acetone remarkably affects the snow crystal habit.