(8) I came to regard the tufts of ef- florescent crystals on the black ice as an indication of its thickness. When the tufts are long and are closely crowded together, the ice is safe to go out upon. When they are short and sparsely scattered over the surface or are absent, the ice is too thin to support one. They are more salty than the ice itself and are formed by the gradual slow freezing in the air at a lower temperature than that of the formation of the sheet ice; of the residual salt left in the interstices of the sheet ice which is gradually extrud- ed as the ice contracts, under the influence of temperatures 10 (or 15) de- grees F and more below the freezing