It is well known that the liquid inclusions commonly found in quartz, fluor-spar, and other minerals resemble in some degree those that occur in crystals of sodium chloride, alum, and many salts. Chloride of sodium in particular shows innumerable cavities of varying size, cubical in form (negative crystals), and generally containing an enclosed bubble, especially if heated gently after the crystals are formed. The smaller cavities show Brownian movement of the enclosed bubble in a very striking manner. (See Fig. 1, which gives a diagrammatic view of a salt crystal as seen through the microscope.)