Hollow glass microspheres as measured by a gas pycnometer
"Average density" is the quotient obtained by dividing the mass of a sample of hollow glass microspheres by the true volume of that mass of hollow glass microspheres as measured by a gas pycnometer. "True volume" is the aggregate total volume of the glass bubbles (not the bulk volume); "glass" refers to all synthetic amorphous inorganic solids or melts that can be used to form amorphous solids, where the raw materials used to form such glass includes various oxides and minerals; "HOLLOW GLASS MI