Abstract: For the preparation of high purity glass powder having a mean particle size of .ltoreq.10 .mu.m, glass powder having a larger particle size up to 300 .mu.m is ground to the desired particle size in a stirred mill with glass grinding elements in the presence of a grinding liquid comprising water or preferably a mixture of at least 50% by weight of water and at least one water-soluble, oxygen-containing organic compound having 1 to 5 carbon atoms in the molecule, e.g., tert.-butyl alcohol; the ground slurry is then frozen, and the solvent is subsequently removed from the frozen slurry by freeze-drying. A resultant glass powder with a mean particle size d.sub.50 of 0.5 to 2 .mu.m is particularly suitable as a filler for synthetic resins in the dental sector.

Abstract: The speed of loaded and unloaded driverless vehicles traveling along an inclined path is controlled by a hydraulic retarder mechanism which dissipates the kinetic energy of the vehicle. The retarder mechanism is velocity sensitive and projects into the path of part of the vehicle. The retarder mechanism slows down fast moving vehicles but has little or no effect on slow moving vehicles.