Examples of suitable desiccants are inorganic salts, such as calcium chloride and sodium carbonate, organic polymers, such as the product obtainable under the trade name Kollidion 90 F, and silicon-dioxide-containing desiccants, such as silica gel, zeolites and desiccants which are obtainable under the trade name Tixosil 38, Sipernat 22 S or Aerosil 200.