Abstract: Solid glass preforms are made by externally heating a tubular preform by a plasma which is reciprocated along the hollow tube. The plasma gas consists entirely or partly of a molecular gas (N.sub.2 or O.sub.2, for example). The hollow preform is preferably rotated during collapsing.

Abstract: A method of manufacturing a low-pressure mercury vapor discharge lamp comprising a discharge tube (1), to the inner wall of which is applied a layer of luminescent material (4) by means of a suspension of the luminescent material, which suspension is brought into contact with this inner wall in such a manner that a layer of suspension adheres thereto, the non-adhered suspension disappearing from the tube, after which the layer is dried by heating a zone (14) of this layer extending along the tube circumference by means of a high-frequency electric field which from a starting point near the upper end of the tube performs such a movement with respect to the tube that the zone moves towards the lower end of the tube.

Abstract: An isothermal heating device comprises an annular tubular body providing a heating chamber for objects. Between the inner wall and the outer wall of this tubular body there is provided a plurality of separate ducts which are situated in a ring-shape about the heating chamber and which extend parallel to the tubular body axis. These ducts contain an evaporable heat transport medium and are in communication with each other preferably at one end of the tubular body. Such interconnection is in turn connected with a gas buffer reservoir containing an inert control gas.