Abstract: In a discharge lamp sealing apparatus 30, luminescent substances are charged into an arc tube 11 through an opening 13b thereof, and an electrode member 15 is then inserted into the arc tube 11 through the opening 13b. A lower end of the arc tube 11 is supported by a support jig 57 in a state that a glass ring 16c is placed around the circumference of the opening 13b. The arc tube 11 is set in an air-tight condition by a feeding conduit 51 and inserted into a heating unit 40. The heating unit 40 fuses the glass ring 16c with heat of an infrared lamp and thereby seals the opening 13b. During the sealing process, one end of the arc tube 11 is supported by the support jig 57. The support jig 57 is mainly made of a material having a greater thermal conductivity than that of the material of the arc tube 11, for example, a metal material like Al or Cu. This arrangement enables heat to be readily conducted from the arc tube 11 to the support jig 57 and accordingly prevents a temperature rise in the arc tube 11.

Abstract: Light-transmissive ceramics are disclosed composed of a plurality of oxides each having a negative standard Gibbs energy of formation (.DELTA.Gf.sup.0) with an absolute value greater than 1581.9 KJ/mol. The light-transmissive ceramics are produced by subjecting the oxides to hot isostatic pressing at a high temperature under a high pressure after they have been subjected to primary sintering. The light-transmissive ceramics of Al.sub.2 O.sub.3 /La.sub.2 O.sub.3 are of a composite structure with particles of La.sub.2 O.sub.3 being present between particles of Al.sub.2 O.sub.3, rather than Al.sub.2 O.sub.3 and La.sub.2 O.sub.3 forming a solid solution. In the composite structure, the particles of La.sub.2 O.sub.3 have sizes that are 1/3 to 1/30 of the particles of Al.sub.2 O.sub.3. Maximum size particles of Al.sub.2 O.sub.3 have a longer axis of 30 .mu.m or less and a shorter axis of 20 .mu.m or less, and maximum size particles of La.sub.2 O.sub.3 or the like have a longer axis of 10 .mu.