. . . "As a first step in making the porous, self-supporting, air electrode tube of this invention, powdered oxides, carbonates, or other compounds that form oxides upon heating, such as oxalates, of low cost lanthanide mixtures, such as an unfinished lanthanide concentrate mainly comprising a mixture of at least La, Ce, Pr and Nd, for example, a mixture of at least La2 O3, CeO2, Pr6 O11, and Nd2 O3 or c" .