that, with different cements, give expansions having a greater spread than is desired. It is hoped that the cause of this can be found and that this spread can be eliminated or reduced.
MELTING POINT OF ALPHA-ALUMINA. 'The value usually quoted in the literature for the melting point of alumina (A1203) is 2,05 Β°0 C., which was published by C. W. Kanolt in I9I 4. Several investigators have since published values ranging from 2,OOI Β° C. to 2,o45 Β° C.. Alumina is not only an important constituent in many industrial products but has become essential in super-duty spark plug insulators, refractories for special high temperature applications, and also for insulators in the field of electronics. Also, a large number of systems which have been, or are being, investigated for the determination of phase relations include alumina as one of the end members. Consequently, it was considered important to attempt again the establishment of the true melting point of alumina of high purity and under carefully controlled conditions.
Three samples were availalNe, all of which contained over 99.9 per cent. of A1208. The specimens were heated in an oxidizing atmosphere, and under atmospheric pressure, using a furnace in which an electric current, passed through resistors made of thoria and ceria, produced the necessary temperature. The heating rate ranged from I Β° C. to 5 Β° C. per minute and observations were made with an optical pyrometer.
In seven tests, the minimum temperature at which melting was seen to begin was 1,99 °° C. and the maximum was 2,oio ° C. Although spectrographic analyses of four of the specimens, made after the melting tests, showed that they had "picked up" measurable impurity from the furnace atmosphere, especially silica and magnesia, the indications are that these melted materials contained about 99 per cent. of A120,.
As a result of these tests, which were made by R. F. Geller and P. J. Yavorsky and are reported in the April number of the Journal of Research (RP I649), it is bel~ieved that alumina melts within the range 2,000 Β° to 2,03 Β°o C.