FOR some time chemists have been doubtful what value to assign to the atomic weight of beryllium. Some years ago Prof. Emmerson Reynolds determined the specific heat of this metal to be 0.642; this number multiplied into 9.1 gave 5.8 as the atomic heat of beryllium; in other words it confirmed the generally accepted atomic weight. In 1878 Nilson and Pettersson re-determined the specific heat of beryllium, and found the number 0.408 for the temperature interval 0°–100°; hence these chemists concluded that the atomic weight of the metal must be increased by one-half (13.6 × 0.408 = 5.6). If Be = 9.1 the oxide of beryllium is BeO, and the metal is placed in the magnesium group; but if Be = 13.6 the oxide is Be2,O3, and the metal is placed in the aluminium group. But there is no place in Mendelcjeff's classification of the elements according to the magnitude of their atomic weights for a metal with the atomic weight 13.6, forming an oxide M2O3, and exhibiting the properties of beryllium. The value of Mendelejeff's classification is however so great that chemists were not inclined to alter the atomic weight of beryllium except upon most cogent evidence.