IN the periodic classification of Mendeleeff there are vacancies for two elements with atomic weights about 99 and 187, which the author supposed might accompany unsuspected, molybdenum (96) and tungsten (184) in minerals. Slight differences in chemical behavior were found in specimens of tungstic acid prepared from different sources, and a complexity of the element tungsten was inferred. The abnormal specimens showed spectra identical with normal tungsten compounds. Similar results were found with molybdenum compounds, so that the spectroscopic evidence negatived the idea of complexity of tungsten and molybdenum. The author suggests the existence of isotopic forms of these elements after the analogy of lead.—Note in Jour. Soc. Chem. Ind. on an article by M. Gerber in Monit Scient.