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 work of savants was done in discovering as exactly as possible the weights of these atoms. This done, many laborious attempts were made, with only partial success, to link the atomic weights with the properties, etc., of the chemical elements. Newlands, De Chancourtois, Odling, Gmelin, and others made attempts to formulate the law, but success was left to the genius of Mendeléeff. He arranged the elements then known in series, standing in a sort of arithmetical progression according to their atomic weights, and chemical and physical properties. He says in his great work (loc. cit.) that "even the physical properties of selenium and its compounds, not to speak of their composition, determined by the group in which it occurs, may be foreseen with a close approach to reality from the properties of sulphur, tellurium, arsenic, and bromine. In this manner it is possible to foretell the properties of still unknown elements."

In the series some gaps presented themselves, and so great was Mendeléeff's confidence in the periodic law, that he at once predicted the discovery of new elements to fill these gaps. This was one of the most daring prophecies ever made in science—and in an exact science too, the science of weighing and measuring—but events have justified the seeming audacity.

In 1871 the prediction was made that three elements would be discovered having certain properties to fit into the gaps of Mendeléeff's table, and to these undiscovered