—for there were a few of which, not enough was yet accurately known to determine their subjection to the rule—arranged in the order of their atomic weights and in groups or periods showing their relations and analogies. These periods might be said to be self-constituted; for, without departing from the orderly arrangement which Mendeleef had declared to exist, they so fell in line as to exhibit the very likenesses and differences which he had insisted upon as a part of his theory. Arranging them in parallel columns, it appeared that the several members of each period were substances that showed no similarity or community of chemical properties with one another; but that the members of the different periods showed an unmistakable parallelism with the corresponding members of the previous period. The columns also exhibited a regular gradation of electro-chemical properties, the most electro-positive elements occupying the places at their heads, and the extreme electro-negative elements the bottom places. The results of later discoveries and more accurate determinations have all been to confirm the correctness of the tabulation and the periodic theory. Thus scandium, gallium, and germanium, when discovered and examined, were found to fit into vacant places in the table, and to possess the atomic weights and the properties which the authors had predicted should belong to the elements falling in those places; and Mendeleef was able to say, in his Faraday lecture, delivered twenty years after the first suggestion of his theory, "When, in 1871, I described to the Russian Chemical Society the properties, clearly defined by the periodic law, which such elements ought to possess, I never hoped to live to mention their discovery to the Chemical Society of Great Britain as a confirmation of the exactitude and the generality of the periodic law." Up to the time of the formulation of this law. Prof. Thorpe says in his article: "The determination of the atomic value or valency of an element was a purely empirical matter, with no apparent necessary relation to the atomic value of other elements. But to-day this value is as much a matter of a priori knowledge as is the very existence of the element or any one of its properties. Striking examples of the aid which the law affords in determining the substituting value of an element are presented in the cases of indium, cerium, yttrium, beryllium, scandium, and thorium. In certain of these cases, the particular value demanded by the law, and the change in representation of the molecular composition of the compounds of these elements, have been confirmed by all those experimental criteria on which chemists are accustomed to depend. . . . The law has, moreover, enabled many of the physical properties of the elements to be referred to the principle of periodicity. At the Moscow Congress of Russian Physicists, in Au-
