264 THE POPULAR SCIENCE MONTHLY. 



— for tliere 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 arrange- 

 ment 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 jjart 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 ex,hibited 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 Men- 

 deleef 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 pos- 

 sess, I never hoped to live to mention their discovery to the 

 Chemical Society of Great Britain as a confirmation of the ex- 

 actitude 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 rela- 

 tion 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 exist- 

 ence of the element or any one of its properties. Striking exam- 

 ples of the aid which the law affords in determining the substi- 

 tuting 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 de- 

 pend. . . . The law has, moreover, enabled many of the physical 

 properties of the elements to be referred to the principle of peri- 

 odicity. At the Moscow Congress of Russian Physicists, in Au- 



