DMITRI IVANOVITSCH MENDELEEFF. 851 



ment of the Periodic Law. In March, 1869, before the Russian 

 Chemical Society, he set forth his conchisions as follows : — 



"1. The elements, if arranged according to their atomic weights^ 

 exhibit an evident periodicity of properties. 



"2. Elements which are similar as regards their chemical proper- 

 ties have atomic weights which are either of nearly the same value 

 (e. g., platinum, iridium, osmium) or which increase regularly (e. g., 

 potassium, rubidium, caesium). 



"3. The arrangement of the elements, or of groups of elements in 

 the order of their atomic weights corresponds to their so-called valen- 

 cies as well as, to some extent, to their distinctive chemical properties 

 — as is apparent among other series in that of lithium, beryllium, 

 boron, carbon, nitrogen, oxygen and fluorine. 



"4. The elements which are the most widely diffused have small 

 atomic weights. 



"5. The magnitude of the atomic weight determines the character 

 of the element just as the magnitude of the molecule determines the 

 character of a compound body. 



" 6. We must expect the discovery of many yet unknown elements, 

 for example, elements analogous to aluminium and silicon, whose 

 atomic weight would be between 65 and 75. 



" 7. The atomic weight of an element may sometimes be amended 

 by a knowledge of those of the contiguous elements. Thus, the 

 atomic weight of tellurium must lie between 123 and 126, and cannot 

 be 128. 



"8. Certain characteristic properties of the elements can be fore- 

 told from their atomic weights." 



In his Faraday Lecture, June 4th, 1889, he describes, in dramatic 

 fashion, the verification of his predictions : — 



"Before the promulgation of this law the chemical elements were 

 mere fragmentary, incidental facts in Nature; there was no special 

 reason to expect the discovery of new elements, and the new ones which 

 were discovered from time to time appeared to be possessed of quite 

 novel properties. The law of periodicity first enabled us to perceive 

 undiscovered elements at a distance which formerly was inaccessible 

 to chemical vision; and long ere they were discovered new elements 

 appeared before our eyes possessed of a number of well-defined 

 properties. We now know three cases of elements whose existence 

 and properties were foreseen by the instrumentality of the periodic 

 law. I need but mention the brilliant discovery of gallium, wliich 

 proved to correspond to eka-aluminium of the periodic law, by 



