A CENTURY OF CHEMISTRY. HI 



may be said in a sentence that the general result of 

 chemical work, since Mendelejeff and Meyer stated 

 the Periodic Law in 1869, has been to show that " al- 

 most every well-defined and comparable property of 

 the elements appears as a periodic function of the 

 atomic weights '^ (Ostwald). The atomic volume 

 shows the periodic variation most clearly (Meyer), 

 the melting point of the elements varies periodically 

 (Carnelley), the same holds true of the specific gra- 

 vities, the magnetic properties of elements depend on 

 the position occupied in the periodic system (Carnel- 

 ley), there is also a periodicity in the amount of heat 

 developed in the formation of the chlorides, bromides, 

 and iodides (Laurie) ; these must serve as illustra- 

 tions of the manifold justification which the theory 

 has received. 



The Test of Prophecy. — In regard to vital phenom- 

 ena where the operative factors are usually complex 

 and numerous, there are few who would be willing to 

 submit their favourite generalisations to the severe 

 test of using them as a basis for prophecy, as the as- 

 tronomer, for instance, can do with some security. 

 But this severe test Mendelejeff did apply to his 

 periodic law. 



In his arrangement of elements into groups and 

 series, Mendelejeff was compelled to leave certain 

 blanks. He asserted that these would be filled up 

 by the discovery of new elements. 



" He was able to foretell the atomic weights and 

 other properties of these elements from their posi- 

 tion in the system, with the aid of the properties ob- 

 served in the groups and series, which, like a system 

 of co-ordinates, could be called in to assist. Three 

 such blanks occurred in the first five series, and these 

 he indicated as representing the positions of eka- 



