no PROGRESS OF SCIENCE IN THE CENTURY. 



pendently reached the same conclusion: — That the 

 properties of the elements are periodic functions of 

 their atomic weights. " If all the elements be ar- 

 ranged in the order of their atomic weights a peri- 

 odic repetition of properties is obtained. This is ex- 

 pressed by the law of periodicity; the properties of 

 the elements, as well as the forms and properties of 

 their compounds, are in periodic dependence, or, ex- 

 pressing ourselves algebraically, form a periodic 

 function of the atomic weights of the elements." * 

 " If all the elements are arranged in the order of 

 their atomic weights in a series, their properties will 

 so vary from member to member that after a definite 

 number of elements has been passed either the first 

 or very similar properties will recur." f This 

 was the conclusion which Mendelejeff and Meyer ex- 

 pounded. 



Let us state the general idea once more. When 

 the elements are arranged according to the magnitude 

 of their atomic weights, " the elements following one 

 another show apparently no regularity in properties, 

 but after the lapse of a certain period the chemical 

 and physical behaviour of the elements now suc- 

 ceeding each other strongly recall that of the previ- 

 ous group, in fact, repeat it. The elements which 

 resembled one another were therefore united into 

 groups or natitral families, and these in their turn 

 were distinguished from the periods, which com- 

 prised the elements whose atomic weights lay be- 

 tween those of two successive members of a natural 

 family." X 



Scientific Justification of the Periodic Law. — It 



* Mendelejeff, Principles of Chemistry, Vol. II., trans, by 

 Kamensky and Greenaway, 1891, p. 16. 



t Ostwald, General Chemistry, trans, p. 35. 



i E. von Meyer, History of Chemistry, trans. 1891, p. 347. 



