312 HISTORY OF CHEMISTRY. [LECTURE XV. 



(what must be regarded as specially important) pointed out 

 clearly the advantages of considerations of the kind. It is for 

 this reason that his paper at once made a great sensation, 

 whereas that of Newlands remained quite unnoticed. 81 



In the tabulation adopted by Mendelejeff, the elements 

 are arranged according to their atomic weights ; but they are 

 further arranged into divisions in such a way that the elements 

 which are analogous to one another fall into vertical columns 

 and form groups, whilst each set of from seven to ten elements 

 succeeding one another in a horizontal line in the order of 

 their atomic weights, constitutes a short period within which 

 the properties (physical as well as chemical) progressively vary. 

 Two successive horizontal series form a long period, in con- 

 nection with which it is to be noted that, in the groups, the 

 analogies between elements of the even, and also of the uneven, 

 horizontal series, are greater amongst themselves than those 

 between elements which belong partly to the even and partly 

 to the uneven series. 



Of the applications of the "periodic law," the two following 

 have attained special importance: (T) The determination or 

 correction of the atomic weights of insufficiently investigated 

 elements, and (2) the prediction of the properties of unknown 

 elements. 



With respect to the first of these applications the following 

 facts must be mentioned here. In conformity with the pro- 

 posal of Awdejeff, 82 Mendelejeff assumed the atomic weight of 

 beryllium to be 9, and placed this element in a group along 

 with magnesium ; whereas it had hitherto been regarded by 

 many chemists as a metal akin to aluminium, and of atomic 

 weight 13.5. This new view called forth a prolonged dis- 

 cussion, which terminated, however, with the complete triumph 

 of Mendelejeff s opinion. 83 



81 With respect to the question of priority, compare Newlands, Chem. 

 News. 32, 21, 192; L. Meyer, Berichte. 13, 259; Mendelejeff, ibid. 13, 

 1796. f2 Fogg- Ann. 56, 101 ; compare also KJatzo, J. pr. Chem. 106, 

 227. 83 Nilson and Fettersson, Berichte. II, 381 ; 13, 1451 ; 17, 987 ; 

 L. Meyer, ibid. 13, 1780; Reynolds, ibid. 13, 2412 ; Nilson, ibid. 13, 2035. 



