THE PERIODIC LAW 149 



of hydrogen occur only in the fourth, fifth, sixth, and seventh 

 groups, four atoms of hydrogen combining with one of each 

 element of the fourth group, and this amount decreasing 

 until we find the halogens in the seventh group univalent 

 toward hydrogen. The first group includes the most electro- 

 positive elements, and there is a steady transition toward 

 the electro-negative end of the series in the seventh group, 

 while the eighth group shows a rather sudden return toward 

 the electro-positive side. The majority of the compounds 

 derived from elements at the left end of the table are soluble, 

 colorless, and volatile, whereas these properties change from 

 left to right until we find the maximum of insolubility, color, 

 and resistance to heat in the lower right hand of the table. 

 It is also possible to select analogous compounds of the differ- 

 ent elements and find those of similar properties falling within 

 a well-marked zone upon the chart. Mendeleeff, in his origi- 

 nal essay, added the following: (1) The elements which have 

 the lowest atomic weights are those most widely distributed 

 in nature, and also represent the most typical characteristics 

 found in the second series of the table; (2) the atomic weight 

 determines the character of an element; (3) from a considera- 

 tion of their position in the system new analogies can be dis- 

 covered between elements; (4) it may be expected that new 

 elements should be discovered to fill blank spaces within 

 the table, and their properties can be predicted from a con- 

 sideration of those of the adjacent elements; (5) errors in the 

 assumed atomic weights may be detected through an irregu- 

 larity in the position of the element in the periodic system. 

 All of these statements have been verified, and the im- 

 mediate acceptance of Mendeleeff' s views was facilitated 

 especially by the sensational discovery of a number of ele- 

 ments whose properties agreed accurately with those pre- 

 dicted by Mendeleeff. Thus gallium, germanium, and scan- 

 dium had been completely described with respect to their 



