696 SCIENCE PROGRESS 



into two natural families, the successive members of which occur 

 alternately. 



The most rational way of representing the arrangement is 

 not on a sheet, but on a cylinder or octagonal prism. A spiral 

 or solid helix is traced down the cylinder or prism, each turn 

 of the screw corresponding with the addition of eight or ten 

 elements arranged in their appropriate vertical columns. 



The doubling of the periodicity indicated by Mendeleef 

 leads to the arrangement of two natural families in each vertical 

 series. Let us examine a pair of these related families as re- 

 gards their capacities for yielding organic derivatives. 



(a) The Silicon Family 



It will be most convenient to start with the fourth vertical 

 series. Here we find, as the initial member, carbon itself, the 

 essential element of organic compounds. Following this'element 

 is silicon, which exhibits certain points of resemblance, but also 

 many points of difference. 



Silicon is followed successively by titanium and germanium, 

 and the question arises which of these is to be placed in the 

 same family as silicon. 



Germanium and tin, the next metal of the series, resemble 

 silicon in forming feebly acidic hydroxides existing in colloidal 

 forms ; their oxides have the same general formula R0 2 as silica, 

 and they yield volatile chlorides RC1 4 decomposable by water. 



Titanium and zirconium also resemble silicon in many of 

 their naturally occurring compounds ; they likewise form feebly 

 acidic gelatinous hydroxides and yield volatile chlorides and 

 bromides decomposed by water. 



Silicon evidently has affinities with both series, but the best 

 criterion of relationship is the capacity for forming organic 

 derivatives. This non-metal readily yields organic derivatives, 

 and a large number of these compounds have been described 

 by Friedel, Ladenburg, Emerson Reynolds, and more recently 

 by Kipping. 



Titanium and zirconium have, however, evaded all attempts 

 to combine them with hydrocarbon radicals ; they are found 

 directly associated with carbon in their carbides, but these 

 compounds are excluded from the present consideration. 



Turning to the germanium-tin series, we find that all the 

 elements of this group yield organic derivatives, and this pro- 



