and the Occurrence of the Elements in Nature. 197 



form the gradual passage from even series to odd series, or 

 rather from the falling to the rising portions of the curve, 

 since the elements of this group always form the minima of 

 the curve. 



§ 7. After the above somewhat lengthy but necessary 

 introduction, we may now proceed to the proper subject of the 

 paper. This may be divided into three heads: — (1) the redu- 

 cibility of the elements to the free state from their state of 

 combination ; (2) occurrence of the elements in the free state 

 in nature ; (3) occurrence of the elements in the combined 

 state in nature. 



§ 8. (1) Eeducibility of the Elements. — Elements belong- 

 ing to odd series are, as a ride, easily reducible to the free state, 

 whilst those belonging to even series are only reducible to the free 

 state ivith difficulty. 



The only exceptions to this rule are as follows : — {a) 0, N, 

 0, Na, Mg, Al, and Si, for explanation of which see § 5. 

 (b) The elements of Group VIII., for which see § 6. 



§ 9. (2) Occurrence of the.Elements in the Free State 

 in Nature. — This is, of course, closely connected with the 

 property of reducibility already considered, so that we may 

 expect a similar relation to hold good. An inspection of the 

 Table in fact shows that — 



Elements belonging to even series {except 0, N, 0, and 

 Group VIII.) never occur in the free state in nature ; whereas 

 elements belonging to odd series generally and sometimes fre- 

 quently do so occur. 



Thus the following elements of odd series are frequently 

 found in the free state: — Cu, Ag, Au, Hg, As, Sb, Bi, S, Se, Te; 

 whilst Pb and Sn sometimes occur native, Ga, In, Tl, and 

 Cd are distributed so sparsely that we can scarcely say whether 

 they occur free or not. 



Of elements belonging to odd series which have not been 

 found free, 01, Br, I, F, Zn, and P are therefore the only 

 notable exceptions, since Na, Mg, Al, and Si may be accounted 

 for" by § 5. 



As regards the exceptions among even series, C, N, and 

 fall under § 5, and Group VIII. under § 6. It may further be 

 said in reference to this group, that all the elements belonging 

 to it except Co occur native ; Fe and Ni, however, only in 

 meteorites. Of the three sets of triads belonging to this group, 

 the tendency to occur in the free state increases as the average 

 atomic weight of each triad increases. Thus the triad Os, Ir, 

 Pt are more likely to occur free than the triad Eu, Bh, Pd ; 

 and the latter more likely than the triad Fe, Co, Ni. 



