Influence of Atomic Weight. 313 



another, and those belonging to odd series also Lear great 

 resemblance to one another ; whereas the corresponding mem- 

 bers of even and of the following uneven series, though they 

 have many points of resemblance, yet differ from one another 

 in character much more than the corresponding members of 

 two even or two uneven series. Thus, in group II., Ca, Sr, 

 and Ba are nearly related, and also Mg, Zn, and Cd ; but Ca, 

 though in many respects it resembles Zn, yet it differs from it 

 far more than it does from Sr or Ba. This theory of odd and 

 even members of a group is well illustrated by the degree of 

 reducibility of the elements to the free state,— those elements 

 (as Ca, Sr, Ba, Ti, Zr, V, Cr, Mn, Ta, Nb, W, B, Be, &c.) be- 

 longing to even series being obtained in the free state with 

 comparative difficulty, whereas elements belonging to odd 

 series, as Cu, Ag, Au, Zn, Cd, Hg, Tl, Ir, Sr, Pd, Sn, Bi, &c, 

 are easily reducible. I also hope to show shortly that all ele- 

 ments belonging to even series are paramagnetic, whilst those 

 belonging to odd series are diamagnetic. We must therefore 

 distinguish between Series and Groups, and between the even 

 and odd members of a group. 



We have also to distinguish between a long period and a 

 short period. A short period contains but seven members ; 

 thus from Li to F and from Na to CI form two short periods. 

 A long period, on the other hand, embraces seventeen mem- 

 bers : thus from K to Br and from B,b to I form two long- 

 periods ; for we have to pass from K over sixteen elements 

 before we come to one, viz. Cs, in which the properties of K 

 are more or less repeated, whereas starting from Li, we have 

 but to pass over six elements before we come to Na, or a point 

 at which the properties of Li are in some measure repeated. 



Another point to be noticed is that the last or seventh mem- 

 ber of an odd series, as CI, is very different, and in fact oppo- 

 site in properties to the first member of the next even series, 

 as K ; whereas the seventh member of an even series, as Mn, 

 has in many respects considerable resemblance to the first 

 member of the next odd series, as Cu. Also in between the 

 seventh member of an even series and the first member of the 

 next odd series we have a somewhat peculiar group, viz. the 

 eighth, in which there are three (or four) elements in each even 

 series ; and these elements stand midway in their chemical 

 and physical properties between the preceding member of the 

 seventh group and the first member of the next series. In other 

 words, there appears to be an abrupt change to exactly oppo- 

 site properties as we pass from the end of an odd series to the 

 beginning of the following even series, as from CI to K, Br 

 to Rb, &c. ; whereas we pass gradually from the seventh to the 

 eighth groups of an even series, and thence to the first mem- 

 bers of the following odd series. 



