xvm THE CLASSIFICATION OF THE ELEMENTS 461 



subdivided into two distinct families. Unfortunately, the 

 distinction between the odd and even series is not a sharp 

 one, since the alkali-metals are found in series two, three, 

 four, six, and the halogens in series two, three, five, seven. 

 This difficulty may be diminished by placing the elements 

 of series two and three either on the right, or on the left 

 according to the relation which they show to the elements 

 of the later series ; or the elements of series two and three 

 may all be placed in the centre of the column as in Table D, 

 p. 462. 



Mendeleeff also recognised that whilst there were " sharp 

 differences between the last members of the odd series 

 (halogens), and the first members of the even series (metals 

 of the alkalis)," yet " the last members of the even series 

 resemble in many respects. . .' . the first members of the 

 odd series" (loc. cit. pp. 145, 146). Thus there was a 

 sharp break between Cl and K or Br and Rb, but a gradual 

 transition from Cr and Mn to Cu and Zn. Moreover, even 

 this interval was bridged over by a group of three metals, 

 which Mendeleeff placed together in an additional Group 

 VIII. The even and odd series were thus linked together 

 by a remarkably smooth TRANSITION SERIES of elements 

 (loc. cit. p. 146) : 



Cr = 52, Mn = 55, Fe = 56, Co = 59, Ni = 59, Cu = 63, Zn = 65. 

 Mendele'eff therefore proposed to describe the triad of 

 Group VIII with the octave or short period on either 

 side as a LONG PERIOD (loc. cit. p. 146) of 7+3 + 7 = 17 

 elements. Similar long periods were recognised on either side 

 of the triads Ru= 104, Rh= 104, Pd= 1 06 and Os= 193 (?), 

 Ir=i 95 (?), Pt=i 9 7. 



A periodic classification of the elements. A modern 

 classification of the elements which embodies all the essential 

 features of the tables of Lothar Meyer and Mendeleeff is set 

 out in Table D. 



