in the Periodic Series of the Elements. Ill 



The atomic heat affords another proof of Lothar Meyer's 

 classification of sodium with the alkali metals, rather than 

 that of Mendeleeff, who, followed by W. Preyer, places it in 

 the copper group. Its chemical affinities to potassium are 

 very close ; thus E. Auge (Comptes Rendus, 1890, ex. p. 1139) 

 obtained a sodium alum, by regulating both concentration 

 and temperature. 



To get comparable results, the specific heats must be 

 observed at a constant temperature. With regard to melting- 

 point, magnesium presents an irregularity in the zinc group, 

 if the usual melting-point, 430°, is adopted ; but the number 

 has been corrected by more recent observation, restoring com- 

 plete harmony to the series. The order of melting-points of 

 Sn and Pb in Group IV., and of Sb and Bi in Group V., 

 both apparent anomalies, will be seen to favour, rather than 

 contradict, the laws of atomic heat and fusion ; for in both 

 cases the atomic heats are reversed to correspond with the 

 melting-points. 



I have placed cobalt in IY. b, and nickel in VI.fr, notwith- 

 standing Zimmermann's values for their atomic weights ; and 

 in doing so, as before, I have been guided entirely by the 

 atomic heats, leading to MendeleefFs original classification, 

 which placed Co before Ni on purely chemical grounds, from 

 the analogies of the former to Eh and Ir, and of the latter to 

 Pd and Pt (Ann. Supp. viii. p. 133 et seq.). Zimmermann 

 (1S86) finds M = 58'71, Co = 58'89. 



The above tables appear to establish the following laws : — 



1. In groups I. and II. (which I will call the metallic 

 groups), as the atomic weight increases, both the melting- 

 point and the atomic heat decrease. 



2. In groups III. and IV. and all the a groups (termed 

 collectively intermediate groups) the atomic heat increases 

 with the atomic weight, while the melting-point decreases ; 

 the former being characteristic of non-metals, the latter of 

 metals. 



3. In groups V., VI., and VII. (the non-metallic groups), 

 as the atomic weight increases, so do both atomic heat and 

 melting-point. 



4. In the b groups the melting-point increases with the 

 atomic weight, whilst the atomic heat decreases ; the former 

 being a non-metallic, the latter a metallic characteristic. 



The a may be called copper groups, and the b iron groups. 



It is to be noted that both groups IV. and V. become 

 irregular in the same way when their last members are 

 reached, these having more marked metallic characters. Thus 

 both Sn and Pb in group IV., and Sb and Bi in V. 7 have 



