THE PERIODIC SYSTEM 275 



and S 2 Cl2 (p. 252), are completely decomposed by water, and the 

 actions are not, in general, reversible. The halides of the typical 

 metals are not extensively hydrolyzed (see p. 369), and with 

 those that are not typical, the action is reversible. 



The distinction is not perfectly sharp, however. Thus, zinc 

 gives both salts like the sulphate, Zn.S0 4 , and chloride, Zn.Cl 2 , and 

 compounds like sodium zincate Na.HZn02. 



Classification by Atomic Weights. Newlands (1863-4) dis- 

 covered a surprising regularity that became apparent when the 

 elements then known were placed in the order of ascending atomic 

 weight. Omitting hydrogen (at. wt. 1) the first seven were: 

 lithium (7), glucinum (9), boron (11), carbon (12), nitrogen (14), 

 oxygen (16), fluorine (19). These are all of totally different 

 classes, and include first a metal forming a stongly basic hydroxide, 

 then a metallic element of the less active sort, then five non-metals 

 of increasingly negative character, the last being the most active 

 non-metal known. The next element after fluorine (19) was 

 sodium (23), which brings us back sharply to the elements that 

 form strongly basic hydroxides. Omitting none, the next seven 

 elements were sodium, (23), magnesium (24.4), aluminium (27), 

 silicon (28.4), phosphorus (31), sulphur (32), chlorine (35.5). 

 In this series there are three metals of diminishing positive char- 

 acter, followed by four non-metals of increasing negative activity, 

 the last being a halogen very like fluorine. On account of the 

 fact that each element resembles most closely the eighth element 

 beyond or before it n the list, the relation was called the law of 

 octaves. After chlorine the octaves become less easy to trace. 



That this periodicity in chemical nature is more than a coinci- 

 dence is shown by the fact that the valence and even many phys- 

 ical properties, such as the specific gravity, show a similar fluc- 

 tuation in each series. In the first two series the compounds with 

 other elements are of the types: 



