224 MINERALOGY 



metals, strong bases, and only take the part of a base when com- 

 bining with other metals or acids to form salts. The solubility of 

 their salts is graded and decreases from the top to bottom, or from 

 lithium to caesium. Similar salts crystallize in the same system 

 and usually with the same symmetry, and with angles and ele- 

 ments very closely related. While" the alkali metals form a very 

 closely related group of elements, possibly more closely related than 

 some other groups of the table, it is true, however, that those ele- 

 ments falling under each other in the vertical columns are very 

 closely related in their chemical properties. In a considera- 

 tion of .those elements which fall in the continuous line, as from 

 lithium to fluorine, there is a continuous gradation from the very 

 basic, on the one hand, metals which are alwaj^s found taking the 

 part of a base in the formation of salts, to those which, on the other 

 hand, as fluorine, are always found taking the part of an acid 

 when forming salts. Between these two extremes there are ele- 

 ments which may take the part either of an acid or a base ; this is 

 particularly true of those elements, like aluminium, which are 

 found near the middle, between the two extremes. 



It has long been recognized that all salts were composed of two 

 distinct components, the acid radicle, or acid anhydride, and the 

 base, or oxide of the metal. This was Berzelius's conception of a 

 salt, and the formulae of minerals were formerly written with this 

 conception in view, as barite, BaO, SO 3 . Here BaO is the oxide 

 of the base, barium, and SO 3 is the oxide of the acid-forming ele- 

 ment, sulphur. Indeed at the present time the analyses of rocks 

 and minerals are still reported in this form as BaO = 65.7 and 

 SO 3 = 34.3 per cent. 



By the study of the behavior of salts, or more especially elec- 

 trolytes, in solution and under the influence of an electric current, 

 it has been demonstrated that salts do not break down into the two 

 components of a basic oxide, as BaO, on the one hand, and acid 



oxide, as SO 3 , on the other, but they are dissociated into ions, inde- 



+ + 

 pendent components, one of which is formed by the metal Ba or a 



+ 



complex like NH 4 , taking the place of a metal, charged with a posi- 

 tive charge of electricity and traveling toward the negative pole ; 

 these are termed cations. The anion is that component which 



carries a negative charge, in this case SO4, and travels toward the 



positive pole or anode. Barium sulphate thus Breaks up in the two 



+ + 

 ions Ba and SO 4 . 



