482 Prof. Odling on the Natural Groupings 



Na NO^ isomorphous with Ca^ CO^ in calc-spar, 

 and 



KNO^ isomorphous with Ca'^ CO^ in arragonite. 



Group VII. Magnesium, Zinc, Cadmium. 



These three metals are generally distributed in three separate 

 groups, notwithstanding that many of their analogies are ex- 

 tremely well known. In this group, as in the preceding, we have 

 the remarkable sequence of the atomic weights : — 



Mg = 12-16 100-36 __ ,_ 



Zn = 32-5 —g— = 33-45 mean. 



Cd = 55-7 



Sum 100-36 Mean difference =20? 



The three atomic volumes, as determined by Kopp, present a 

 similar sequence : — 



]\Ig = 40 



Zn = 58 179 .„ _ 



Cd =_81 -3- =^9-6 mean. 



Sum 179 



The number for magnesium is the saline volume. Those for 

 zinc and cadmium represent both the primitive and the saline 

 volumes. All three metals are soft, white, readily fusible, mal- 

 leable, and crystallize in octahedra. Magnesium is the least 

 volatile ; zinc, from its volatility, is prepared by distillation ; 

 while cadmium is one of the most volatile of all the metals. 

 Unlike the metals of the two preceding groups, the metals mag- 

 nesium, zinc, and cadmium are all permanent in dry air, and 

 acquire only a film of oxide in moist air. All three are combus- 

 tible at a high temperature. All the metals of this group are 

 remarkable for the facility with which they precipitate other 

 metals, iron, copper, and tin for example. Magnesium decom- 

 poses boiling water, though very slowly ; zinc requires a slight 

 acidulation, cadmium a stronger acidulation. Each metal forms 

 only one class of salts. The salts are for the most part colour- 

 less, like those of the two former groups. The three oxides are 

 insoluble in water. The three hydrates, MgHO, ZnHO, and 

 CdHO, are insoluble in water but soluble in ammonia. The 

 three sulphides are insoluble in water. Sulphide of magnesium 

 is soluble in acetic acid, — of zinc in very dilute chlorhydric acid, 

 — of cadmium in stronger chlorhydric acid. Sulphide and sul- 

 phydrate of zinc correspond with magnesian sulphide and sulphy- 

 drate in having a white colour. The sulphides of zinc and mag- 

 nesium are the only two insoluble sulphides that are white. The 



