of the Elements. 483 



corresponding salts are for the most part isomorphous, and pre- 

 sent great analogy of properties. The magnesian salts are most 

 neutral, then those of zinc, then those of cadmium. The three 

 chlorides are deliquescent. The hydrated chlorides are decom- 

 posed by heat, hydrochloric acid being liberated, — most readily in 

 the case of magnesium, less so in that of zinc, least of all in that 

 of cadmium. The three sulphates are soluble in water, and are 

 decomposed by heat, — that of cadmium most readily, then that 

 of zinc, and that of magnesium only at a white heat. The sul- 

 phates of magnesium and zinc are isomorphous, Mg^ S0'*.7H^0 

 with Zn2 SO'*. ZH^O. The sulphate of cadmium crystallizes with 

 four and eight atoms of water. The double sulphates of magne- 

 sium, zinc, and cadmium, with potassium, or sodium, or ammo- 

 nium, are isomorphous,— MgKSO^ eH^O, ZnKS^O*. eH^O, 

 and CdKSO*. 6H^0. The crystalline ammonio-phosphate of zinc 

 corresponds in composition with the ammonio-phosphate of magne- 

 sium dried at lOOX.; Zn2NH4P0''.H20 withMg^NH^PO^.H^O. 

 All three carbonates, borates, and phosphates are insoluble in 

 water but soluble in chlorhydric acid. The respective nitrates 

 and chlorates of magnesium and zinc correspond, — 



MgClQs. 3H20 with ZnClO^ . SH^O, 

 and 



MgNO^ . SH^O with ZnNO^ . SH^O. 



Nitrate of cadmmm crystallizes with two atoms of water. 



Zinc hydrate differs from the magnesian and cadmic hydrates 

 in being soluble in potash. Moreover, cadmic oxide and sul- 

 phide diflfer from the magnesian and zincic oxides and sulphides 

 in colour. 



The members of this seventh group, more particularly mag- 

 nesium, present some general relations to those of the sixth, but 

 not in any way sufficient to warrant the usual disposal of mag- 

 nesium in the calcic rather than in the zincic family. Magne- 

 sium differs from calcium and agrees with zinc in the character 

 of its metal, of its hydrate, of its sulphide, of its sulphate, and 

 of its hydrated chloride. Magnesite is isomorphous with bitter- 

 spar, calc-spar, and calamine. Moreover, the constitution of 

 bitter-spar, Ca M^j; CO^, tends rather against, than in favour of 

 the association of calcium and magnesium, as in mineral che- 

 mistry we rarely have legitimate double salts of two metals belong- 

 ing to the same natural family. In the solubility of their sul- 

 phates, the members of this group correspond rather with those 

 of the fifth than of the sixth group. 



These three groups present a very curious relation in their 

 atomic weights : — 



