268 METALS AND THEIR COMBINATIONS. 



ble closely. Kubidium occurs in carnallite of the Stassfurt beds, and is obtained 

 as rubidium alum, from the mother-liquors after the potassium chloride is 

 crystallized out. Csesium takes fire in air at the ordinary temperature, and it 

 is the most electropositive of all metals. Kubidium takes fire in air and decom- 

 poses water with greater energy than does potassium, the hydroxide formed, 

 Rb(OH), having even stronger basic properties than potassium hydroxide. 

 Both rubidium and caesium have a marked power of forming double salts. All 

 the salts are white and soluble in water. Probably the one most often used 

 medicinally is ccesium-rubidium-ammonium bromide, (CsEb)Br 2 .3NH 4 Br. Rubid- 

 ium bromide, RbBr, and iodide, Rbl, have been recommended as substitutes for 

 the corresponding potassium salts. Cesium bromide, CsBr, has also been used. 



23. AMMONIUM. 

 NH^IS (17.93). 



General remarks. The salts of ammonium show so much resem- 

 blance, both in their physical and chemical properties, to those of the 

 alkali-metals, that they may be studied most conveniently at this 

 place. 



The compound radical NH 4 acts in these ammonium salts very 

 much like one atom of an alkali-metal, and, therefore, frequently has 

 been looked upon as a compound metal. The physical metallic prop- 

 erties (lustre, etc.) of ammonium cannot be fully demonstrated, as it 

 is not capable of existing in a separate or free state. There is known, 

 however, an alloy of ammonium and mercury, which may be obtained 

 by dissolving potassium in mercury, and adding to the potassium- 

 amalgam thus formed, a strong solution of ammonium chloride, when 

 potassium chloride and ammonium-amalgam are formed. The latter 

 is a soft, spongy, metallic-looking substance, which readily decomposes 

 into mercury, ammonia, and hydrogen : 



HgK + NH 4 C1 = KC1 + NH 4 Hg; 

 NH 4 Hg = NH 3 + H + Hg. 



The source of all ammonium compounds is ammonia NH 3 , or am- 

 monium hydroxide, NH 4 OH, both of which have been considered 

 heretofore. 



A solution of ammonia has much weaker basic properties than a solution 

 of sodium or potassium hydroxide has. In a normal solution (about 1.7 per 

 cent.) only about 0.4 per cent, of the ammonia molecules are dissociated into 

 NH 4 - and (OH) / ions. There is much free NH 3 , besides the NH 4 OH which 

 results from union of NH 3 with water. It is only the ionized portion of the 

 NH 4 OH which shows basic properties. 



The ionic equation for the neutralization of ammonia water with an acid is 

 this: 



NH 4 -,-f OH' + H- + d' -> NH 4 - + Cl' + H 2 O. 

 As fast as (OH) 7 and H' ions unite to form water, more NH 4 OH dissociates 



