PLATINUM MOLYBDENUM. 197 



phuric acid, but is dissolved by nitrohydrochloric acid, by free 

 chlorine or bromine, and by mercury, with which it forms an 

 amalgam. 



Gold is generally trivalent, as in auric chloride, AuCl 3 , but 

 most likely also univalent in some compounds, as in aurous 

 chloride, AuCl. 



Auric chloride, AuCl 3 . Obtained by dissolving pure gold in 

 nitrohydrochloric acid and evaporating the solution. A mix- 

 ture of equal parts of auric chloride and sodium chloride is offi- 

 cinal under the name of chloride, of gold and sodium. It is an 

 orange-yellow, very soluble powder. 



Analytical reactions. 



1. Add hydrosulphuric acid to solution of gold : brown auric 

 sulphide, Au 2 S 3 , is precipitated, which is soluble in ammonium 

 sulphide. 



2. Add ferrous sulphate to solution of gold and set aside for 

 a few hours : metallic gold is precipitated as a dark powder, 

 which, by fusion, is converted into a metallic mass. 



Platinum, Pt = 194.4. Platinum, like gold, is found in nature 

 in the free state, the chief supply being derived from the Ural 

 mountains, where it is found associated with a number of 

 metals (iridium, ruthenium, osmium, palladium, rhodium) re- 

 sembling platinum in their properties. 



Platinum is of great importance and value on account of its 

 high fusing-point and its resistance to the action of most chemical 

 agents, for which reason it is used in the manufacture of vessels 

 serving in chemical operations. 



Platinum, when dissolved in nitrohydrochloric acid, forms 

 platinic chloride, PtCl 4 , a salt frequently used as a reagent for 

 potassium or ammonium salts, with which it forms insoluble 

 double chlorides of the composition PtCl 4 .2KCl and PtCl 4 . 

 2NH 4 C1. By heating the latter salt sufficiently it is decomposed, 

 and metallic platinum is left as a gray spongy mass. 



Molybdenum, Mo : 95.5. This metal is found in nature 

 chiefly as sulphide, MoS 2 , from which, by roasting, molybdic 

 oxide, Mo0 3 , is obtained. The oxide, when dissolved in water, 



