368 METALS AND THEIR COMBINATIONS. 



its salts platinum is in the anion, as PtCl,/'. The chloride, PtCl 4 , is 

 obtained by heating the chloroplatinic acid in a current of chlorine 

 at 360 C. Its solution in water gives red, non-deliquescent crystals 

 of the composition, H 2 PtCl 4 O.4H 2 O. 



Chloroplatinous acid, H 2 PtCl 4 , results when platinous chloride, 

 PtCl 2 , is dissolved in hydrochloric acid. The potassium salt, 

 K 2 PtCl 4 , is used in making platinum prints in photography. The 

 corresponding barium platinocyanide, BaPt(CN) 4 .4H 2 O, forms light 

 yellow crystals. Screens coated with this salt become luminous when 

 Rontgen rays (#-rays) fall upon them. Such fluorescent screens are 

 used for observing x-ray pictures. Ultra-violet and radium rays also 

 affect the screens. 



Iridium, Ir = 191.5. This element has been mentioned as one of the metals 

 which accompany platinum in nature. It is obtained from the material left 

 in the working of platinum ores. 



Iridium has a grayish- white color and resembles polished steel ; it is harder, 

 more brittle, specifically heavier and less fusible than platinum. The increased 

 hardness, rigidity and elasticity which iridium imparts to platinum makes the 

 alloy a valuable dental material. Gold pens are often tipped with iridium 

 which renders them more durable. 



Compact pieces of iridium are insoluble in all acids; when finely divided it 

 dissolves in nitro-hydrochloric acid. It is for these reasons that most of the 

 iridium is left in the residue of the material from which platinum has been 

 extracted. Several oxides, hydroxides and chlorides of iridium are known. 



Molybdenum, Mo = 95.3. This metal is found in nature chiefly as sul- 

 phide, MoS. 2 , from which, by roasting, molybdic oxide. MoO 3 , is obtained. The 

 oxide, when dissolved in water, forms an acid which combines with metals, 

 forming a series of salts termed molybdates. Of interest is ammonium molyb- 

 date, a solution of which in nitric acid is an excellent reagent for phosphoric- 

 acid, with which it forms a yellow precipitate, insoluble in acids, soluble in 

 ammonium hydroxide. 



QUESTIONS. How is antimony found in nature, and what are the proper- 

 ties of this metal ? State the composition of antimonous sulphide, and its 

 color when crystallized and amorphous. How do hydrochloric acid and alkali 

 hydroxides act upon antimonous sulphide? Mention the two chlorides of 

 antimony and state their properties. How is antimonous oxide made, and for 

 what is it used? Give tests for antimony. State the use made of tin in the 

 metallic state; mention the two chlorides of tin, and the use of stannous chlo- 

 ride. Describe processes for refining gold by the dry and wet methods. How 

 are gold and platinum found in nature ; by what acid may they be dissolved, 

 and what is the composition of the compounds formed? Which is the most 

 important compound of molybdenum, and what is its use? 



