526 SMITH'S INTERMEDIATE CHEMISTRY 



metals all fall in the last group of the periodic system (see pp. 

 278, 282). Iridium is harder than platinum, and is alloyed with 

 it for special purposes (pen-points and vessels to resist fluorine). 

 A platinum-indium alloy is used for the international standards of 

 length and mass, kept in Paris. A palladium-gold alloy (palau) 

 has recently been devised as a substitute for platinum in labo- 

 ratory crucibles. 



Exercises. 1. Make an equation for : (a) the action of con- 

 centrated sulphuric acid on silver; (b) the decomposition of 

 silver chloride by light. 



2. Make an equation for: (a) the displacement of gold from 

 potassium chloraurate by silver; (b) the decomposition of chloro- 

 platinic acid, and of (c) ammonium chloroplatinate by heat; (d) 

 the action of zinc on chloroplatinic acid. 



3. What would be the advantages and disadvantages of using 

 gold instead of platinum for crucibles? 



