OXY-SALTS AND HALOID SALTS. 49 



especially when heated. Its ductility is so great that 

 Dr. Wollaston succeeded in making a wire of it the two 

 thousandths droVu") of an incn m diameter. In fusibility 

 this metal and mercury stand at opposite ends of the 

 scale. While mercury may be said to melt at 39 be- 

 low zero, platinum can not be melted in the hottest 

 furnace, as is shown in the crucibles of the chemist that 

 are made of it. The metals iridium, rhodium, and os- 

 mium, which are found in company with platinum, are 

 very hard, and are used for points to gold pens. There 

 is a mineral called iridosmine, because composed of irid- 

 ium and osmium, which is the hardest alloy known. 

 This mineral, which is found in scales, is also used for 

 pointing gold pens. 



CHAPTER VI. 



OXY-SALTS AND HALOID SALTS. 



95. The two Classes of Salts. Those salts which are 

 formed by the union of acids with oxyds are called oxy- 

 salts, because they have oxygen in their composition. 

 There is, besides, a class of salts which contain no oxy- 

 gen, but are composed of a metal and some other element. 

 Common salt is an example. This is composed of chlo- 

 rine and sodium, and is a chlorid of sodium. This is the 

 principal salt of this class, and hence the name given to 

 the whole class, haloid, from two Greek words, hals, sea- 

 salt, and eidos, like. Some of the oxy-salts have been al- 

 ready treated of in other connections, as the sulphates in 

 the chapter on sulphur and its compounds, from their 

 natural association with the sulphurets. There are also 

 some which will be reserved for another chapter, as the 

 silicates, on the same principle of natural grouping. 



96. Nitrate of Potash. This salt, commonly called ni- 

 tre or saltpetre, is produced in India and South America 

 from the decomposition of animal substances in the soil 



C 



