254 GENERAL SCIENCE 



oxygen. Sodium reacts vigorously with water to form 

 sodium hydroxide and hydrogen. The heat evolved 

 melts the sodium which forms into a ball and floats on 

 the water. 



Experiment 69. Place a small piece of sodium in a wide- 

 mouthed bottle which is two thirds full of water and cover the 

 bottle with a piece of glass. Test the gas formed in the top of 

 the bottle with a flame. In this experiment care should be taken 

 to have the hands dry as well as all apparatus used in handling 

 sodium. 



Salt, a compound of sodium, sodium chloride, is found 

 in sea water and in extensive deposits at Stassfurt and 

 Reichenhall in Germany, in Cheshire, England, and in 

 several parts of the United States. It is a necessary 

 article of diet. Salt may be obtained from salt water by 

 evaporating the water. It crystallizes in white cubes. 

 Other useful compounds of sodium are sodium nitrate 

 (Chile saltpeter) and sodium bicarbonate, which is 

 commonly known as baking soda. 



Silver. - - The chief supply of silver is obtained from 

 ores of lead and copper. Silver is mined extensively in 

 the western part of the United States, 'Mexico, and 

 Australia. For silverware and coins, silver is alloyed 

 with 10 per cent of copper ; that is, they are " 900 fine." 

 Sterling silver is 925 fine or it is 92| per cent pure 

 silver. Articles may be plated with silver by the method 

 described under electroplating in Chapter XI. Mirrors 

 are silvered by cleaning the surface of the glass and pour- 

 ing over it a solution of silver nitrate, ammonium hy- 

 droxide, and some reducing agent, such as formaldehyde. 

 The film of silver which adheres to the glass is dried and 

 varnished to protect it. Photographic plates and films 

 are made by washing them with an emulsion of silver 



