POTASSIUM AND SODIUM. 219 



304. Borax. Chemically this substance is sodium bibo- 

 rate. It is found native in some of the lakes of Asia and of 

 California, and is also prepared by neutralizing with sodi- 

 um carbonate boracic acid obtained from hot springs in 

 Italy. It contains half its weight of water of crystalliza- 

 tion, having the composition Na 2 B 4 O 7 -f 10H 2 O. Borax is 

 much used in the trades for soldering. If you hold with 

 pincers over a spirit-lamp a piece of copper on which are 

 placed a bit of tin and of iron wire, the tin will melt, but 

 will not adhere to either metal. But if you smear the 

 three metals over with a paste made of moistened borax, 

 and repeat the experiment, you will find that the wire is 

 firmly soldered to the copper. The explanation is this : 

 Metals will adhere to each other only w r hen they have a 

 pure surface ; but heating them always produces at once 

 a film of oxide, and so prevents their adhering. Now the 

 borax serves to keep the surfaces bright by forming with 

 this oxide a sort of melted glass, which is easily pushed 

 aside by the melted solder. There are various substances 

 used in soldering, and they all act by removing in some 

 way the oxides produced by the heat. 



305. Soda Saltpetre. This salt, sodium nitrate, NaNO 3 , 

 resembles common saltpetre. It is found in large quanti- 

 ties in South America, where extensive plains are covered 

 with it, and it is exported to other countries under the 

 name of Chili saltpetre. It has the same amount of oxy- 

 gen in it that nitre has, and parts with it as readily, as is 

 shown by its brisk deflagration on glowing coals. But it 

 will not answer in place of nitre in gunpowder, simply be- 

 cause it is strongly disposed to attract water from the air. 

 Keeping the powder dry would be difficult if one of its 

 ingredients be deliquescent. 



306. Ammonium. With the group of metals we are study- 

 ing, the salts of ammonium may be conveniently ranged. 



