228 MAWS CONQUEST OF SUBSTANCES 



the dead seas of the prehistoric past that our vast stores of 

 salt come. The waters of the Mediterranean and of our own 

 Great Salt Lake are led into shallow basins, where, after 

 evaporation by the heat of the sun, they leave a residue of 

 salt. By far the largest quantity of salt, however, comes 

 from the seas which no longer exist, but which in far remote 

 ages dried up and left behind them their burden of salt. 

 Deposits of salt formed in this way are found scattered 

 throughout the world, and in our own country are found in 

 greatest abundance in New York. The largest salt deposit 

 known has a depth of one mile and exists in Germany. 



Salt is indispensable on our table and in our kitchen, but 

 the amount of salt used in this way is far too small to account 

 for a yearly consumption of 4,000,000 tons in the United 

 States alone. The manufacture of soap, glass, bleaching 

 powders, baking powders, washing soda, and other chemicals 

 depends on salt, and it is for these that the salt beds are mined. 



210. Baking Soda. Salt is by all odds the most important 

 sodium compound. Next to it come the so-called carbon- 

 ates : first, sodium carbonate, which is already familiar to us 

 as washing soda ; and second, sodium bicarbonate, which is 

 an ingredient of baking powders. These are both obtained 

 from sodium chloride by relatively simple means ; that is, by 

 treating salt with the base, ammonia, and with carbon dioxide. 



Washing soda has already been discussed. Since baking 

 powders in some form are used in almost all homes for the 

 raising of cake and pastry dough, it is essential that their 

 helpful and harmful qualities be clearly understood. 



The raising of dough by means of baking soda bicarbon- 

 ate of soda is a very simple process. When soda is heated, 

 it gives off carbon dioxide gas ; you can easily prove this for 

 yourself by burning a little soda in a test tube, and testing 

 the escaping gas in a test tube of limewater. When flour and 



