When an acid is mixed with a base, the result is a substance more or less similar in its properties to common salt; for this reason all compounds formed by the neutralization of an acid with a base are called salts.  If, instead of hydrochloric acid (HCl), we use an acid solution of potassium tartrate, and if instead of caustic soda we use bicarbonate of soda (baking soda), the result is a brackish liquid as before, but the salt in the liquid is not common salt, but Rochelle salt.  Different combinations of acids and bases produce different salts.  Of all the vast group of salts, the most abundant as well as the most important is common salt, known technically as sodium chloride because of its two constituents, sodium and chlorine.

We are not dependent upon neutralization for the enormous quantities of salt used in the home and in commerce.  It is from the active, restless seas of the present, and from 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 carbonates:  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—­bicarbonate 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 water alone are kneaded and baked in loaves, the result is a mass so compact and hard that human teeth are