LESSONS IN CHEMISTRY. 151 



exerts less strain on the gun. The early history of powder is 

 obscure. Our first knowledge of its use as a military agent dates 

 from the seventh century, while its first use as a propelling agent 

 was in Spain in the 12th century. It was known and used in Eng- 

 land early in the 1.4th century. 



Potassium and its compounds have been derived from the rocks 

 through the medium of plants, but sodium and its compounds 

 are derived from seawater, which derived them from the older 

 rocks by solution. While land plants gather up potassium com- 

 pounds along with some sodium compounds from the soils, 

 water takes up sodium compounds, with some potassium com- 

 pounds, and carries them down to the sea. These sodium com- 

 pounds are taken up to some extent by seaweeds, and we can 

 obtain sodium compounds from sea plants as we obtain po- 

 tassium compounds from land plants; but it is more profit- 

 able to take them directly from the seawater, or from beds of 

 rock salt which are supposed to have been deposited from sea- 

 water. 



Rock salt is sometimes mined as other rock is mined ; some- 

 times water is let into the mine, then pumped out after it has dis- 

 solved its load of salt. Again in connection with these salt beds 

 natural brines are abundant, which flow out as springs or may 

 be pumped out for use. In some places seawater is pumped into 

 shallow ponds or lakes and allowed to evaporate in the sun until 

 it becomes strong enough to pay for artificial evaporation. In 

 Russia, sometimes a portion of the pure water is allowed to 

 freeze out of the brine. Seawater, rock salt, and natural brines, 

 besides the sodium chloride, contain also magnesium and potas- 

 sium chlorides, and sulphates of magnesia and lime. But as the 

 water is evaporated, most of the sodium chloride crystallizes out, 

 leaving the other substances in solution with iodine, bromine, 

 etc. From the sodium chloride, chlorine, hydrochloric acid and 

 the metal sodium with its various compounds are derived. So- 

 dium chloride, common salt, formula NaCl, is too well known to 

 need any description. Its usefulness in preserving meat, and as 

 an article of food for man and beast make it almost essential to 



