SALT&. 



141 



6tlier, arid it extinguishes flame instantaneously. Water may 

 be made by pressure to absorb three times its bulk of this 

 gas ; by which it acquires an acidulous and not unpleasant 

 taste. Soda water, cider, and other fermented liquors owe 

 their briskness and sparkling to the presence of this gas. 

 Fatal accidents often happen from the burning of charcoal 

 in chambers, for wherever charcoal is burned this gas is 

 always formed. It so often occupies the bottoms of wells, 

 that workmen ought not to venture into such places without 

 previously letting down a lighted candle. If the candle 

 burns they may enter it with safety ; if not, a quantity of 

 quick-lime should be let down in buckets, and gradually 

 sprinkled with water. As the lime slakes, it will absorb the 

 gas, and the workmen may afterwards descend in safety. 



The number of acids that are well known amounts to more 

 than forty, and their uses are so many and important that 

 it is impossible to enumerate them. They are indispensable 

 to various arts and manufactures; they are employed for 

 culinary purposes, and for medicine ; they act an important 

 part in the great elaboratory of nature, and form a great 

 proportion of many of the mountainous districts of the globe 

 in their various combinations. 



The precise number of the salts is not known, but they 

 probably amount to more than two thousand. The different 

 salts are known from each other by the peculiar figure of 

 their crystals, by their taste, and other distinctive or specific 

 characters. The separation of salts from the water in which 

 they may be dissolved, is generally effected by evaporation 

 and cooling. When a certain portion of the water of solu- 

 tion is evaporated, and the remainder left in a proper tem- 

 perature at rest, the salts will shoot into crystals, and will be 

 found dispersed through the water at the bottom and at the 

 sides of the vessel, and sometimes also on the surface of the 

 solution. Their crystallization is owing to the abstraction 

 of the heat or water by which they were dissolved. Crys- 

 tallized salts are liable to changes in their appearance by 

 exposure to atmospheric air. Some have so great an af- 

 finity for water that they absorb it with avidity from the at- 

 mosphere, and thus becoming moist or liquid, they are said 

 to deliquesce. Others, having less affinity for water than 

 atmospheric air has, lose their water of crystallization by 

 exposure, and readily fall into powder. Such salts are said 



