The Microscope. 87 



All the imbedded cavities are empty. You may search them 

 over and not see a particle of inclosed matter. But on the surface, 

 where the walls are broken down, they may be seen partly filled with 

 the debris of the crushed salt, which proves that they are actually 

 cavities and not illusory. 



It having been proved that the salt contains only traces of 

 water, it may be inferred that the cavities are filled with a gas or 

 with atmospheric air. Otherwise it would be difficult to account for 

 the explosion when heated. 



On examining the salt after heating, it was found that the 

 transparency was not materially impaired except at those points 

 where the box-like cavities were shattered by the escaping air under 

 pressure. They had lost their beautiful form and had become irre- 

 gular, roughly globular cavities, filled with broken fragments of salt. 

 In every direction from the shattered cavities the substance was fis- 

 sured and fractured, showing the great force exerted by the escaping 

 gas or air. It is a mystery how these beautiful cavities could be 

 formed in so hard and anhydrous a substance as rock salt. 



After actual food and water, salt is one of the most necessary 

 requirements of man and animals, and it is a question if a healthy 

 bodily condition could be long maintained without it. Salt is also 

 largely employed in manufactures and the arts. 



Rock salt is not always so pure as the specimens shown you 

 this evening. In England it is colored red by the oxide of iron it 

 contains. It is also sometimes contaminated by clay and sand, and 

 often by imbedded associate minerals, as gypsum, anhydrite, borax, 

 glauberite and others; still it is seldom, if ever, so impure as salt 

 made from seawater, for which reason it commands a higher price. 

 It dissolves more slowly than the more impure varieties, which prop- 

 erty fits it for certain purposes and uses in the arts. Pure salt does 

 not deliquesce except in a very moist atmosphere. 



Salt obtained artificially contains various impurities which 

 impair its value. These impurities are generally magnesia, gypsum, 

 bromine and iodine, with much organic matter, while rock salt is 

 free from them. This has led to the theory that seawater takes its 

 salt from beds of rock salt, instead of rock salt being deposited from 

 the ocean. This theory is strengthened by the fact that rock salt is 

 sometimes absolutely anhydrous. 



While inferior salt may be extracted from brines found in 

 nearly all countries, rock salt is rather rare. It occurs in very large 

 deposits in England, Poland, Hungary and Germany. In the high 

 mountains of Chili it is met with at an elevation of 9000 feet above 



