Californta Rock Salt. 231 



redness, explodes with great violence, so much so, that the ex- 

 periment is one of considerable danger if the eyes are not pro- 

 tected from the minute flying cubes into which the larger one is 

 broken by the explosion. 



On obtaining these very interesting results, I naturally appealed 

 to the microscope, our favorite instrument, for the cause of the 

 violent decrepitation, in one case, and quiet fusion in the other. 

 Nor did I seek in vain, as I hope to be able to show you this even- 

 ing. I found the phantom lines and reflecting spangles to be mi- 

 nute cavities in the anhydrous salt, all of the same general form 

 but varying in size from those so minute as to be scarcely visible 

 under a two-thirds objective, to others that can be examined in 

 detail. The cavities are box-shaped, mosdy square but some- 

 times slightly oblong. They are generally from four to six times 

 as broad as they are deep. All the angles are rounded, and all 

 the lines marking the sides of the cavities curved, just as we saw 

 others some months ago while examining the beautiful diamond 

 from Amador. Both ihese minerals crystallize in the same sys- 

 tem. 



All the imbedded cavities are empty. You may search them 

 over and not see a particle of inclosed matter. But on the sur- 

 f ice, where the walls are broken down, they may be seen partly 

 fllled 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 heat- 

 ing, it was found that the transparency was not materially im- 

 paired, except at those points where the box-like cavities were 

 shattered by the escaping air under pressure. They had lost 

 I heir beautiful form and had become irregular, roughly globular 

 cavities, filled with broken fragments of salt. In every direction 

 from the shattered cavities the substance was fissured and fractur- 

 ed, showing the great force exerted by the escaping gas or air. 

 It is a mystery how these beautiful cavities ctuild 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 high- 

 er price. It dissolves more slowly than the more impure varie- 



