86 The Microscope. 



esting subject, and I have discovered certain peculiarities in the 

 mineral that I trust will be as interesting to the members of this 

 society as they have proved to me. 



Like the very best quality of rock salt, this mineral occurs in 

 blocks of the utmost transparency. It is quite easy to read printing 

 through a cube several inches in thickness. Some pieces are as clear, 

 colorless and free from mechanical impurities as ice from distilled 

 water, frozen in a vessel of porcelain. Others contain some foreign 

 matter which does not enter into the composition of the salt. When 

 dissolved and filtered the solution is perfectly colorless, and on 

 applying the usual chemical tests, without observing sufficient care 

 to detect minute traces, the salt is found to be almost absolutely 

 pure. The fact that in a somewhat moist atmosphere it does not 

 deliquesce is an additional proof of its chemical purity. 



Some pieces, transparent and colorless, melt at a red heat on 

 platinum foil without decrepitation to a transparent and also color- 

 less fluid, which retains its transparency when cooled. 



Other specimens show faintly opalescent lines meeting each 

 other at right angles. If such a specimen is held at a certain angle 

 in the sunlight, a multitude of reflecting surfaces like imbedded 

 spangles may be seen which glimmer something like adventurine, 

 or glow like a sunstone. It may also be seen that while the faint 

 lines meet at right angles, a dividing line forms a miter like the cor- 

 ner of a door panel. Such a specimen, when heated to redness, 

 explodes with great violence, so much so that the experiment is one 

 of considerable danger if the eyes are not protected 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 evening. I 

 found the phantom lines and reflecting spangles to be minute 

 cavities in the anhydrous salt, all of the same general form, but vary- 

 ing 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, mostly square but sometimes 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 these minerals 

 crystallize in the same system. 



