OBJECTS FOK POLARIZED LIGHT. 449 



Crystals of Murexide, Crystals of Sulphate of Mckel, 



Oxalate of Ammonia, Tartrate of Lime, 



Chromium, Triple Phosphate, 



Lime, Uric Add, 



Soda, Aragonite, 



Nitrate of Ammonia, Asbestos, 



— — Barytes, Avanturine, 



Lead, Granite, 



Potash, Marble, 



Soda, Eaphides Hyacinth, 



Oxalic Acid, Onion, 



Phosphate of Soda, Rhubarb, 



Satin Spar, Sea-sand, 



Sugar, Tremolite, 



of Milk, Wavehte, 



Sulphate of Cadmiiun, Zeolite, 



Copper, Selenite of different thicknesses, 



Magnesia, Salicine. 



Selenite. — As before stated, at page 240, laminsB of this 

 mineral, or of mica, will be required of different thicknesses ; 

 these may readily be split off from a large crystal by a pen- 

 knife or other sharp instrument; for the purpose of being 

 used, the laminse should be placed between two plates of thin 

 glass, either with or without Canada balsam. A piece of 

 plate-glass, three or more inches long, and an inch-and-a-half 

 wide, with a raised edge, and having a thin lamina of selenite 

 cemented upon it, and covered by a piece of thin glass with 

 Canada balsam, is employed by some persons as a selenite 

 stage, and upon this the object that is intended to be examined 

 is placed. 



Crystals of Salts. — These may be easily prepared by crys- 

 tallizing slowly a boiling saturated solution of the salt upon a 

 cold glass slide ; the crystalUzation can be effected very rapidly 

 by warming the under surface of the glass over a lamp. To 

 get the finest crystals, the more slowly the solution is allowed 

 to evaporate the better ; the uncrystaUizable part, or mother- 

 liquor, as it is termed, should be removed either with blotting- 

 paper or a glass tube. Several specimens of each sort should 

 be crystallized on slides, and those selected that show best ; 

 the others may easily be cleaned off. If any of the specimens 

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