Crystals and Polariscope Objects 201 



LESSON XL 

 CRYSTALS AND POLARISCOPE OBJECTS. 



Crystals. — Method 1. — Make a strong solution of the 

 material in distilled water, with the aid of heat if necessary, 

 and filter ; take up a small quantity of the solution in a 

 dipping-tube, and drop it on a cover- glass. Prepare several 

 covers in this way, and allow some to dry slowly, and 

 evaporate others over a spirit-lamp. When dry, add a drop 

 or two of Canada balsam, and mount in the usual way. 



Method 2. — Make a strong solution in distilled water, 

 and add a few drops of gum water or a small piece of 

 gelatine ; mix well, and filter. Apply some of the solution 

 to a cover-glass, and allow it to dry slowly in a place pro- 

 tected from dust. Mount in Canada balsam. 



Method 3. — Place a small piece of the dry crystal on a 

 slide, and apply a cover-glass ; warm over a spirit-lamp 

 until fusion results, press the cover down with a needle, 

 and allow the slide to cool. Clean off the exuded material, 

 and finish off with some good cement. 



Some crystals are soluble in Canada balsam ; in which 

 case, mount in castor-oil. 



Crystallize the specimen on the cover-glass ; make a thin 

 cell with some shellac cement on a slide, and allow it to 

 become perfectly dry ; then apply another coat of cement, 

 and when this has nearly dried, fill the cell with castor-oil. 

 Take up the cover with a pair of forceps, and bring the 

 crystallized surface in contact with the oil, being very 

 careful that no air-bubbles form. Ease it down gently, and 

 when it rests on the cell, give it a press with the point of 

 the forceps ; this will squeeze out the excess of oil and 

 embed the edge of the cover in the cement. Put away to 

 dry; wash off the exuded oil with some turpentine, and 

 apply another coat of shellac cement. 



The following salts, etc., are easily obtained, and they all 

 give very good results : 



