Crystals and Polariscope Objects. 97 



and warm a cover-glass, and apply its edge to the cell ; 

 press down, and be sure that it adheres to the cement all 

 round. Wash away the excess of glycerine with some 

 water, and dry the slide with a soft cloth. When quite dry, 

 apply a ring of Miller's cement, and when this has dried 

 add a coat of any good black varnish. 



LESSON 11. 

 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 

 protected 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 ; if so, mount 

 in castor oil. 



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

 give very good results : 



Chloride of barium.* Sulphate of iron.* Asparagine. 



Chlorate of potash.* Tartrate of soda.* Quinidine. 



Sulphate of copper.* Salicine. Santonine. 



Spermaceti (fuse). Stearine (fuse). Tartaric acid. 



Those marked * are more effective when crystallized in 

 gum or gelatine. 



