176 THE AMERICAN MONTHLY [June, 



Take a clean slide and run a ring of India ink on the 

 back of the slide, as a guide for fixing the cover glass 

 quite central. Place a few drops of the solution from a 

 clean pipette in the center of the other side of the glass 

 and put it away to dry on a shelf that is not quite level, 

 which will cause a part of the solution to drain away to 

 the bottom and corner of the slide ; if the air is very 

 moist it will not readily crystalize, nor give a good re- 

 sult, so that perhaps many attempts will have to be 

 made before you succeed in obtaining anything worthy 

 of being mounted ; but when it is obtained the result 

 will be found to compensate for the trouble taken. 



Do not attempt to hasten the crystalization by warm- 

 ing the slide as even a moderate heat breaks up the crys- 

 tals and spoils them. When, after examination under 

 the microscope with the polarizing apparatus and the 

 selenite film, you find that you have obtained something 

 worthy of being preserved for the cabinet, put the slide 

 on the turn-table, slowly revolve and scrape off the 

 excess with a knife, having the disk a little smaller 

 than the cover-glass which you intend to use; get your 

 cover-glass well cleaned, and to preserve the crystals 

 without any diminution of their brilliancy, put a few 

 drops of the purest petroleum oil on the crystals, and 

 drop the cover on removing the excess of petroleum with 

 a small brush, but be careful not to remove any from 

 under the cover-glass. If you cannot get it pretty clean, 

 use a little spirit or benzole on the brush, wiping the 

 brush after every stroke on a piece of paper, and remov- 

 ing the benzole by dipping the brush in, and removing 

 the excess by draining it against the edge of the cup 

 that holds it. The j)etroleum will soon dry off, when 

 the glass can be fixed by a touch of Miller's Caoutchouc 

 Cement at each side, putting as little as possible on : 

 you must see that the cover is exactly in the center of 

 the ink circle before fastening down. 



