52 THE ART OF PROJECTING 



the wetted surface appears very plain upon the screen. 

 Then wait until the solution begins to evaporate, as it 

 will, from the upper edge first, when crystallization will 

 begin there. See to it that the focus is right, and then 

 gently blow upon the plate, unless the work is going on 

 fast enough. The crystals will shoot out and grow 

 while one looks, until they cover the entire screen with 

 beautiful forms. 



The following are good substances for illustration 

 when dissolved in water : Ammonium chloride ; barium 

 chloride ; copper sulphate ; camphor dissolved in water ; 

 common alum ; urea dissolved in alcohol. 



ICE FLOWERS. 



To exhibit the decrystallization of ice, which was 

 first shown by Tyndall, it will be necessary to saw from 

 a very clear piece of ice a cake three or four inches 

 square, and about a half or three-quarters of an inch 

 thick, cut parallel to the plane of freezing. When first 

 cut, the sawn surface will be too rough for use, but will 

 quickly melt smooth enough by dipping a few seconds 

 in water. The beam of light that falls upon it should 



Fig. 33. 



consist of parallel rays, and the porte lumiere is better 

 for projecting this experiment than any lantern. 



A single lens for an objective, four or five inches 



