196 THE BOOK OF THE LANTERN. 



common salt, i.e., the chloride of sodium, and upon adding 

 to this a small quantity of nitrate of silver in solution, a 

 heavy white precipitate of chloride of silver is thrown 

 down, but as this is perfectly opaque it will only appear on 

 the screen as black clouds. 



Other very beautiful experiments may be performed to 

 demonstrate the crystallisation of various salts. Plates of 

 glass may be prepared beforehand with saturated solutions 

 of the salts, and these plates, slipped into a slide carrier, 

 can be used for projection, giving very fine effects. But 

 by far the most striking way of exhibiting these interesting 

 phenomena is to show' the crystallisation actually in 

 progress. 



This is easy enough if the lantern be furnished with a 

 vertical attachment, but not so easy without such an 

 appendage. But the following experiments can be readily 

 performed with an ordinary lantern. Prepare a saturated 

 solution of sal-ammoniac, and with the help of a camel-hair 

 brush cover a clean glass plate with the liquid ; place this 

 glass on the lantern stage, when the heat from the lamp 

 will speedily cause the water to evaporate and the crystals 

 to form on the glass. It will be noticed that in the crys- 

 tallisation of this salt the branches of the marvellous tree, 

 which grows so rapidly on the screen, always keep at a 

 particular angle to its stem. Another experiment of a like 

 nature is performed by employing a solution of urea in 

 alcohol, in which the crystallisation is quite different, the 

 plate being quickly covered with bundles of fibres which 

 are no longer at right angles to the stem from which 

 they spring, but take all kinds of different directions. 



