THE BOOK OF THE LANTERN. 195 



ammonia, will quickly restore the original colour, and these 

 changes from red to blue, and vice versa, can be continued 

 by adding acid and alkali alternately, as often as may be 

 required. 



If the tank be charged with a solution of sulphate of 

 iron and gallic acid be added to it, a black solution of ink 

 will immediately be produced. Another pretty experiment 

 demonstrates the presence in hard water of various mineral 

 matters which will cause certain chemicals to give a 

 precipitate which they would not do in water that has 

 been freed from mineral matter by distillation. A good 

 plan of showing this is to suspend in a tank a crystal of 

 oxalic acid. As the crystal dissolves in the water long 

 threads of oxalate of lime will be given off by it, forming 

 a very curious appearance on the screen. It may then be 

 shown that by the substitution of distilled for hard water 

 the crystal will dissolve all the same, but these threads will 

 not be given off, because there is no lime present to form 

 them. The action of bleaching powder, commonly called 

 chloride of lime, is well shown by filling the tank with a 

 solution of indigo, which has been acidified with sulphuric 

 acid. Upon adding a solution of the bleaching powder, 

 the sulphuric acid will liberate the chlorine contained in 

 it, and will discharge the blue colour of the indigo, leaving 

 the disc on the screen perfectly white. 



The precipitates caused by the admixture of various 

 chemicals is not effective in the lantern, for the reason that 

 most of these precipitates are opaque, and therefore they 

 look black upon the screen. 



For instance, we may fill a tank with a solution of 

 o 2 



