194 THE BOOK OF THE LANTERN. 



and such experiments are for this reason best performed 

 with a lantern having the simple construction shown in 

 Fig. 49. These experiments are so valuable for educa- 

 tional purposes, and can so easily be shown with ordinary 

 oil-lit lanterns that it is to be hoped that manufacturers 

 will see the necessity of providing for them by the adop- 

 tion of an open stage. 



The composition of Prussian blue can be easily demon- 

 strated by means of the chemical tank. For this experiment 

 we shall require a solution of the yellow prussiate of 

 potash from which the colour takes its name. This is 

 placed in the tank. Have in readiness a solution of sul- 

 phate of iron or green vitriol. On pouring the contents 

 of this bottle by means of a pipette into the tank, a heavy 

 blue precipitate is thrown down, but as this precipitate is 

 opaque the colour is not perceptible on the screen ; but by 

 adding to the blue precipitate a few drops of sulphuric 

 acid, and following this by a little bi-chromate of potash 

 in solution, a brilliant transparent blue is immediately 

 made apparent. The formation of other colours can by 

 reference to any book on chemistry be readily demon- 

 strated. 



The tests for acid and alkaline solutions by means of 

 litmus can be demonstrated in the following way : 



Fill the tank with a solution of litmus or with an in- 

 fusion of purple cabbage, made by slicing a few of the 

 leaves, and pouring boiling water upon them. Place either 

 of these solutions in the tank, when, upon adding a small 

 quantity of acid, the liquid will be seen to turn red ; sub- 

 sequent addition of an alkali, such as a weak solution of 



