244 OPTICAL PROJECTION 



give sufficient colour on the screen, which is the chief thing to 

 be studied, and will depend upon the thickness of the tank : 

 hence a tank of proportionate thickness enables any strength 

 of solution to be used which may be suitable to the reaction. 

 There is also room for ingenuity, in so arranging an experi- 

 ment as to give the reactions in an attractive or striking manner. 

 Thus, a tank may be filled with the well-known infusion of 

 red cabbage, the top of the tank being long enough to extend 

 all along the diameter of the condensers. Then, if we add a 

 drop or two of potass solution at one end, of alum solution 

 in the centre, and hydrochloric acid at the other end, the 

 purple, green, and crimson colours will appear simultaneously. 



The reactions formed by adding potassic ferrocyanide to 

 salts of iron, copper, bismuth, &c., are very impressive. Most 

 of them are better shown with dilute solutions than the fore- 

 going. 



Electric reactions are easily shown by filling a tank with 

 sodium sulphate, coloured with cabbage or litmus, which pro- 

 jects as a blue solution. If the terminals of a small battery 

 are then introduced, the acid and alkaline reactions will appear 

 at the poles. 



The action of heat upon salts of cobalt is prettily shown in 

 the lantern by coating a glass plate with a saturated solution 

 of the chloride in a solution of gelatine. The rosy tint will 

 gradually change to a blue in the heat of the lantern. This 

 may be varied by using with the plate a photographic slide, 

 which if judiciously chosen will give a curious apparent change 

 from day to moonlight ; or if a design be sketched with a 

 weaker solution, the gradual visibility of the so-called sympa^ 

 thetic inks can be readily exhibited. 



The main facts of popular ' domestic chemistry ' are readily 

 demonstrated. Bleaching, for instance, may be illustrated by 

 filling a tank with a solution of indigo in dilute sulphuric 

 acid, and projecting the tank upon the screen. By adding a 

 solution of calcium chloride, the colour will gradually disappear. 



