353 OPTICAL PROJECTION 



solution increases the size of the crystals. When dry, those 

 which do not dissolve in balsam should be covered with 

 another glass cemented down by balsam in benzol, which 

 adds to the transparency. Acetates are generally fugitive, as 

 are some others ; but the vast majority give good results, 

 tartaric acid and salicine being particularly fine. Salicine 

 should be used in saturated solution, with a little alcohol 

 mixed in the water ; the fluid is dried gently over a lamp to 

 a clear amorphous film ; and on more strongly heating this, 

 especially if aided by a few pricks with a sharp needle, the 

 beautiful rosettes with rotating crosses crystallise out. Occa- 

 sional breathing on the film will produce circular ripples. 

 Several others can be treated similarly. Many of the most 

 beautiful crystals for the microscope are too small to show 

 effectively in the polariscope. but are gorgeous objects for the 

 polarising microscope described in Chapter XIII. 



Another class of beautiful crystallisations are formed by 

 melting the substance between two glass discs, and allowing 

 the film to cool. Such are benzoic acid, cinchonine, santo- 

 nine, succinic acid, cinnamic acid, &c., chiefly more or less 

 organic in character. A splendid variety of beautiful slides are 

 easily prepared from crystallisations. 



It often proves interesting to an audience to project coarse 

 specimens of the precious stones, compared with imitations in 

 glass. The difference illustrates that between crystalline and 

 non-crystalline bodies. 



Crystallisation on the screen is a beautiful experiment, 

 easily shown by melting benzoic acid between two glass discs 

 held in a pair of forceps over a spirit lamp, and then quickly 

 placed hot in a frame made for the purpose, and so into the 

 stage. The splendid coloured needles quickly begin to shoot 

 out on the dark field. Another way is to flow a strong solu- 

 tion of urea in gum-water, heated in a test-tube, over a 

 warmed disc, and to place this in the stage, which is held 

 back as open as possible, in order to touch the wet film with 



