MUREXIDE OR PURPURATE OF AMMONIA. 337 



The formation of crystals under the microscope may be 

 watched with the greatest facility. A little common salt 

 (chloride of sodium) dissolved in water, and a drop of the 

 solution placed on a glass slide gently heated over a spirit 

 lamp, or by applying the corner of the slide to the candle, 

 will show the formation of crystals in primitive cubes, ter- 

 minated by quadrangular pyramids. The water slowly 

 evaporates, and the atoms held in solution return to their 

 natural form. 



ACETATE OF COPPER 



is made by dissolving common verdigris in excess of diluted 

 acetic acid, and when crystallized on the slide will exhibit 

 the phenomena of dichromism or double colour, deep blue 

 and yellowish green. % 



SULPHATE OF COPPER. 



Blue vitriol dissolved in water, and likewise treated with 

 a gentle heat, will show the formation of beautiful blue 

 crystals in oblique rhomboidal prisms. 



ALUM 



does not polarize, but gives crystals of the octohedral form. 



OXALURATE OF AMMONIA. 



This is most beautiful in the formation of its crystals ; 

 they appear on the slide as circular discs or very flat spheres, 

 consisting of minute needles radiating from the centre, and 

 sometimes projecting beyond the circumference of the disc. 

 Without the selenite stage these discs are like brilliant 

 little white stars, traversed by a black cross ; with the sele- 

 nite they are splendid objects, the colours often disposed in 

 concentric rings. 



MUREXIDE OR PURPURATE OF AMMONIA 



is an artificial product of the decomposition of uric acid. 

 The crystals are flattened, short, four-sided prisms, of bright 



