112 PREPARATION AND MOUNTING 



to such a degree that the eye can perceive no relationship 

 to exist between them. If a solution of sulphate of iron be 

 made, a small quantity spread evenly upon a slide, and then- 

 suffered to dry whilst in a flat position, the crystals often 

 resemble the fronds of the common fern in shape. But if, 

 whilst the liquid is evaporating, it is kept in motion by 

 stirring with a thin glass rod, the crystals form separately, 

 each rhombic prism having its angles well defined, and 

 giving beautiful colours with the polarized light. Again, 

 pyro-gallic acid, when allowed to flow evenly over the slide 

 in a saturated solution, covers the surface in long needles, 

 which are richly coloured by polarized light ; but if any 

 small portion of dust or other matter should form a nucleus 

 around which these needles may gather, the beauty is 

 wonderfully increased. A form very closely resembling the 

 " eye " of the peacock's tail, both in form and colour, is then 

 produced, which to one uninitiated in crystallography bears 

 very little resemblance to the original crystal. From these 

 simple facts it will be clearly seen that in this, as in every 

 other department, study and experience are needful to give 

 the best results. 



By dropping a saturated solution of any salt into alcohol 

 where it" is not soluble in the alcohol crystals are in- 

 stantaneously produced, and the results are often very curious 

 and beautiful. These crystals can easily be taken up by a 

 pipette deposited upon a slide, and, after having been 

 allowed to dry spontaneously mounted in balsam. 



To obtain anything like uniformity in the formation of 

 crystals upon the glass slide, every trace of grease must be 

 removed by cleaning with liquor potassaa or ammonias im- 

 mediately before using, care also being taken that none of 

 the agent is left upon the slide, otherwise it may in- 

 terrupt and change their relative position, and even their 

 form. 



Amongst those which are generally esteemed the most 

 beautiful, are the crystals of oxalurate of ammonia. The 

 preparation of this salt from uric acid and ammonia is a 



