PREPARATION AND MOUNTING 



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 sepa- 

 rately, 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 satu- 

 rated 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 gatker, 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 crys- 

 tal. 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. 



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 potassse or ammonia immediately before using, 

 care als3 being taken that none of the agent is left upon the slide, 

 otherwise it may interrupt and change their relative position, and 

 even their form. 



Amongst those which are generally esteemed, the most beautiful 

 are the crystals of oxalurate of am nD.iia. The preparation of this 

 salt from uric acid and ammonia is a rather difficult process, and 

 will not on that account be described here ; but when possessed, 

 a small quantity of a strong solution in water must be made, and 

 a little placed on the slide, and evaporated slowly. Part of the 

 salt will then be deposited in circles with the needle-like crystals 

 extending from common centres. Thay should then be mounted 

 in pure Canada balsam ; and, wli3n th-3 best colours are wanted, 

 used with the selenite plate. Of this class of crystal, salicine is 

 a universal favourite, and can be easily procured of most chemists. 

 The crystals may be produced in two ways : A small portion of 

 the salt must be placed upon the slide, and a strong heat applied 

 underneath until fusion ensues; the matter should then be evenly 

 and thinly spread over the surface. In a short time the crystals 

 will form, and are generally larger than those procured by the 



