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 potassze or ammoniz 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 
