With almost every salt the method of _crystallization_ must be modified to obtain the best forms; I may even go further than this, and say that it is possible to change these forms to such a degree that the eye can perceive no relationship to exist betwixt them. If a solution of sulphate of iron is 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.
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 potassæ or ammonia immediately before using, care also 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 ammonia. 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. They should then be mounted in pure Canada balsam; and, when the 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 following process; but the uncertainty is increased a little when fusion is used, which, however, is desirable with many salts. Secondly, make a saturated solution of salicine, which in cold water is effected by adding one part of the salt to eighteen parts of water. Lay a little upon the slide, and allow it to evaporate spontaneously, or with the aid of gentle heat. The crystals are generally uniform, and with ordinary powers quite large enough to afford a beautiful object. The circular shape and gorgeous colours of this crystal have made it so great a favourite that there are few cabinets without it.