Most objects intended for the polariscope may be mounted in Canada balsam; but there are some exceptions to this. Many of the salts are soluble in this medium, or their forms so injured by it, that glycerine or oil has to be used (see Chapter IV.); others must be left in the dry form, as before mentioned; and some few it is impossible to preserve unchanged for any length of time. _Crystals_, however, are amongst the most beautiful and interesting subjects for polarisation; and it is very probable that, by the aid of the polariscope, new and valuable facts are yet to be made known. For one who finds pleasure in form and colour, there is a field here which will only open wider upon him as he advances; and instead of being in anywise a merely mechanical occupation, it requires deep and careful study. The little here said on the subject will show this in some degree. 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.