There is one class of objects for the polariscope which differs in preparation from any we have yet considered, and affords very beautiful specimens. Some of the plants, including many of the grasses and the Equisetaceæ (_i. e._ horsetails), contain so large a quantity of silica, that when the vegetable and other perishable parts are removed, a skeleton of wonderful perfection remains. This skeleton must be mounted in balsam, the method of performing which will now be considered. Sometimes the cuticle of the equisetum is removed from the plant, others dry the stem under pressure, whilst the grasses, of course, require no preparation. The vegetable should be immersed in strong nitric acid and boiled for a short time; an effervescence will go on as the alkalies are being removed, and when this has ceased more acid should be added. At this point the modes of treatment differ; some remove the object from the acid and wash, and having dried, burn it upon thin glass until all appears _white_, when it must be carefully mounted in balsam. I think, however, it is better to leave it in strong acid until all the substance, except the required portion, is removed; but this will take a length of time, varying according to the mass, &c., of the plant. Of course, when this latter method is used, the skeleton must be washed from the acid, &c., before being mounted in balsam. These _siliceous cuticles_ are readily found. The _straws_ of most of the cereals, wheat, oat, &c.; the _husks_, also, of some of these; many _canes_; the equisetum, as before described; and some of the grasses. Many of these are everywhere procurable, so that the student can never want material for a splendid object for the polariscope. In Chapter II. the _scales_ (or hairs) which are often found upon the leaves of plants were mentioned as beautiful objects when mounted dry; but some of these when detached from the leaf--which is easily done by gently scraping it, when dried, with a knife--present brilliant starlike and other forms, if mounted in balsam and used with the polariscope. There is a little danger, when placing the thin glass upon the balsam, of forcing out the scales in the wave of matter which is always ejected; this may be overcome by applying to the slide, previously to placing the objects upon it, an extremely thin covering of the balsam diluted with turpentine as before mentioned, and thus giving them every chance of adherence; or by using the balsam with chloroform, as before noticed. These scales are much more abundant than was formerly supposed, and daily new specimens are discovered; so that the student should always be on the lookout for them in his researches in the vegetable world. Most classes of objects, and the treatment they require when mounting them in balsam, have now been considered. The next chapter will be devoted to the preservative liquids, and the best methods of using them. CHAPTER IV.