_Formalin_ is an excellent preservative fluid; one part to 20,000 is sufficient to prevent fermentation. For the preservation of vegetable sections, a one per cent. solution is required; even the fresh appearance of vegetable structures is preserved for some time when immersed in it. In the nutrient gelatine for biological specimens, if used early, will arrest the liquefaction of the gelatine by bacteria. For hardening it saves time, and is even better than alcohol, chromic acid, pot. bich., and many others. It does not cause shrinkage of the cells. Tissue 1/2 to 3/4 inch thick hardens in twenty-four hours in pure formalin; five to ten per cent. is best for loose tissue. In another method, by which time can be saved, instead of placing the specimen in the _formalin_ and afterwards in mucilage, prior to cutting sections, make the mucilage with two per cent. (or stronger) formalin water, and it will then answer both purposes at the same time. Preparing, Mounting, Cementing and Collecting Objects. Various materials are required for preparing and mounting microscopic objects, as slips of glass, patent flatted plate measuring 3 × 1 inch, thin glass covers, glass cells, preservative media, varnishes, cements, a glazier’s diamond, and a Shadbolt’s turn-table. The glass slides and covers, although sent out packed ready for use, should be immersed in an alkaline solution to ensure perfect freedom from any greasiness derived from touching by the fingers. Dr. Seller recommends a particular solution for this purpose. (See _Formulæ_, Appendix.) Varnishes and cements must be selected with care, as these are not only expected to adhere firmly to the glass slide, but also to resist the action of the preservative fluid in which the specimen may be mounted. Among the numerous preparations employed, I may enumerate Canada balsam, gum dammar, Venice turpentine, Japanners’ gold size, used for closing up cells, asphalte varnish, Brunswick black, shellac, glue and honey, Hollis’ liquid glue, and marine glue. To give a finish to the mounted specimen, coloured varnishes are sometimes resorted to. A red varnish of sealing-wax is made by digesting powdered sealing-wax in strong alcohol. Filter, and place the solution in a dish, and evaporate by means of a sand bath to reduce it to a proper consistency. This is said to resist the action of cedar oil. For white, zinc, cement is the best. This is made of benzole, gum dammar, oxide of zinc, and turpentine. Cole gives another formula, but either of these may be obtained of Squire, who supplies every kind of staining and mounting material. [Illustration: Fig. 262.--Walmsley’s Cell-making Turn-table.]