_Methods of preserving the tissues_.--All water must be removed from the tissue, either by drying or by immersing it in rectified spirits, and then in absolute alcohol, and the alcohol driven off by floating it upon oil of clove or turpentine. The substances used to preserve the tissues are Canada balsam, Dammar balsam, glycerine, Farrant's solution, potassium acetate, spirits, naphtha, and creosote. The section is to be floated on to the slide or placed in position with a camel's hair brush. It should be spread out, and then examined under the microscope for the purpose of improving its position if necessary, or of removing any foreign particles. A drop of the preserving medium is then placed upon it, and another placed on the cover and allowed to spread out. The cover is then taken by a pair of pincers and inverted over the object, and one edge brought to touch the slide at one part of its margin. The cover is then gently lowered, and the whole space beneath the cover filled and the tissue completely saturated. If air bubbles show themselves, raise the cover at one corner and deposit a further quantity of the medium. The slide should be set aside for a few days. First, the excess of the medium must be removed; if it is glycerine, much of it can be removed by a piece of blotting paper, but the cover must not be touched, for it is easily displaced; that near the cover can be replaced by a camel's hair brush. A narrow ring of glycerine jelly should be placed around the edge of the cover, to fix it before the cement is applied. When this has set, a narrow strip of cement is to be put on, just slightly overlapping the edge of the cover and outside the margin of the jelly. Until it has been perfectly secured, a slide carrying glycerine must never be placed in an inclined position, as its cover will slide off. _Preservative media_.--Canada balsam may be prepared as follows: Place some pure Canada balsam in a saucer, and cover with paper to exclude dust; dry it in an oven at a temperature of 150°; when it cools, it will become hard and crystalline. Dissolve this in benzole, and use in the same way as glycerine. Dammar is now used as a substitute for Canada balsam. By its use the tissues are rendered more transparent. To prepare it, dissolve one-half ounce of Dammar rosin and one-half ounce of gum mastic in three ounces of benzole, and filter. This may be used to mount unsoftened bone and tooth, hair, brain, and spinal column, and most tissues that have been hardened in alcohol or chromic acid, which require to have their transparency increased. Glycerine is not adapted for white fibrous tissue or blood vessels, unless they have been hardened in chromic acid, as it causes the white fibers to swell up and lose their normal features. Sections of liver, lung, skin, and alimentary canal show better in glycerine unless they have been stained.