CHLORIDE OF ZINC SOLUTION.--In the “Micrographic Dictionary” this is stated to be “perhaps the best preservative known for animal tissues.” Persons of great experience, however, have given a very different opinion; but it is certainly very useful in many cases where a small degree of coagulating action is not injurious. It is used of strengths varying according to the softness of the parts to be preserved; the average being 20 grains of the fused chloride to 1 oz. of distilled water. To keep this liquid a lump of camphor may be left floating in the bottle. I have heard complaints that this mixture becomes turbid with keeping, but I think this must only be the case when some impurity has got into the bottle. CARBOLIC ACID.--This substance has not been known long enough to warrant any decided opinion upon its merits. A solution of 1 part of acid to 20 of water has been recommended on account of its antiseptic properties. CASTOR OIL.--This is a very useful preservative for crystals and other objects. Many salts are quite destroyed when Canada balsam is used with them; but very few are acted upon by this oil. To use it, it must be dropped in a sufficient quantity to cover the crystal or object to be preserved with a thin coating of oil. It may be necessary sometimes to spread it with a needle or other instrument. The thin glass should then be carefully placed upon it, so that all air may be excluded; and should any oil be forced out, owing to the quantity used being too great, it must be removed with blotting paper. When the edge of the thin glass cover and the surrounding parts of the slide are as clean as possible, a coating of sealing-wax varnish or liquid glue must be applied and allowed to dry. A second or even a third coating may be required, but not before the previous cover is quite dry. These varnishes, however, are very brittle, and it is much safer, as a finish, to use one of the tougher cements--gold-size, for instance--which will render it doubly secure. The above are the principal liquids, &c., used for preserving objects in cells. The different cells may be here mentioned; and it is recommended that these should always be kept some time before use in order that the cement may become perfectly dry; and care must be taken that no cement be used on which the preservative liquid employed has any action whatever.