Glycerine Jelly.—This mixture closely resembles the above, but as the composition differs a little it may be mentioned here. It is strongly recommended by Mr. Lawrance in the Microscopic Journal, where he states “that the beautiful green of some mosses mounted two years ago, is still as fresh as the day they were gathered;” and that this is the only medium he knows which will preserve the natural colour of vegetable substances. He takes a quantity of Nelson’s gelatine, soaks it for two or three hours in cold water, pours off the superfluous water, and heats the soaked gelatine until melted. To each fluid ounce of the gelatine, whilst it is fluid but cool, he adds a fluid drachm of the white of an egg. He then boils this until the albumen coagulates and the gelatine is quite clear, when it is to be filtered through fine flannel, and to each ounce of the clarified solution add 6 drachms of a mixture composed one part of glycerine to two parts of camphor-water.
Goadby’s Fluid.—This is much used in the preservation of animal objects; and seldom, if ever, acts upon the colours. It is thus prepared—Bay salt, 4 oz.; alum, 2 oz.; corrosive-sublimate, 4 grains. Dissolve these in two quarts of boiling water and filter. For delicate preparations some recommend that this mixture be reduced by the addition of an equal quantity of water; but where there is bone or shell in the object the above acts injuriously upon it, in which case this fluid may be used:—Bay-salt 8 oz., corrosive sublimate 2 grains, water 1 quart.
Thwaites’ Liquid.—This is recommended for the preservation of Algæ, &c., as having little or no action on the colour, and is thus prepared:—Take one part of rectified spirit, add drops of creosote enough to saturate it; to this add sixteen parts of distilled water and a little prepared chalk, and filter. When filtered, mix with an equal quantity of camphor-water (as before mentioned), and strain through fine muslin before using.
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.
Cement Cells.—Where the object is not very thick, this kind of cell is generally used. They are easily made with the turntable before described; but when the objects to be preserved are very minute, these cells need not be much deeper than the ordinary circle of cement on the slide. When, however, a comparatively great depth is required, it is sometimes necessary to make the wall of the cell as deep as possible, then allow it to dry and make another addition. Of these cements gold-size is one of the most trustworthy, and may be readily used for the shallow cells. The asphaltum and india-rubber, before noticed, I have found very durable when well baked, and exceedingly pleasant to work with. It may be used of such a thickness as to give space for tolerably large objects. Black japan also is much used. Many cements, however, which are recommended by some writers, are worse than useless, owing to the brittleness which renders their durability uncertain, as sealing-wax varnish, liquid-glue, &c.
The student may feel himself at a loss in choosing the cement which will give him the safest cells, many of them becoming partially or wholly dry in a year or two, as stated in another place. I can only give him a few general directions, and he must then use his own judgment. Of course it would be lost labour to employ any cement upon which the preservative liquid has any action whatever. It is also a good rule to avoid those in whose composition there are any particles which do not become a thorough and intimate portion, as these unreduced fragments will almost certainly, sooner or later, prepare a road by which the liquid will escape; and, lastly, whatever cement he uses, the cells are always better when they have been kept a short time before use, as already stated.
Gutta-percha Rings have been recommended by some, as affording every facility for the manufacture of cells for liquids; but they cannot be recommended, as, after a certain length of time, they become so brittle as to afford no safeguard against ordinary accidents.