PRESERVATIVE LIQUIDS, ETC., PARTICULARLY WHERE CELLS ARE USED.  
There are many objects which would lose all their distinctive peculiarities if allowed to become dry, especially those belonging to the fresh-water Alg?, many animal tissues, and most of the very delicate animal and vegetable substances in which structure is to be shown. These must be preserved by immersion in some fluid; but it is evident that the fluid must be suited to the kind of matter which it is intended to preserve. As it often requires much study and trouble to obtain microscopic objects of this class, it is well that their preservation should be rendered as perfect as possible; and for this reason the CELLS, or receptacles of the fluids, should be so closed that all possibility of escape should be prevented. The accomplishment of this is not so easy a matter as it might appear to the inexperienced.
Before giving any directions as to the manipulation required in mounting the objects, we must consider the different liquids and cells which are requisite for their preservation. Of the former there are a great number, of which the principal may be mentioned.
Distilled Water is strongly recommended by many for Diatomace? and other Protophytes. It has been, however, stated that confervoid growths often disturb the clearness of the liquid, and on this account various additions are made to it. A lump of camphor is often left in the bottle, so that the water may dissolve as much as possible. One grain of bay-salt and one of alum are added to each ounce of water; or a drop or two of creosote shaken up with the84 ounce of water, which should be afterwards filtered. These additions are often made; perhaps each of them good for certain objects.
Glycerine.—Some affirm this to be one of the best preservative liquids, especially for vegetable objects; but others think that it is much better when diluted with two parts of camphor-water, prepared as above.E
E Dr. Carpenter says:—“Glycerine has a solvent power for carbonate of lime, and should not be employed when the object contains any calcareous structure. In ignorance of this fact, the author (Dr. C.) employed glycerine to preserve a number of remarkably fine specimens of the pentacrinoid larva of the Comatula, whose colours he was anxious to retain; and was extremely vexed to find, when about to mount them, that their calcareous skeletons had so entirely disappeared, that the specimens were completely ruined.”
Glycerine and Gum.—This is also believed to be a very good liquid for vegetable tissues, and is thus prepared:—
Pure gum-arabic 1 oz.
Glycerine 1  ”
Water (distilled) 1  ”
Arsenious acid 1? grains.
Dissolve the arsenious acid in the cold water, then the gum, add the glycerine, and mix without bubbles.
Deane’s Compound.—This is usually deemed about the best medium for preserving Alg?, mosses, &c., and is thus prepared:—Soak 1 oz. of best gelatine in 4 oz. of water until the gelatine becomes soft, when 5 oz. of honey heated to boiling-point are added; boil the mixture, and when it has cooled, but not enough to become stiff, add ? oz. rectified spirit with which 5 or 6 drops of creosote have been well mixed, and filter the whole through fine flannel. This compound when cold forms a stiff jelly, the use of which will be described elsewhere.
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 “that85 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 Alg?, &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 coagulating86 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.
87Cement 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.
88 Often the cells must necessarily be of a large size, and for this reason are made by taking four strips of glass of the thickness and depth required, and grinding the places where these are to meet with emery, so as to form a slightly roughened but flat edge. The glass strip must also be ground on the side where it meets the plate, and each piece cemented with the marine glue mentioned in Chapter I. in the following manner:—On that part of the glass to which another piece is to be attached should be laid thin strips of the glue; both pieces must then be heated upon a small brass table, with the aid of the spirit-lamp, until the strips become melted; the small piece is then to be taken up and placed upon the spot to which it is to be attached, and so on until the cell is completed. It will be found necessary to spread the glue over the surface required with a needle or some other instrument, so that an unbroken line may be presented to the wall of the cell, and no bubbles formed. Too great a heat will “burn” the marine-glue, and render it brittle; care must be therefore taken to avoid this.
When shallow cells are required, those which are made by grinding a concavity in the middle of an ordinary slide will be found very convenient. The concavities are cut both circular and oblong; and the surface being flat, the cover is easily fastened upon it. These are now cheap, and are very safe as to leakage.
Circular cells with a flat bottom used to be made by drilling a hole through glass of the required thickness, and fixing this upon an ordinary slide with marine-glue; but the danger of breakage and the labour were so great that this method is seldom used now, and, indeed, the rings about to be mentioned do away with all necessity of it.
Glass Rings.—Where any depth is required, no method of making a cell for liquids is so convenient as the use of glass rings, which are now easily and cheaply procurable. They are made of almost every size and depth, and, except in very extraordinary cases, the necessity for building cells is completely done away with. These rings have both edges89 left roughened, and consequently adhere very well to the slide, this adherence being generally accomplished by the aid of marine glue, as before noticed with the glass cells. Gold-size has been occasionally used for this purpose; and the adherence, even with liquid in the cell, I have always found to be perfect. This method has the advantage of requiring no heat, but the gold-size must be perfectly dry, and the ............