cleanse the slides, of course answer equally well for cleansing the hands. The most ready solvent for balsam is Ether; but the ordinary use of this being interdicted by its costliness, and by the quickness with which it is dissipated by evaporation, Alcohol or Oil of Turpentine may be used in its stead.

131. Preservative Fluids.-Objects which would lose their characters in drying, can of course only be preserved in anything like their original condition, by mounting in fluid; and the choice of the fluid to be employed in each case will depend upon the character of the object, and the purpose aimed at in its preservation. As specific directions will be given hereafter in regard to most of the principal classes of Microscopic preparations, little more will be required in this place, than an enumeration of the preservative fluids, with a notice of their respective qualities. For very minute and delicate Vegetable objects, especially those belonging to the orders Desmidiaceae and Diatomaceæ, nothing seems to produce less alteration in the disposition of the endochrome, or serves better to preserve their color, than Distilled Water; provided that, by the complete exclusion of air, the vital processes and decomposing changes can be alike suspended. This method of mounting, however, is liable to the objection that confervoid growths sometimes make their appearance in the preparation; and it is preferable to make some addition to the water, which shall be unfavorable to their de velopment. Saturation with camphor, or shaking up with a few drops of creasote, will sometimes answer; but if the preservation of color be not an object, nothing is better than the addition of about a tenth part of alcohol; and where the loss of color would be objectionable, the solution of a grain of bay-salt and a grain of alum in an ounce of water will give it the requisite qualities. For larger preparations of Algae, &c., what is called Thwaites's Fluid may be employed; this is prepared by adding to one part of rectified spirit as many drops of creasote as will saturate it, and then gradually mixing up with it in a mortar some prepared chalk with 16 parts of water; an equal quantity of water saturated with camphor is then to be added, and the mixture, after standing for a few days, is to be carefully filtered. A liquid of this kind also serves well for the preservation of many animal preparations; but it is said by Dr. Beale to become turbid when thus employed in large quantity; and he recommends the following modification. Mix 3 drachms of creasote with 6 ounces of wood naphtha, and add in a mortar as much prepared chalk as may be necessary to form a smooth thick paste; water must be gradually added to the extent of 64 ounces, a few lumps of camphor thrown in, and the mixture allowed to stand for two or three weeks in a lightly covered vessel, with occasional stirring; after which it should be filtered, and preserved in well-stopped bottles. Of late years, Glycerine has been much employed as a preservative fluid; it allows the colors of vegetable substances to be

Some

retained, but, unless much diluted, it alters the disposition of the endochrome; and confervoid growths are apt to make their appearance in it. The best proportion seems to be one part of glycerine to two parts of camphor-water. The preparation known as Deane's Gelatine is one of the most convenient media for preserving the larger forms of Confervæ and other Microscopic Algae, as well as sections of such as are still more bulky. This is prepared by soaking 1 oz. of gelatine in 4 oz. of water until the gelatine is quite soft, and then adding 5 oz. of honey previously raised to boiling heat in another vessel; the whole is then to be made boiling hot, and when it has somewhat cooled, but is still perfectly fluid, 6 drops of creasote and oz. of spirit of wine, previously mixed together, are to be added, and the whole is to be filtered through fine flannel. This composition, when cold, forms a very stiff jelly; but it becomes perfectly fluid on the application of a very slight warmth, and may then be used like any other preservative liquid, care being taken, however, that the slide and the glass cover are themselves gently warmed before it comes into contact with them. A mixture of gelatine and glycerine has been employed for the same purpose. Vegetable Anatomists make great use of a solution of Chloride of Calcium (muriate of lime) in three parts of water; its chief advantage being, that owing to its deliquescent property, it does not dry up, even when the cell is not perfectly closed in. It has, however, the disadvantages of not preserving colors, and of altering the disposition of the cell-contents by its endosmotic power. For the preservation of microscopic preparations of Animal tissues, a mixture of one part of Alcohol and five of water will generally answer very well, save in regard to the removal of their colors. Where the preservation of these is aimed at, the best medium will usually be Goadby's Solution, which is made by dissolving 4 oz. of bay salt, 2 oz. of alum, and 4 grains of corrosive sublimate, in 4 pints of boiling water; this should be carefully filtered before it is used; and for all delicate preparations it may be diluted with an equal bulk, or even with twice its bulk of water. This solution must not be used where any calcareous texture, such as shell or bone, forms part of the preparation; and one of Mr. Goadby's other solutions (8 oz. of bay salt and 2 grs. of corrosive sublimate, to a quart of water,or, in cases where the coagulating action of corrosive sublimate on albuminous matters would be an objection, the substitution of 20 grains of arsenious acid) may be used in its stead; or Thwaites's fluid, or Dr. Beale's modification of it, or Deane's Gelatine may be tried. It is often quite impossible to predicate beforehand what preservative fluid will answer best for a particular kind of preparation; and it is consequently desirable, where there is no lack of material, always to mount the same object in two or three different ways, marking on each slide the method

employed, and comparing the specimens from time to time, so as to judge how each is affected.

132. Of Mounting Objects in Fluid.-As a general rule, it is desirable that objects which are to be mounted in fluid, should be soaked in the particular fluid to be employed, for some little time before mounting; since, if this precaution be not taken, air-bubbles are very apt to present themselves. It is sometimes necessary, in order to secure the displacement of air contained in the specimen, to employ the air-pump in the mode already directed (§ 128); but it will sometimes be found sufficient to immerse the specimen for a few minutes in alcohol (provided that this does not do any detriment to its tissues), which will often penetrate where water will not make its way; and when the spirit has driven out the air, the specimen may be removed back to water, which will gradually displace the spirit. When Deane's Gelatine is used, however, all that can be done, will be to drain the object of its superfluous water before applying the liquefied medium; but as air-bubbles are extremely apt to arise, they must be removed by means of the air-pump, the gelatine being kept in a liquid state by the use of a vessel of hot water, as in the case of Canada balsam. In dealing with the small quantities of fluid required in mounting microscopic objects, it is essential for the operator to be provided with the means of transferring very small quantities from the vessel containing it, to the slide, as well as of taking up from the slide what may be lying superfluous upon it. The straight and curved-pointed "dipping-tubes" (Fig. 51, A, B) may be made to answer this purpose; but it is much better that tubes for this purpose be furnished with a bulb, like that of the Chemist's "pipette," and that their orifices be drawn to a fine point. The fluid is drawn into the tubes by suction, and expelled by the pressure of the breath; the curved-pointed tube will generally be the best for introducing fluid beneath the glass cover, and the straight-pointed for simply filling cells or for taking up superfluous fluid. The Author has of late found very great convenience in the use of a small glass syringe, the orifice of which is slightly curved and drawn to a fine capillary point; for as the syringe works independently of the mouth, its orifice may be applied in any way that may be found convenient; and when the mouth is freed from the efforts of suction and ejection, the eyes can be better employed in watching the operation. Besides the pipettes and the syringe, some blotting-paper, of the most bibulous kind that can be procured, will be found very useful.

133. There are certain objects of extreme thinness, which require no other provision for mounting them in fluid, than an ordinary glass slide, a thin glass cover, and some gold-size or asphalte (§ 120). The object having been laid in its place, and a drop of the fluid laid upon it (care being taken that no airspace remains beneath the under side of the object and the sur

face of the slide), the glass cover is then to be laid upon it, one side being first brought into contact with the slide, and the other being gradually lowered, in such a manner that the air shall be all displaced before the fluid. If any air-bubbles remain in the central part of the space between the cover and the slide, the former must be raised again, and more fluid should be introduced; but if the bubbles be near the edge, a slight pressure on that part of the cover will often suffice to expel them, or the cover may be a little shifted so as to bring them to its edge. There are some objects, however, whose parts are liable to be displaced by the slightest shifting of this kind; and it is more easy to avoid making air-bubbles, by watching the extension of the fluid as the cover is lowered, and by introducing an additional supply when and where it may be needed, than it is to get rid of them afterwards without injury to the object. When this end has been satisfactorily accomplished, all that is needed is first to remove all superfluous fluid from the surface of the slide, and from around the edge of the cover, with a piece of blotting-paper, taking care not to draw away any of the fluid from beneath the cover, or (if any have been removed accidentally) to replace what may be deficient; and then to make a circle of asphalte or gold-size around the cover, taking care that it "wets" its edges, and advances a little way upon its upper surface. When this first coat is dry, another should be applied, particular care being taken that the cement shall fill the angular furrow at the margin of the cover. In laying on the second coat, it will be convenient, if the cover be round, to make use of the whirling-table (Fig. 63); and if the slide be so carefully laid upon it, that the glass cover is exactly concentric with its axis, the whirling-table may be used even for the first application of the varnish; a slight error in this respect, however, may occasion the displacement of the cover. By far the greater number of preparations which are to be preserved in liquid, however, should be mounted in a "Cell" of some kind, which forms a well of suitable depth, wherein the preservative liquid may be retained. This is absolutely necessary in the case of all objects, whose thickness is such as to prevent the glass cover from coming into close approximation with the slide; and it is desirable, wherever that approximation is not such as to cause the cover to be drawn to the glass slide by capillary attraction, or wherever the cover is sensibly kept apart from the slide by the thickness of any portion of the object. Hence it is only in the case of objects of the most extreme tenuity, that the "cell" can be advantageously dispensed with;-the danger of not employing it, in many cases in which there is no difficulty in mounting the object without it, being that after a time the cement is apt to run in beneath the cover, which process is pretty sure to continue, when it may have once commenced.

134. Cement-Cells.-When the Cells are required for mounting very thin objects, they may be advantageously made of varnish

only, by the use of the ingenious little instrument (Fig. 63) contrived by Mr. Shadbolt. This consists of a small slab of ma

FIG. 63.

Shadbolt's Turn-table for making Cement-Cells.

hogany, into one end of which is fixed a pivot, whereon a circular turntable of brass, about three inches in diameter, is made to rotate easily, a rapid motion being given to it by the application of the fore-finger to the milled head seen beneath. The glass slide being laid upon the turn-table, in such a manner that its two edges shall be equidistant from the centre (a guide to which is afforded by a circle of an inch in diameter, traced upon the brass), and being held by the springs with which it is furnished, a camel-hair pencil dipped in the varnish to be used (Brunswick black or Asphalte is the best) is held in the right hand, so that its point comes into contact with the glass, a little within the guiding circle just named. The turn-table being then put into rotation with the left hand, a ring of varnish of suitable breadth is made upon the glass; and if the slide be set aside in a horizontal position, this ring will be found, when dry, to have lost the little inequalities it may have at first presented, and to present a very level surface. If a greater thickness be desired than a single application will conveniently make, a second layer may be laid on after the first is dry. It is convenient to prepare a number of these cells at once, since, when "the hand is in," they will be made more dexterously than when the operation is performed only once; and it will be advantageous to subject them to the warmth of a slightly heated oven, whereby the flattening of their surface will be more completely assured. The Microscopist will find it a matter of great convenience to have a stock of these cells always by him, ready prepared for use.

135. Thin Glass Cells.-For the reception of objects too thick for varnish cells, but not thicker than ordinary thin glass, it is advantageous to construct cells of glass; and these may be made in one of two ways, either by grinding down the cross sections of glass tubes (§ 137) until they have been reduced to the desired thinness, or by perforating a plate of thin glass with an aperture of the desired size; and then cementing the ring or the plate to the glass slide with marine glue. The former plan is liable to the objection, that in reducing the glass rings to the desired thinness, they are extremely liable to crack or break, and that their attainable forms are limited. The latter will generally answer very well, if care be taken in the selection of a flat piece of thin glass; and the perforation, if due precaution be employed, may be made of any size or form that may be desired. For making round cells, the perforated pieces that sometimes re