lost among the lamellæ of cork which are removed at the same time. The special methods of preparation which are required in the case of the various substances, of which sections may be conveniently cut by this instrument, will be described under their several heads.

108. Grinding and Polishing of Sections.-Substances which are too hard to be sliced with a cutting instrument in the manner last described,-such as bones, teeth, shells, corals, fossils of all kinds, and even some recent vegetable tissues,-can only be reduced to the requisite thinness for Microscopical examination, by grinding down thick sections, until they become so thin as to be transparent. The general method of making such preparations will be here described; but those special details of management which particular substances may require, will be given when these substances are respectively described. The first thing to be done, will usually be to procure a section of the substance, as thin as it can be safely cut. Most substances not siliceous may be divided by the fine saws used by artisans for cutting brass; but there are some bodies (such as the enamel of teeth, and porcellanous shells), which, though merely calcareous, have their mineral particles arranged in such a peculiar state of aggregation, as to make it very difficult and tedious to divide them in this mode; and it is much the quicker operation to slit them with a disk of soft iron (resembling that used by the lapidary) charged at its edge with diamond-dust, which may be driven in an ordinary lathe. Where waste of material is of no account, a very expeditious method of obtaining pieces fit to grind down, is to detach them from the mass with a strong pair of "cutting-pincers," or, if it be of small dimensions, with "cutting-pliers;" and a flat surface must then be given to it, either by holding it to the side of an ordinary grindstone, or by rubbing it on a plate of lead (cast or planed to a perfect level) charged with emery, or by a strong toothed file, the former being the most suitable for the hardest substances, the latter for the toughest. There are certain substances, especially calcareous fossils of wood, bone, and teeth, in which the greatest care is required in the performance of these preliminary operations, on account of their extreme friability; the vibration produced by the working of the saw or the file, or by grinding on a rough surface, being sufficient to disintegrate even a thick mass, so that it falls to pieces under the hand; such specimens, therefore, it is requisite to treat with great caution, dividing them by the smooth action of the wheel, and then rubbing them down upon nothing rougher than a very fine "grit." Where (as often happens) such specimens are sufficiently porous to admit of the penetration of Canada balsam, it will be desirable, after soaking them in tur

The following directions do not apply to Siliceous substances; as sections of these can only be prepared by those who possess a regular Lapidary's apparatus, and who have been specially instructed in the use of it.

pentine for a while, to lay some liquid balsam upon the parts through which the section is to pass, and then to place the specimen before the fire or in an oven for some little time, so as first to cause the balsam to run in, and then to harden it; by this means the specimen will be rendered much more fit for the processes it has afterwards to undergo. It not unfrequently happens, that the small size, awkward shape, or extreme hardness of the body, occasions a difficulty in holding it either for cutting or grinding; in such a case, it is much better to attach it to the glass in the first instance, by any side that happens to be flattest; and then to rub it down by means of the "hold" of the glass upon it, until the projecting portion has been brought to a plane, and has been prepared for permanent attachment to the glass. This is the method which is generally most convenient to pursue with regard to small bodies; and there are many which can scarcely be treated in any other way, than by attaching a number of them to the glass at once, in such a manner as to make them mutually support one another.1

109. The mode in which the operation is then to be proceeded with, depends upon whether the section is to be ultimately set up in Canada balsam (§ 125), or is to be mounted dry (§ 122), or in fluid (§ 132). In the former case, the following is the plan to be pursued. The flattened surface is to be polished, by rubbing it with water on a "Water-of-Ayr" stone, on a hone or "Turkey" stone, or on a new stone recently introduced under the name of the "Arkansas" stone; the first of the three is the best for all ordinary purposes; but the two latter, being much harder, may be employed for substances which resist it. When this has been sufficiently accomplished, the section is to be attached with Canada balsam to a slip of thick, well-annealed glass; and, as the success of the final result will often depend upon the completeness of its adhesion to this, the means of most effectually securing that adhesion will now be described in detail. Some Canada balsam, previously rendered somewhat stiff by the evaporation of part of its turpentine, is to be melted on the glass slip, so as to form a thick drop, covering a space somewhat larger than the area of the section; and it should then be set aside to cool, during which process, the bubbles that may have formed in it will usually burst. When cold, its hardness should be tested, which is best done by the edge of the thumb-nail; for it should be with difficulty indented by its pressure, and yet should not be

1 Thus, in making horizontal and vertical sections of Foraminifera, as it would be impossible to cut them through, they must be laid close together in a bed of hardened Canada balsam on a slip of glass, in such positions, that, when rubbed down, the plane of section shall traverse them in the desired directions; and one flat surface having been thus obtained for each, this must be turned downwards, and the other side ground away.

2 As the flatness of the polished surface is a matter of the first importance, that of the stones themselves should be tested from time to time; and whenever they are found to have been rubbed down on any one part more than on another, they should be flattened on a paving-stone with fine sand, or on the lead-plate with emery.

so resinous as to be brittle. If it be too soft, as indicated by its too ready yielding to the thumb nail, it should be boiled a little more; if too hard, which will be shown by its chipping, it should be re-melted and diluted with more fluid balsam, and then set aside to cool as before. When it is found to be of the right consistence, the section should be laid upon its surface, with the polished side downwards; the slip of glass is next to be gradually warmed until the balsam is softened, special care being taken to avoid the formation of bubbles; and the section is then to be gently pressed down upon the liquefied balsam, the pressure being at first applied rather on one side than over its whole area, so as to drive the superfluous balsam in a sort of wave towards the other side, and an equable pressure being finally made over the whole. If this be carefully done, even a very large section may be attached to glass, without the intervention of any airbubbles; if, however, they should present themselves, and they cannot be expelled by increasing the pressure over the part beneath which they are, or by slightly shifting the section from side to side, it is better to take the section entirely off, to melt a little fresh balsam upon the glass, and then to lay the section upon it as before.

110. When the section has been thus secured to the glass, and the attached part thoroughly saturated (if it be porous) with hard Canada balsam, it may be readily reduced in thickness, either by grinding or filing as before, or, if the thickness be excessive, by taking off the chief part of it at once by the slitting-wheel. So soon, however, as it approaches the thinness of a piece of ordinary card, it should be rubbed down with water on one of the smooth stones previously named, the glass slip being held beneath the fingers with its face downwards, and the pressure being applied with such equality, that the thickness of the section shall be (as nearly as can be discerned) equal over its entire surface. As soon as it begins to be translucent, it should be placed under the Microscope (particular regard being had to the precaution specified in §86), and note taken of any inequality; and then, when it is again laid upon the stone, such inequality may be brought down, by making special pressure with the fore-finger upon the part of the slide above it. When the thinness of the section is such as to cause the water to spread around it between the glass and the stone, an excess of thickness on either side may often be detected, by noticing the smaller distance to which the liquid extends. In proportion as the substance attached to the glass is ground away, the superfluous balsam which may have exuded around it will be brought into contact with the stone; and this should be removed with a knife, care being taken, however, that a margin be still left round the edge of the section. As the section approaches the degree of thinness which is most suitable for the display of its organization, great care must be taken that the grinding process be not carried too far; and fre

quent recourse should be had to the Microscope, which it is convenient to have always at hand, when work of this kind is being carried on. There are many substances whose intimate structure can only be displayed in its highest perfection, when a very little more reduction would destroy the section altogether; and every Microscopist who has occupied himself in making such preparations, can tell of the number which he has sacrificed in order to attain this perfection. Hence if the amount of material be limited, it is a good rule to stop short as soon as a good section has been made, and to lay it aside-"letting well alone"-whilst the attempt is being made to procure a better one; if this should fail, another attempt may be made, and so on, until either success has been attained, or the whole of the material has been consumed, the first section, however, still remaining: whereas, if the first, like every successive section, be sacrificed in the attempt to obtain perfection, no trace will be left to "show what has been." In judging of the appearance of sections in this stage under the Microscope, it is to be remembered that its transparence will subsequently be considerably increased by mounting in Canada balsam (§ 125); this is particularly the case with fossils, to which a deep hue has been given by the infiltration of some coloring matter; and with any substances whose particles have a molecular aggregation, that is rather amorphous than crystalline. When a sufficient thinness has been attained, the section may generally be "mounted" in Canada balsam; and the mode in which this must be managed, will be detailed hereafter (§ 129). 111. As there are certain substances, however, the view of whose structure is impaired by mounting in Canada balsam, and which should therefore be mounted either dry or in fluid, a different method of procedure must be adopted with them. If tolerably thin sections of them can be cut in the first instance, or if they are of a size and shape to be held in the hand whilst they are being roughly ground down, there will be no occasion to attach them to glass at all; is is frequently convenient to do this at first, however, for the purpose of obtaining a "hold" upon the specimen; but the surface which has been thus attached, must afterwards be completely rubbed away, in order to bring into view a stratum which the Canada balsam shall not have penetrated. As none but substances possessing considerable toughness, such as bones and teeth, can be treated in this manner, and as these are the substances which are most quickly reduced by a coarse file, and are least liable to be injured by its action, it will be generally found possible to bring the sections to a considerable thinness, by laying them upon a piece of cork or soft wood held in a vice, and operating upon them first with a coarser and then with a finer file. When this cannot safely be carried further, the section must be rubbed down upon that one of the fine stones already mentioned (§ 109), which is found best to suit it; as long as the section is tolerably thick, the finger may be used to press and move it but as soon as the finger

itself begins to come into contact with the stone, it must be guarded by a flat slice of cork, or by a piece of gutta percha, a little larger than the object. Under either of these, the section may be rubbed down until it has been reduced to the requisite degree of tenuity; but even the most careful working, on the finest-grained stone, will leave its surface covered with scratches, which not only detract from its appearance, but prevent the details of its internal structure from being as readily made out, as they can be in a polished section. This polish may be imparted, by rubbing the section with putty powder (peroxide of tin) and water, upon a leather strap, made by covering the surface of a board with buff-leather, having three or four thicknesses of cloth, flannel, or soft leather beneath it; this operation must be performed on both sides of the section, until all the marks of the scratches left by the stone shall have been rubbed out; when the specimen will be fit for mounting, after having been carefully cleansed from any adhering particles of putty powder.

112. Chemical Actions.-One important part of the preparation of Microscopic objects, is often effected by the use of Chemical Reagents. These may be employed, either for the sake of removing substances of which it is desired to get rid, in order to bring something else into view; or for the sake of detecting the presence of particular substances in the object under examination. Thus, in order to obtain the animal basis of Shell, Bone, Tooth, &c., it is necessary to dissolve away the calcareous portion of these tissues by the use of acids; a mixture of nitric and muriatic acids is preferable; and this should be added, little by little, to a considerable bulk of water, until a disengagement of gas be perceived to commence from the surface of the specimen. Care should always be taken not to hurry the process by adding too much acid, since, when the animal membrane is of very delieate consistence, it is liable to be dissolved; and in some cases it is better to allow the action to go on for many weeks, adding only a drop or two of acid at a time. When siliceous particles are to be removed (such as those which form the lorica of the Diatomacea), for the sake of leaving the organic membrane in a state adapted to separate examination, hydrofluoric acid must be employed as the menstruum. It is sometimes necessary, on the other hand, to get rid of the organic matter, for the sake of obtaining the mineral particles in a separate state, as in the case of the spicules of Sponges, Gorgoniæ, &c., this may be done either by incineration, or (which is generally preferable) by boiling or macerating for a long time in a solution of caustic potash. In separating from Guano, again, the siliceous skeletons of Diatomacere, &c., which it may contain, muriatic and nitric acids are largely used, to dissolve away every part of the mass on which they will act; the microscopic organisms for which search is made, being contained in a few grains of sediment which are left when a pound of pure guano is thus treated.