fine soft powder, such as starch. Before using it, however, the Microscopist should be careful to clean it thoroughly; not merely for the sake of removing foulnesses which would interfere with the view of the object, but also for the sake of getting rid of adherent starch-grains, the presence of which might lead to wrong conclusions, and also of freeing the surface from that slight greasiness, which, by preventing it from being readily wetted by water, frequently occasions great inconvenience in the mounting of objects in fluid. The thicker pieces may be washed and wiped without much danger of fracture, if due care be employed; but the thinner require much precaution; and in cleansing these, the simple method devised by Mr. Spencer will be found very useful. This consists in the use of a pair of round flat disks, about 1 inch in diameter, made of wood or metal covered with chamois leather, and furnished with handles; for when a piece even of the thinnest glass is laid upon one of these, it may be rubbed clean with the other, and any amount of pressure may be used, without the least risk of breaking it. Previously to doing this, however, it will be advantageous to soak the pieces for a time in strong sulphuric acid, and then to wash them in two or three waters; if greasiness, however, be their chief fault, they should be soaked in a strong infusion of nutgalls; with which it will be also advantageous to cleanse the surface of glass slides that are to be used for mounting objects in liquid.

120. Varnishes and Cements.-There are three very distinct purposes, for which cements that possess the power of holding firmly to glass, and of resisting, not merely water, but other preservative liquids, are required by the Microscopist; these being (1) the attachment of the glass covers to the slides or cells containing the object, (2) the formation of thin cells of cement only, and (3) the attachment of the glass plate or tube-cells to the slides. The two former of these purposes are answered by liquid cements or varnishes, which may be applied without heat; the last requires a solid cement of greater tenacity, which can only be used in the melted state. The Varnishes used for mounting objects in liquid, should always be such as contain no mixture of solid particles. This is a principle on which the Author, from an experience of many years, is disposed to lay great stress; having often made trial, at the recommendation of friends, of varnishes which were said to have been greatly improved by thickening. with litharge or lamp-black; and having always found that, although they might stand well for a few weeks or months, they became porous after a greater lapse of time, allowing the evaporation of the liquid and the admission of air. He has himself found none more durable than that known as japanner's Goldsize, which may be obtained at almost every color-shop; for although this, when newly made, is apt to be somewhat too thin, so as to tend to run in beneath the glass cover, it may be easily

prevented by employing very liquid Gold-size, and by using it in extremely small quantity in the first instance; since whenever the glass cover lies perfectly flat on its bed, and the fluid beneath extends to the edges, the thin layer of this varnish dries very quickly, without any tendency to run in. When this has completely set, a second layer should be applied; and a layer of Asphalte over the whole will add to its security, and improve the appearance of the mounting. The danger of running in appears to the Author to be the greatest, when, in consequence of the use of old and viscid gold-size, the layer is too thick, and is long in drying. His experience leads him to distrust Asphalte when used alone, as being liable to admit air after a lengthened period of drying. He has recently learned from Mr. Tomes, that he finds saturated solution of arsenious acid to be a very good medium for mounting delicate preparations of animal structures. If it be allowed to become so thick, however, as not to be easily worked by the brush, it is quite unfit for use. There are few preservative liquids with which goldsize may not be employed; since it is not acted on by any aqueous solution, and resists moderately diluted spirit; oil of turpentine being its only true solvent. Many Microscopists prefer the solution of shell-lac in naphtha, which is sold under the name of Liquid Glue; this dries more quickly than gold-size, but is more brittle when completely hardened, and does not, in the Author's opinion, adhere so firmly and enduringly to glass; and it is, moreover, more easily acted on by diluted alcohol than the preceding. Of late, a solution of Asphalte in drying-oil or turpentine, sometimes known under the name of "Brunswickblack," has come much into use. It is extremely easy and pleasant to work with, and dries quickly; but it is brittle when dry, and is disposed to crack, not merely when subject to any "jar," but also (after some time) spontaneously. This evil may be corrected, according to Mr. Brooke, by adding to it a little solution of Caoutchouc in mineral naphtha. Oil of turpentine is the solvent for this varnish, as for gold-size; and brushes which have been used with either, may be cleansed by that menstruum; those which have been used with liquid glue, may be cleansed with naphtha. For mounting objects dry (§ 122), or for giving a finished appearance to mountings which have been made by one or other of the foregoing cements, varnishes may be used, which, from containing coloring particles, or from being acted on by the preservative liquids employed, could not be safely laid on in the first instance. Among the most convenient of this kind, are varnishes made by dissolving red or black or any other colored Sealing-wax in strong alcohol; these are more to be recommended for their appearance, however, than for their

The Author has preparations mounted with gold-size as much as twelve years ago, which have remained perfectly free from leakage; the precaution having been taken, to lay on a thin coat of varnish every two or three years.

tenacity, being very apt to lose their hold upon the glass after a time; and the Author, having suffered much injury to his preparations from trusting to them, would recommend that, even in mounting objects dry, some other cement should be first used, by which the glass cover should be attached to the slide, the sealing-wax varnish being only laid on as a finish. If a black varnish be desired for such a purpose, it may be readily made by mixing gold-size with a small quantity of lamp-black; this dries quickly, and is free from brittleness; but, for the reason already mentioned, it should not be used in the first instance to mount objects in fluid, although it may be laid as a finish over gold-size or asphalte. For making cement-cells (§ 134), either asphalte, gold-size, or liquid glue may be employed, the first being on the whole preferable; the varnish termed Black Japan also makes very good and durable cells, if the glasses to which it has been applied be exposed to the heat of an oven, not raised so high as to cause them to "blister."

121. Although Canada balsam has been sometimes used as a cement, and has the advantage of being worked with extreme convenience, yet it is so apt to crack when hardened by time, that a slight "jar" will cause the cell to spring away from the glass to which it has been attached. Hence, if employed at all for fixing cells to glass slides, its use should be limited to those plate-cells which afford a large surface of attachment (§ 136), or to those very thin tube-cells (§ 135), which cannot be so conveniently attached with marine glue, and of which the cover may be secured to the slide by spreading the ring of gold-size round the margin of the cell itself (§ 138). Care should be taken, in applying the Canada balsam, that it be sufficiently hardened by heat, but that it be not so heated as to become brittle (§ 109): the general method of using it for this purpose, is the same as that which must be practised in the case of marine glue. The superfluous balsam left after pressing down the cell, is to be removed, first by scraping with a heated knife, and then with a rag dipped in oil of turpentine, after which it is desirable to give the glass surface a final cleansing with alcohol. For all kinds of cells ($$ 135, 137) except those just mentioned, the proper cement is Marine Glue, which is a mixture of shell-lac, caoutchouc, and naphtha, now extensively employed; being distinguished by its extraordinary tenacity, and by its power of resisting solvents of almost every kind. Different qualities of this substance are made for the several purposes to which it is applied; that which is most suitable to the wants of the Microscopist, is known in commerce as GK 4. As this cement can only be applied hot, and as it is a great saving of trouble to attach a considerable number of cells at the same time, a Mounting-Plate should be provided, which will furnish the requisite heat to several slides at once. Such a surface may be afforded by the top of a stove; but it is better to have one which can be used at all seasons, and the heat

pre

of which can be precisely regulated at pleasure. A very simple apparatus much used for this purpose, consists of a small table of brass or iron plate, about 6 inches long and 2 broad, with legs about 4 inches high, either screwed into its four corners, or so jointed to them as to fold down; this is set over a small spiritlamp, the flame of which is regulated to give the heat required. The Author has found it much preferable, however, to lay the plate on one of the rings of a small "retort-stand" (used in Chemical operations), which admits of being shifted to any height that may be desired, so that the heat applied may be cisely graduated; or, if a gas lamp be employed for the ordinary purposes of illumination, its stem may be fitted with a slidingring, which will carry either a hot plate or a water-bath.' It is more convenient, however, to have two such plates, laid on two rings; one being allowed to cool with the slides upon it, whilst the other is being heated. The glass slides and cells which are to be attached to each other, must first be heated on the mounting-plate; and some small cuttings of marine-glue are then to be placed, either upon that surface of the cell which is to be attached, or upon that portion of the slide on which it is to lie, the former being perhaps preferable. When they begin to melt, they may be worked over the surface of attachment by means of a needle-point; and in this manner, the melted glue may be uniformly spread, care being taken to pick out any of the small gritty particles which this cement sometimes contains. When the surface of attachment is thus completely covered with liquefied glue, the cell is to be taken up with a pair of forceps, turned over and deposited in its proper place on the slide; and it is then to be firmly pressed down with a stick (such as the handle of the needle) or with a piece of flat wood, so as to squeeze out any superfluous glue from beneath. If any air-bubbles should be seen between the cell and the slide, these should if possible be got rid of by pressure, or by slightly moving the cell from side to side; but if their presence results, as is sometimes the case, from deficiency of cement at that point, the cell must be lifted off again, and more glue applied at the required spot. Sometimes, in spite of care, the glue becomes hardened and blackened by overheating; and as, in this case, it will not stick well to the glass, it is preferable not to attempt to proceed, but to lift off the cell from the slide, to let it cool, and then to repeat the process. When the cementing has been satisfactorily accomplished, the slides should be allowed to cool gradually, in order to secure the firm adhesion of the glue; and this is readily accomplished, in the first instance, by pushing each, as it is finished, towards one of the extremities of the plate, which is of course cooler than the centre. If two plates are in use, the heated plate may then be readily moved away upon the ring which supports 1 Both these fittings are adapted to the Gas lamp supplied for the use of Microscopists by Mr. S. Highley ($ 75).

it, the other being brought down in its place; and as the heated plate will be some little time in cooling, the firm attachment of the cells will be secured. If, on the other hand, there be only a single plate, and the operator desire to proceed at once in mounting more cells, the slides already completed should be carefully removed from it, and laid upon a wooden surface, the slow conduction of which will prevent them from cooling too fast. Before they are quite cold, the superfluous glue should be scraped from the glass with a small chisel or awl; and the surface should then be carefully cleansed with a solution of potash, which may be rubbed upon it with a piece of rag covering a stick shaped like a chisel. The cells should next be washed with a hard brush and soap and water, and may be finally cleansed by rubbing with a little weak spirit and a soft cloth. In cases in which appearance is not of much consequence, and especially in those in which the cell is to be used for mounting large opaque objects, it is decidedly preferable not to scrape off the glue too closely round the edges of attachment, as the "hold" is much firmer, and the probability of the penetration of air or fluid much less, if the immediate margin of glue be left, both outside and inside the cell.

122. Mounting Objects Dry.-There are certain objects, which, even when they are to be viewed by transmitted light, are more advantageously seen when simply laid on glass, than when they are immersed either in fluid or in balsam. This is the case especially with sections of bones and teeth, much of whose internal structure is obliterated by the penetration of fluid; and also with the scales of Lepidopterous and other Insects, whose minute surface-markings are far more distinct when thus examined, than when treated in any other way.' For preserving such objects, it is of course desirable that they should be protected by a cover; and this must be so attached to the glass slide, as to keep the object in place, besides being itself secured. For this purpose sealing-wax varnish is often used, but is unsuitable on account of its brittleness when dry; gold size mixed with lamp-black is much to be preferred, and, if carefully laid on, will not tend to run in between the cover and the slide. If the object have any tendency to curl up, or to keep off the cover from the slide by its own "spring," it will be useful, while applying the varnish, to make use of pressure, such as that afforded by the little implement represented in Fig. 62. This pressure should

It is affirmed by two high authorities on all that relates both to the theoretical and practical action of Object-glasses of large aperture (namely, Prof. Robinson and Mr. Wenham), that the effect of mounting delicate test-objects in Canada balsam is practically to reduce the aperture, since no rays can diverge after passing through a stratum of this substance, at a greater angle than 85° or 90°. Hence they recommend that the *difficult" Diatomacea should not be mounted in balsam, if they are to be viewed by objectives of 120° or 130°. Their position is disputed, however, by Prof. Bailey (U. S.), who affirms that balsam-mounted specimens are preferable as test objects. Those who are interested in this question, will do well to consult the papers of these gentlemen, in the 2d and 3d volumes of the "Quart. Journ. of Microsc. Science."