The term “dry” is used when the object to be mounted is not immersed in any liquid or medium, but preserved in its natural state, unless it requires cleaning and drying. I have before stated that thorough cleanliness is necessary in the mounting of all microscopic objects. I may here add that almost every kind of substance used by the microscopist suffers from careless handling. Many leaves with fine hairs are robbed of half their beauty, or the hairs, perhaps, forced into totally different shapes and groups; many insects lose their scales, which constitute their chief value to the microscopist; even the glass itself distinctly shows the marks of the fingers if left uncleaned. Every object must also be thoroughly dry, otherwise dampness will arise and become condensed in small drops upon the inner surface of the thin glass cover. This defect is frequently met with in slides which have been mounted quickly; the objects not being thoroughly dry when enclosed in the cell. Many of the cheap slides are thus rendered worthless. Even with every care it is not possible to get rid of this annoyance occasionally.
For the purpose of mounting opaque objects “dry,” discs were at one time very commonly made use of. These are circular pieces of cork, leather, or other soft substance, from one-quarter to half inch in diameter, blackened with varnish or covered with black paper, on which the object is fixed by gum or some other adhesive substance. They are usually pierced longitudinally by a strong pin, which serves for the forceps to lay hold of when being placed under the microscope for examination. Sometimes23 objects are affixed to both sides of the disc, which is readily turned when under the object-glass. The advantage of this method of mounting is the ease with which the disc may be moved, and so present every part of the object to the eye save that by which it is fastened to the disc. On this account it is often made use of when some particular subject is undergoing investigation, as a number of specimens may be placed upon the discs with very little labour, displaying all the parts. But where exposure to the atmosphere or small particles of dust will injure an object, no advantage which the discs may possess should be considered, and an ordinary covered cell should be employed. Small pill-boxes have been used, to the bottom of which a piece of cork has been glued to afford a ground for the pin or other mode of attachment; but this is liable to some of the same faults as the disc, and it would be unwise to use these for permanent objects.
Messrs. Smith and Beck have lately invented, and are now making, a beautiful small apparatus, by means of which the disc supporting the object can be worked with little or no trouble into any position that may prove most convenient, whilst a perforated cylinder serves for the reception of the discs when out of use, and fits into a case to protect them from dust. A pair of forceps is made for the express purpose of removing them from the case and placing them in the holder.
All dry objects, however, which are to be preserved should be mounted on glass slides in one of the cells (described in Chapter I.) best suited to them. Where the object is to be free from pressure, care must be taken that the cell is deep enough to ensure this. When the depth required is but small, it is often sufficient to omit the card, leather, or other circles, and with the “turn-table” before described, by means of a thick varnish and camel-hair pencil, to form a ring of the desired depth; but should the varnish not be of sufficient substance to give such “walls” at once, the first application may be allowed to dry, and a second24 made upon it. A number of these may be prepared at the same time, and laid by for use. When liquids are used (see Chapter IV.), Dr. Carpenter recommends gold-size as a good varnish for the purpose, and this may be used in “dry” mountings also. I have used the asphaltum and india-rubber (mentioned in Chapter I.), and found it to be everything I could wish. The cells, however, must be thoroughly dry, and when they will bear the heat they should be baked for an hour at least in a tolerably cool oven, by which treatment the latter becomes a first-rate medium. All dry objects which will not bear pressure must be firmly fastened to the slide, otherwise the necessary movements very often injure them, by destroying the fine hairs, &c. For this purpose thin varnishes are often used, and will serve well enough for large objects, but many smaller ones are lost by adopting this plan, as for a time, which may be deemed long enough to harden the varnish, they exhibit no defect, but in a while a “wall” of the plastic gum gathers around them, which refracts the light, and thus leads the student to false conclusions. In all finer work, where it is necessary to use any method of fixing them to the slide, a solution of common gum, with the addition of a few drops of glycerine (Chapter I.), will be found to serve the purpose perfectly. It must, however, be carefully filtered through blotting paper, otherwise the minute particles in the solution interfere with the object, giving the slide a dusty appearance when under the microscope.
When mounting an object in any of these cells, the glass must be thoroughly cleaned, which may be done with a cambric handkerchief, after the washing mentioned in Chapter I. If the object be large, the point of a fine camel-hair pencil should be dipped into the gum solution, and a minute quantity of the liquid deposited in the cell where the object is to be placed, but not to cover a greater surface than the object will totally hide from sight. This drop of gum must be allowed to dry, which will take a few minutes. Breathe then upon it two or three times, holding the slide25 not far from the mouth, which will render the surface adhesive. Then draw a camel-hair pencil through the lips, so as to moisten it slightly (when anything small will adhere to it quite firmly enough), touch the object and place it upon the gum in the desired position. This must be done immediately to ensure perfect stability, otherwise the gum will become at least partially dry and only retain the object imperfectly.
When, however, the objects are so minute that it would be impossible to deposit atoms of gum small enough for each one to cover, a different method of proceeding must be adopted. In this case a small portion of the same gum solution should be placed upon the slide, and by means of any small instrument—a long needle will serve the purpose very well—spread over the surface which will be required. The quantity thus extended will be very small, but by breathing upon it may be prevented drying whilst being dispersed. This, like the forementioned, should be then allowed to dry; and whilst the objects are being placed on the prepared surface, breathing upon it as before will restore the power of adherence.
When gum or other liquid cement has been used to fix the objects to the glass, the thin covers must not be applied until the slide has been thoroughly dried, and all fear of dampness arising from the use of the solution done away with. Warmth may be safely applied for the purpose, as objects fastened by this method are seldom, if ever, found to be loosened by it. As objects are met with of every thickness, the cells will be required of different depths. There is no difficulty in accommodating ourselves in this—the deeper cells may be readily cut out of thick leather, card, or other substance preferred (as mentioned in Chapter I.). Cardboard is easily procured of almost any thickness; but sometimes it is convenient to find a thinner substance even than this. When thin glass is laid upon a drop of any liquid upon a slide, every one must have observed how readily the liquid spreads betwixt the two: just so when26 any thin varnish is used to surround an object of little substance, excessive care is needed lest the varnish should extend betwixt the cover and slide, and so render it worthless. The slightest wall, however, prevents this from taking place, so that a ring of common paper may be used, and serve a double purpose where the objects require no deeper cell than this forms.
Many objects, however, are of such tenuity—as the leaves of many mosses, some of the Diatomace?, scales of insects, &c.—that no cell is requisite excepting that which is necessarily formed by the medium used to attach the thin glass cover to the slide; and where the slide is covered by the ornamental papers mentioned in Chapter I., and pressure does not injure the object, even this is omitted, the thin glass being kept in position by the cover; but slides mounted in this manner are frequently injured by dampness, which soon condenses upon the inner surfaces and interferes both with the object and the clearness of its appearance.
The thin glass, then, is to be united to the slide, so as to form a perfect protection from dust, dampness, or other injurious matter, and yet allow a thoroughly distinct view of the object. This is to be done by applying to the glass slide round the object some adhesive substance, and with the forceps placing the thin glass cover (quite dry and clean) upon it. A gentle pressure round the edge will then ensure a perfect adhesion, and with ordinary care there will be little or no danger of breakage. For this purpose gold-size is frequently used. The asphalt and india-rubber varnish also will be found both durable and serviceable. Whatever cement may be used, it is well to allow it to become in some measure “fixed” and dried; but where no cell or “wall” is upon the slide, this is quite necessary, otherwise the varnish will be almost certain to extend, as before mentioned, and ruin the object. It may be stated here that gold-size differs greatly in its drying powers, according to its age, mode of preparation, &c. (Chapter IV.)
27 Should any object be enclosed which requires to be kept flat during the drying of the cement, it will be necessary to use some of the contrivances mentioned in Chapter I.
When the slide is thus far advanced, there remains the “finishing” only. Should the student, however, have no time to complete his work at once, he may safely leave it at this stage until he has a number of slides which he may finish at the same time. There are different methods of doing this, some of which may be here described.
If ornamental papers are preferred, a small circle must be cut out from the centre a little less than the thin glass which covers the object. Another piece of coloured paper is made of the same size, and a similar circle taken from its centre also, or both may be cut at the same time. The slide is then covered round the edges with paper of any plain colour, so that it may extend about one-eighth of an inch over the glass on every side. The ornamental paper is then pasted on the “object” surface of the glass, so that the circle shows the object as nearly in the centre as possible, and covers the edges of the thin glass. The other coloured paper is then affixed underneath with the circle coinciding with that above. And here I may observe, that when this method is used there is no necessity for the edges of the slide to be “ground,” as all danger of scratching, &c., is done away with by the paper cover.
Many now use paper covers, about one and a half inches long, on the upper side of the slide only, with the centre cut out as before, with no other purpose than that of hiding the edge of the thin glass where it is united to the slide.
The method of “finishing,” however, which is mostly used at the present time, is to lay a coating of varnish upon the edge of the thin glass, and extend it some little way on the slide. When a black circle is required, nothing serves the purpose better than the gold-size and lamp-black, or the asphalt and india-rubber varnish, neither of which is liable to chip; but when used for this, the latter should be rather28 thinner, as before advised. Some of these varnishes are preferred of different colours, which may be made by using the different kinds of sealing-wax, as described in Chapter I.; but they are always liable to the defects there mentioned. This circle cannot be made in any other way but by one of those contrivances which have now centred in Shadbolt’s turntable. A very little practice will enable the young student to place his slide so that the circle may be uniform with the edge of the thin glass.
The slide is now complete, except the addition of the name and any other particulars which may be desirable. For this purpose one of the methods described in Chapter I. must be employed.
Amongst the various classes of microscopic objects now receiving general attention, the Diatomace? may be placed in a prominent position. They afford endless opportunities of research, and some very elaborate works have already been issued concerning them. Professor Smith’s may be mentioned as one containing, perhaps, the best illustrations. The young student may wish to know what a diatom is. The “Micrographic Dictionary” gives the following definition:—“A family of confervoid Alg?, of very peculiar character, consisting of microscopic brittle organisms.” They are now looked upon by almost all of our scientific men as belonging to the vegetable kingdom, though some few still assign them to the animal. They are almost invariably exceedingly small, so that the unaided eye can perceive nothing on a prepared slide of these organisms but minute dust. Each separate portion, which is usually seen when mounted, is termed a “frustule,” or “testule:” this consists of two similar parts, composed of silica, between and sometimes around which is a mass of viscid matter called the “endochrome.” They are found in almost every description of water, according to the variety: some prefer sea-water, others fresh, and many are seen nowhere but in that which is a mixture of both, as the mouths of rivers, &c. Ditches, ponds, cisterns, and indeed almost every reservoir,29 yield abundance of these forms. They are not, however, confined to “present” life; but, owing to the almost indestructible nature of their siliceous covering, they are found in a fossil state in certain earths in great abundance, and are often termed “fossil Infusoria.” Upon these frustules are generally to be seen lines, or “markings,” of different degrees of minuteness, the delicacy of which often serves the purpose of testing the defining power of the object-glasses. Some of the frustules are triangular, others circular, and, indeed, of almost every conceivable shape, many of them presenting us with exquisitely beautiful designs.
The markings, however, are seldom seen well, if at all, until the frustules are properly prepared, the different methods of accomplishing which will be given a little further on.
The collection of the fresh diatoms is so closely connected with their preservation, that a few notes may be given upon it before we pass on. For this purpose a number of small bottles must be provided, which may be placed in a tin box, with a separate apartment for each, so that all chance of breakage may be done away with. The diatoms are generally of a light brown colour; and where they are observed in the water, the bottle may be so placed, with the mouth closed by the finger, that when the finger is withdrawn the water will rush in, carrying the diatoms also. If they are seen upon plants, stones, or any other substance, they may generally be detached and placed in the bottle. When there is a green covering upon the surface of the water, a great quantity of diatoms is usually found amongst it; as also upon the surface of the mud in those ponds where they abound. In these cases, a broad flat spoon will be found very useful, and one is now made with a covering upon the broader portion of it to protect the enclosed matter from being so readily carried off whilst bringing it to the surface again. Where there is any depth of water, and the spoon will not reach the surface of the mud, the bottle must be united to a long rod, and being30 then carried through the upper portion with the mouth downwards, no water will be received into it; but on reaching the spot required, the bottle-mouth may be turned up, and thus become filled with what is nearest.
From the stomachs of common fish—as the cod, sole, haddock, &c.—many specimens of Diatomace? may be obtained, but especially from the crab, oyster, mussel, and other shell-fish. Professor Smith states that from these curious receptacles he has taken some with which he has not elsewhere met. To remove them from any of the small shell-fish, it is necessary to take the fish or stomach from the shell, and immerse it in strong hot acid (nitric is the best) until the animal matter is dissolved, when the residue must be washed and treated as the ordinary Diatomace? hereinafter described.
Many diatoms are seen best when mounted in a dry state, the minute markings becoming much more indistinct if immersed in liquid or balsam; and for this reason those which are used as test objects are usually mounted dry. Many kinds also are now prepared in this way, as opaque objects to be examined with the lieberkuhn, and are exquisitely beautiful. Others, however, are almost invariably mounted in balsam; but as these will be again referred to in Chapter III., and require the same treatment to fit them for the slide, it will not be out of place to describe the cleaning and preparation of them here. As before stated, there is much matter surrounding them which must be got rid of before the “siliceous covering” can be shown perfectly. As, however, we may first wish to become acquainted in some degree with what we have to do, it is well to take a small piece of talc, and place a few of the diatoms upon it. This may be held over the flame of the spirit-lamp until all the surrounding matter is burnt away, and a tolerable idea may be thus obtained as to the quality of our treasure.
In some cases it is well to use this burning operation alone in mounting specimens of diatoms, when they may be placed in their natural state upon the thin glass, burnt for31 awhile upon the platinum plate, hereafter described, and mounted dry or in balsam.
In the preparation and cleaning of Diatomace?, there is little satisfaction unless these operations have been successfully performed, as a very small portion of foreign matter seriously interferes with the object. The mode of preparing them varies even amongst the most experienced. It will be found, therefore, most satisfactory to examine the principal of these separately, although it may be at the risk of some little repetition.
The method which is the most frequently made use of is the following:—Place the “gathering” containing the Diatomace? in a small glass or porcelain vessel, add strong nitric acid, and, by the aid of Bunsen’s burner or spirit-lamp, boil for some minutes. From time to time a drop of the mixture may be put upon a slide, and examined under the microscope to see if all foreign matter be got rid of. When the valves are clean, the vessel containing them must be filled with water, and the whole left for an hour or two, so that all the diatoms may settle perfectly. The liquid must then be poured off carefully, or drained away by the aid of a syphon, so that none of the diatoms are removed with it. Indeed, it is well to examine the liquid drained off each time with the microscope, as the finer forms are frequently lost in the washings. The vessel must then be refilled with pure water, allowed to settle, and drained as before. This washing must be repeated until a drop being placed upon a slide and evaporated leaves no crystals. When it is desirable to preserve the diatoms in this state before mounting (which process will be described in another place), they may be placed in a small phial with a little distilled water.
There are many cases in which the above method will not effect a perfect cleansing, as certain substances with which the diatoms are frequently mixed are not soluble in nitric acid. For this reason the following method is resorted to:—Take a quantity of the matter containing the Diatomace? and wash first with pure water, to get rid of all32 the impurities possible. Allow this to settle perfectly and decant the water. Add hydrochloric acid gradually, and when all effervescence has subsided, boil for some minutes by aid of the lamp. When cool and the particles have subsided, decant the hydrochloric and add nitric acid. The boiling must then be repeated until a drop of the liquid when placed under the microscope shows the valves or “frustules” clean. After allowing the diatoms to settle, the acid must be decanted, and pure water substituted. The washing must be repeated as in the former process until all the remains of crystals or acid are removed, when the specimens may be preserved in small phials as stated above.
Such are the usual modes of treating the Diatomace?, but there are certain cases in which particular methods are required to give anything like perfect results. Persons of great experience combine a variety of treatments, and thus obtain better and more uniform specimens. Perhaps it will be advantageous to give the young student the process adopted by one of the most successful preparers of these objects; but I will first state the different methods of mounting the cleaned diatoms dry: how to employ Canada balsam and fluid in their preservation will be elsewhere described.
It was before stated that the diatoms when cleansed might be preserved in small phials of distilled water. When required for mounting, shake the phial, and with a thin glass tube or rod take up a drop of the fluid and spread it upon the surface of the slide in the desired position. This must then be allowed to dry gradually, or by the aid of the lamp if necessary, without being shaken or interfered with, otherwise uniformity of dispersion will be prevented. When thoroughly dry, a thin ring of one of the adhesive varnishes—gold-size will be found as good as any—may be drawn round the diatoms, and allowed to dry in a slight degree. The slide and thin glass cover should then be warmed and the latter gently pressed upon the ring of varnish until the adhesion all round is complete.
33 As some of the diatoms require object-glasses of extremely high power, and, consequently, short focus, to show them, they must be as close to the outer surface of the cover as possible. For this reason they are sometimes placed upon the under side of the thin glass, as follows. Clean the surfaces of the slide and cover, and with the rod or pipe place the liquid containing the diatoms upon the thin glass, and dry as before. Trace the ring to receive the cover upon the slide, and when almost dry, warm both and proceed as above. Whichever of these methods is employed, the outer ring of coloured varnish may be applied as elsewhere described and the slide finished.
The diatoms are also sometimes mounted betwixt two thin glasses, as described in Chapter I., so that the light by which they are examined may receive as little interference as possible, and that an achromatic condenser may be brought into focus under the slide.
Of the various modes of cleaning and mounting the Diatomace?, I believe that the following may be safely recommended as affording results of the very best quality. My friend, Mr. T. G. Rylands, gave it to me as that which he prefers, and I can safely say that his numerous slides are at least equal to any I have ever seen. I will give it just as I received it from him, though there may be some little repetition of what has been said elsewhere, as he does not appropriate any part of it as his own. He says:—In this branch of mounting, general rules alone can be laid down, because the gatherings may contain iron, lime, fine silt, or vegetable matter under conditions for special treatment, and consequently the first step should be to experiment on various kinds.
In gathering diatoms much labour is saved by judgment and care; hence it is desirable to get acquainted with them in their growing condition, so that when recognised upon the sands or other spots they may be carefully removed by the aid of the spoon or small tin scoop before described. When growing upon alg? or other plants, the plants and diatoms34 together may be carried home, in which case they must be simply drained and not washed or pressed, in order that the diatoms be not lost. As it is always desirable to examine the gathering on the ground, a “Gairdner’s hand microscope” with powers from 80 to 200 diameters will be found very useful. The best gatherings are those which represent one species abundantly. Those which are mixed may be rejected, unless they are seen to contain something valuable or important, as the object should be not so much to supply microscopical curiosities as to collect material which is available for the study of nature.
The gathering when carried home should always be carefully examined before anything is done with it; not only on account of the additional information thus acquired, but also because it often happens that a specimen should be mounted in fluid (see Chapter IV.) in the condition in which it is gathered, as well as cleaned and mounted in balsam (Chapter III.) and dry.
Where the gathering is taken from sand, the whole may be shaken up in water as a preliminary operation, when much of the sand will be separated by its own weight. The lime test, however, should be applied, viz.—a small portion of hydrochloric acid, and if there be effervescence it must be dissolved out by this means. From Alg? and other weeds diatoms may be detached by agitating the whole together in a weak solution of nitric acid—about one of pure acid to twenty or thirty of water, as it must be sufficiently weak to free the diatoms without destroying the matter to which they adhere. The diatoms may then be separated by sifting through coarse muslin, which will retain the Alg?, &c. The process of cleaning will vary according to circumstances. Some gatherings require to be boiled only a few minutes in nitric acid; but the more general plan where they are mixed with organic or other foreign matter, is to boil them in pure sulphuric acid until they cease to grow darker in colour (usually from a half to one minute), and then to add, drop by drop to avoid explosions,35 a cold saturated solution of chlorate of potash until the colour is discharged, or, in case the colour does not disappear, the quantity of the solution used is at least equal to that of the acid. This operation is best performed in a wide-mouthed ordinary beaker glass,B a test-tube being too narrow. The mixture whilst boiling should be poured into thirty times its bulk of cold water, and the whole allowed to subside. The fluid must then be carefully decanted and the vessel re-supplied once or twice with pure water, so as to get rid of all the acid. The gathering may then be transferred to a small boiling glass or test-tube, and—the water being carefully decanted—boiled in the smallest available quantity of nitric acid, and washed as before. This last process has been found necessary from the frequent appearance of minute crystals, which cannot otherwise be readily disposed of without the loss of a considerable proportion of diatoms.
B These glasses are round, about six inches high, and usually contain about eight ounces. They are rather wider at the bottom, tapering gradually to the top, and may be generally procured at the chemists, &c.
I may here mention that the washing glasses used by Mr. Rylands are stoppered conical bottles varying in capacity from two ounces to one quart; the conical form being employed to prevent the adherence of anything to the side; they are “stoppered” to render them available in the shaking process about to be described.
The gathering, freed from acid, is now put into two inches depth of water, shaken vigorously for a minute or two, and allowed to subside for half an hour, after which the turbid fluid must be carefully decanted. This operation must be repeated until all the matter is removed which will not settle in half an hour. The fluid removed should be examined by a drop being put upon a slide, as in some cases very light diatoms have been found to come off almost pure in one or more of these earlier washings. The quantity of water and time of subsidence given may be taken36 generally, but may require to be modified according to circumstances and the judgment of the operator. By the repetition and variation of this process—the shaking being the most important part—the gathering, if a pure one, will be sufficiently clean. If, however, it contains a variety of species and forms, it may require to be divided into different densities.
In some cases, however, it is best to divide the gathering as a preliminary operation, which may be done by agitating it in a quantity of water and decanting what does not readily subside. The heavier and the lighter portions are then to be treated as two separate boilings. But when the cleansing has been carried to the above stage and this division is required, the plan must be somewhat as follows:—The gathering must be shaken in a test-tube with six inches of water, and then allowed to subside until one inch at the top remains pure. About three inches are then to be carefully withdrawn by a pipette, when the tube may be filled up and the operation repeated. The three lower inches also may then be decanted and examined. The gathering is thus divided into three portions, viz.—that which was withdrawn by the pipette, that which remained floating in the lower three inches of water in the tube, and that which had settled at the bottom. An examination of these will inform the operator how to obtain that particular density of gathering which he desires, and how far it is worth while to refine this process of elutriation; for in cases of necessity any one, or all three, of these densities may be operated upon in the same way to separate a particular diatom.
As occasional aids, it may be remarked, that in some cases liquor ammoni? may be used in place of water, as it often separates fine dirt, which is not otherwise easily got rid of. Some fossil deposits require to be treated with a boiling solution of carbonate of soda to disintegrate them; but this operation requires great care, lest the alkali should destroy the diatoms. Vegetable silicates also sometimes require to37 be removed by a solution of carbonate of soda; but as the frustules of the diatoms themselves are but vegetable silica, even more care is required in this case. It may be well to mention, that some diatoms are so imperfectly siliceous that they will not bear boiling in acid at all. Some of these may be allowed to stand in cold nitric acid some time, whilst others of a smaller and more delicate character should, when possible, be treated with distilled water alone.
We will now consider the mode of mounting the prepared diatoms, which, if used dry (as described in this chapter), should be carefully washed two or three times with the purest distilled water. In this branch, as in every other, each collector gives preference to that method in which he is an adept. Thus the diatoms may be placed on the under side of the cover, to be as near to the object-glass as possible, or upon the slide itself; and each plan has its advocates. Whichsoever of these is used, nothing seems more simple to the novice than a tolerably equal dispersion of the objects upon the slide or cover; but this is by no means so readily accomplished, consequently I give Mr. Rylands’ method, as his slides are perfect in this respect also. He always places the diatoms upon the thin glass cover. It is not sufficient, as is frequently thought, to take a drop of liquid containing the cleansed material and spread it upon the cover or slide, as without some additional precaution that uniform and regular distribution of the specimens is not obtained which is desirable. In order to effect this, let a drop of the cleansed gathering be diluted sufficiently for the purpose—how much must be determined in each case by experiment—and let the covers to be mounted be cleaned and laid upon the brass plate. (See Chapter I.) By means of a glass tube, about one-twelfth of an inch in diameter, stopped by the wetted finger at the upper end, take up as much of the diluted material as will form a moderately convex drop extending over the whole cover. When all the covers required are thus prepared,38 apply a lamp below the brass plate, and raise the temperature to a point just short of boiling. By this means the covers will be dried in a few minutes, and the specimens equally distributed over the whole area. The spread of the fluid upon the covers is facilitated by breathing upon them; and, to insure uniformity, care must be taken to avoid shaking them whilst drying. The best plan is to mount at least half a dozen at once.
Before mounting, Mr. Rylands always burns the diatoms upon the glass at a dull red heat, whether they are used with balsam or dry. This burning, he says, is not only an additional cleaning process, but it effectually fixes the diatoms, and prevents them floating out if mounted with balsam. The thinnest covers may be burnt without damage if they are placed upon a small piece of platinum foil of the size required, which should be about one-hundredth of an inch thick, perfectly flat, and having three of its edges slightly bent over, so as to prevent its warping with the heat. The small flame of a spirit-lamp, or, where there is gas, a Bunsen’s burner, may be employed. The cover should be shaded from direct daylight, that the action of the flame may be observed more perfectly. Care must then be taken to raise the temperature only to the dull red heat before mentioned. The cover will then be in a fit state for mounting as required.
It has been stated in another place that it is assumed the operator is not mounting diatoms simply as microscopic objects, but as instructive specimens. It is not, therefore, sufficient to take a single slide a............