The nature and use of this substance has been before spoken of, so the method of working with it may be at once described. Perfect dryness of the objects is, if possible, more necessary in this mode of mounting than any other, as dampness remaining in the object will assuredly cause a cloudiness to make its appearance in a short time after it is fixed. Where pressure does not injure the specimens, they are most successfully treated when first dried betwixt the leaves of a book, or in any other way which may prove most convenient, as noticed in Chapter II.
Before describing the methods of proceeding with any particular objects, general rules may be given which should be observed in order to succeed in this branch of mounting.
As the object is to be thoroughly immersed in the balsam, it is evident that when it has once been covered, so it must remain, unless we again free it by a process hereafter mentioned, which is very troublesome; and on this account there must be nothing whatever in the balsam except the object. The inexperienced may think this an unnecessary caution; but the greatest difficulty he will meet with is to get rid of minute bubbles of air, perhaps invisible to the naked eye, which appear like small globules when under the microscope, and render the slide unsightly, or even worthless. Ten objects out of eleven contain air, or at least are full of minute holes which are necessarily filled with it; so that if they should be immersed in any liquid of thick consistency, these cells of air would be imprisoned, and become bubbles. The air, then, must be got rid of, and this57 is usually accomplished by soaking for some time in turpentine, the period required differing according to the nature of the object. In some cases, the turpentine acts upon the colour, or even removes it altogether, so that it must be watched carefully. Often, however, this is an advantage, as where the structure alone is wanted, the removal of the colouring matter renders it more transparent. There are objects, however, which retain the air with such tenacity that soaking alone will not remove it. If these will bear heat without being injured, they may be boiled in turpentine, or even in balsam, when the air will be partly or totally expelled. But where heat is objectionable, they must be immersed in the turpentine, and so submitted to the action of the air-pump. Even with this aid, sometimes days are required to accomplish it perfectly, during which time the air should be exhausted at intervals of five or six hours, if convenient, and the objects turned over now and then.
Sometimes the objects are so minute that it is impossible to submit them to any soaking, and in this case they must be laid upon the slide at once, and the turpentine applied to them there. But it must not be forgotten that there are some few which are much better mounted in such a way that the balsam may thoroughly surround, and yet not penetrate, the substance more than necessary. Sections of teeth are amongst these, which will be noticed in another place, and some insects (see Dr. Carpenter) when required to show the “ramifications of the trache?.”
Having freed the object, then, from these two enemies—dampness and air—we must proceed to mount it.
The slide must first be cleaned; then on the centre a quantity of balsam must be placed with a bluntly-pointed glass rod, according to the size of the object about to be mounted. To this a slight heat must be applied, which will cause any bubbles to rise from the surface of the slide, so that they may be readily removed with a needle. The object should be freed from all air by steeping in turpentine, as before described, and then from superfluous liquid by a58 short drainage, and carefully laid upon, or where it is practicable thrust into, the balsam, prepared on the slide as above. In the former case, or where the balsam has not totally covered the object, a small quantity must be taken, warmed, and dropped upon it, and any bubbles removed by the needle as before. To cover this, the thin glass must be warmed, and beginning at one side, allowed to fall upon the balsam, driving a small “wave” before it, and thus expelling any bubbles which may remain. This is quite as safely performed (if not more so) by making a solution of balsam in turpentine of the consistency of thick varnish. The thin glass cover may be slightly coated with this, and will then be much less liable to imprison any air, which frequently happens when the cover is dry. Bubbles, however, will sometimes make their appearance in spite of all care; but when the object is comparatively strong, they may be removed by keeping the slide rather warm, and working the cover a little, so as to press them to one side, when they should be immediately removed with a needle point, otherwise they are again drawn under.
Where the slide requires keeping warm for any length of time, a hot-water bath is sometimes made use of, which is simply a flat tin, or other metal case, with a mouth at the side, that when the hot water is introduced it may be closed up, and so retain its warmth for a long time. In working, the slide is laid upon it, and so admits of longer operations, when required, without growing cold. Sometimes a spirit-lamp is placed under it to keep up an equal heat through excessively long processes. Where the time required, however, is but short, a thick brass plate is sometimes used (see Chapter I.), which is heated to any degree that is required, and the slide placed upon it.
Some objects, which are so thin that they are usually floated upon the slide, as before stated, require no steeping in turpentine or other liquid. These are best mounted by covering with a little diluted balsam, and after this has had time to penetrate the substance, ordinary balsam59 is laid upon it, and the slide finished in the usual manner.
I have stated that the balsam is usually applied to the slide and objects with a “bluntly-pointed glass rod;” but for the purpose of drawing the balsam from the bottle, and conveying it to the desired place, Dr. Carpenter uses a glass syringe with a free opening. These are his instructions:—“This (the syringe) is most readily filled with balsam, in the first instance, by drawing out the piston, and pouring in balsam previously rendered more liquid by gentle warmth; and nothing else is required to enable the operator at any time to expel precisely the amount of balsam he may require, than to warm the point of the syringe, if the balsam should have hardened in it, and to apply a very gentle heat to the syringe generally, if the piston should not then be readily pressed down. When a number of balsam objects are being mounted at one time, the advantage of this plan in regard to facility and cleanliness (no superfluous balsam being deposited on the slide) will make itself sensibly felt.”
When the “mounting” is thus far accomplished, the outer “wall” of balsam may be roughly removed after a few hours have elapsed; but great care is necessary lest the cover be moved or interfered with in any way. In this state it may be left for the final cleansing until the balsam becomes hard, which takes place sooner or later, according to the degree of warmth it has been subjected to. A mantel-piece, or some place about equal to it in temperature, is the best suited to this purpose; and when the requisite hardness is attained, it may be proceeded with as follows:—With a pointed knife the balsam must be scraped away, taking care that the thin glass be not cracked by the point getting under it. If used carefully, the knife will render the slide almost clean; but any minute portions which still adhere to the glass must be rubbed with linen dipped in turpentine or spirit. If the balsam is not very hard, these small fragments are readily removed by folding a piece of paper tightly in a triangular form with many folds, and60 damping the point with which the glass is rubbed. As the paper becomes worn with the friction, the balsam will be carried off with it. In some cases I have found this simple expedient very useful.
Sometimes the object to be mounted is of such a thickness as to require a cell. For this purpose glass rings are used (as described in Chapter IV.), and filled with balsam. The best mode of doing this is thus described by Mr. T. S. Ralph in the Microscopic Journal:—“The question was asked me when I was in England, if I knew how to fill a cell with Canada balsam and leave behind no air-bubbles? I replied in the negative; but now I can state how to accomplish this. Fill the cell with clear spirit of turpentine, place the specimen in it, have ready some balsam just fluid enough to flow out of the bottle when warmed by the hand; pour this on the object at one end, and, gradually inclining the slide, allow the spirit of turpentine to flow out on the opposite side of the cell till it is full of balsam; then take up the cover, and carefully place upon it a small streak of Canada balsam from one end to the other. This, if laid on the cell with one edge first, and then gradually lowered until it lies flat, will drive all the air before it, and prevent any bubbles from being included in the cell. It can be easily put on so neatly as to require no cleaning when dry. If the cover is pressed down too rapidly, the balsam will flow over it, and require to be cleaned off when hardened, for it cannot be done safely while fluid at the edges.”
Sometimes with every care bubbles are enclosed in the balsam, injuring objects which are perhaps rare and valuable. The whole slide must then be immersed in turpentine until the cover is removed by the solution of the balsam; and the object must be cleansed by a similar steeping. It may then be remounted as if new in the manner before described.
The balsam and chloroform described in Chapter I. is thus used; and where the object is thin, the mounting is very easily accomplished. When the object is laid upon the61 slide with a piece of glass upon it, and the balsam and chloroform placed at the edge of the cover, the mixture will gradually flow into the space betwixt the glasses until the object is surrounded by it, and the unoccupied portion filled. The chloroform will evaporate so quickly that the outer edge will become hard in a very short time, when it may be cleaned in the ordinary way. Sometimes the balsam is dissolved in the chloroform without being first hardened; but this is only to render it more fluid, and so give the operator less chance of leaving bubbles in the finished slide, as the thicker the medium is, the more difficult is it to get rid of these intruders.
It has been before mentioned that some have objected to chloroform and balsam, believing that it became clouded after a certain time. Perhaps this may be accounted for in part by the fact that almost all objects have a certain amount of dampness in them. Others are kept in some preservative liquid until the time of mounting, and these liquids generally contain certain salts (Chapter IV.). If this dampness, as well as all traces of these salts, however small, are not totally removed—the former by drying, the latter by repeated washings—the addition of chloroform will render the balsam much more liable to the cloudiness than when balsam alone was used, as before mentioned.
This mode of employing the balsam, however, will not be always applicable, as chloroform acts upon some substances which balsam alone does not. Some salts are even soluble in it, the crystals disappearing after a few days or weeks, whereas in the balsam alone they are quite permanent. Experience is the only guide in some cases, whilst in others a little forethought will be all that is required.
The particular methods used for certain objects may be now entered upon. Many of the Diatomace? and fossil Infusoria, as they are sometimes termed, are mounted dry, and cleaned in the way described in Chapter II. Others are almost always placed in balsam, except where they are intended to be used with the lieberkuhn and dark background, by which62 means some of them are rendered exquisitely beautiful. The usual way of mounting them in balsam is as follows:—Take a drop of the water containing them, place it upon the slide, and evaporate over the lamp, whilst with a needle they may be dispersed over any space desired. When they are thoroughly dry, drop a little balsam on one side, and exclude the bubbles. The slide may then be warmed to such a degree that the balsam, by lifting the glass at one end, will be carried over the specimens, which may then be covered with thin glass, made warm as before described. Where the objects are quite dry, and loose upon the glass, it requires great care in placing the cover upon them, otherwise they are forced to one edge, or altogether from under it, in the wave of the balsam. For this reason, Professor Williamson adds a few drops of gum-water to the last washing, which causes them to adhere sufficiently to the glass to prevent any such mishap.
Mr. T. G. Rylands’ method differs in some degree from the above, and is, to use his own words, as follows:—Thick balsam is preferable, and the burnt covers (see Chapter II.) to be mounted are laid in a convenient position with the diatoms upwards. The slides required having been carefully cleaned and marked on the under side with a ring of ink, by the aid of a turntable about half an inch in diameter to point out the centre, a drop of benzole is applied by a large pin to the diatoms on the cover, so as to exclude the air from the valves and frustules. The slide is then held over the lamp, and when warm, a sufficiently large drop of balsam is put upon it, and heated until it begins to steam. If small bubbles appear, a puff of breath removes them. The slide being held slightly inclined from the operator, and the drop of balsam becoming convex at its lower edge, the cover is brought in contact with it at that point, gradually laid down, pressed with the forceps, and brought to its central position. When cool the superfluous balsam (if any) is removed with a heated knife-blade, the slide cleaned with a little turpentine, and finished by washing in a hand-basin63 with soap and water. In this process there is no delay if the balsam be sufficiently thick, as the slide may be cleaned off almost before it is cold.
It is now well known that from common chalk it is an easy matter to obtain interesting specimens of Foraminifera. Scrape a small quantity of chalk from the mass and shake it in water; leave this a few minutes, pour the water away and add a fresh quantity, shake up as before, and repeat two or three times. Take a little of the residue, and spread it upon the slide, and when quite dry add a little turpentine. When viewed with a power of two hundred and fifty diameters this will generally show the organisms very well. If it is desired to preserve the slides, they may be then mounted in Canada balsam. Mr. Guyon, in “Recreative Science,” observes that the accumulation of the powder, by the action of the rain or exposure to the atmospheric action, at the foot or any projection of the chalk cliffs, will afford us better specimens than that which is “scraped,” as the organisms are less broken in the former.
When the Foraminifera are of a larger size, though transparent enough to be mounted in balsam, the air must be first expelled from the interior, otherwise the objects will be altogether unsatisfactory. To accomplish this they must be immersed in turpentine and submitted to the action of the air-pump. So difficult is it to get rid of this enemy that it is often necessary to employ three or four exhaustions, leaving them for some time under each. When all air has given place to the turpentine, they must be mounted in the ordinary way.
Of all objects which are commonly met with, few are such general favourites as the Polycystin?, and deservedly so. Their forms are most beautiful, and often peculiar—stars varying in design, others closely resembling crowns; the Astromma Aristotelis like a cross, and many whose shapes no words could describe. The greater part, perhaps, of those which are usually sold, is from the rocky parts of Bermuda; but they are also found in Sicily, some parts of64 Africa and America. They are usually mounted in balsam, but are equally beautiful mounted “dry” and used with the lieberkuhn. They require as much care in cleaning as the Diatomace?, but the process is a different one. Sometimes this is effected by simply washing until they are freed from all extraneous matter, but this is seldom as effectual as it should be. In the Microscopic Journal Mr. Furlong gives the following method of treatment as the best he knew:—
Procure—
A large glass vessel with 3 or 4 quarts of water.
New tin saucepan holding 1 pint.
2 thin precipitating glasses holding 10 oz. each.
Take 3 oz. of dry “Barbadoes earth” (lumps are best), and break into rather small fragments. Put 3 or 4 oz. of common washing soda into the tin and half fill it with water. Boil strongly, and having thrown in the earth, boil it for half an hour. Pour nine-tenths of this into the large glass vessel, and gently crush the remaining lumps with a soft bristle brush. Add soda and water as before, and boil again; then pour off the liquid into the large vessel, and repeat until nothing of value remains. Stir the large vessel with an ivory spatula, let it stand for three minutes, and pour gently off nine-tenths of the contents, when the shells will be left, partially freed only, like sand.
2nd Process.—Put common washing soda and water into the tin as before, and having placed the shells therein, boil for an hour. Transfer to the large vessel as before, and after allowing it to stand for one minute pour off. Each washing brings off a kind of “flock,” which seems to be skins.
3rd Process.—Put the shells in precipitating glass and drain off the water until not more than ? oz. remains. Add half a teaspoonful of bicarbonate of soda, dissolve, and then pour in gently 1 oz. of strong sulphuric acid. This liberates the “flock,” &c., and leaves the shells beautifully transparent. Wash well now with water to get rid of all salts and other soluble matter.
65 Some of the large shells are destroyed by this method, but none that are fit for microscopic use. An oblique light shows these objects best.
These are sometimes treated in the manner described in Chapter II. where the diatoms are spoken of, but many forms are liable to be injured by this severe process.
It has been before stated that some of the zoophytes may be mounted dry, and others examined as opaque or transparent objects according to their substance. They are very interesting when examined in the trough whilst living, but to preserve many of them for future examination they must be mounted in some preservative medium. Sometimes this may be one of the liquids mentioned in Chapter IV., but if possible they should be kept in balsam, as there is less danger of injury by accident to this kind of slide. This method of mounting presents some difficulties, but I think that all agree as to the trustworthiness of Dr. Golding Bird’s information on the subject, which appeared in the Microscopic Journal. Of this, space forbids me to give more than a condensed account, but I hope to omit nothing of moment to the reader for whom these pages are written.
After stating that there are few who are not familiar with these exquisite forms, and have not regretted the great loss of beauty they sustain in dying, he informs us that from their so obstinately retaining air in the cells and tubes when dried, it is hardly practicable to get rid of it; and they also shrivel up very seriously in the process of drying. The following plan, however, he has found almost faultless in their preparation.
To preserve them with extended tentacles, they should be plunged in cold fresh water, which kills them so quickly that these are not often retracted. The specimens should be preserved in spirit until there is leisure to prepare them; if, however, they have been dried, they should be soaked in cold water for a day or two before being submitted to the following processes:—
1. After selecting perfect specimens of suitable size, immerse66 them in water heated to about 120° and place them under the receiver of an air-pump. Slowly exhaust the air, when bubbles will rise and the water appear to be in a state of active ebullition. After a few minutes re-admit the air and again exhaust, repeating the process three or four times. This will displace the air from most, if not all, of the class.
2. Remove the specimens and allow them to drain upon blotting-paper for a few seconds; then place them in an earthen vessel fitted with a cover, and previously heated to about 200°. This heat may be easily got by placing the vessel for a short time in boiling water, wiping it immediately before use with a thick cloth. The specimens are then dropped into this, covered with the lid, and immediately placed under the receiver of the air-pump, and the air rapidly exhausted. By this means they are dried completely, and so quickly that the cells have no time to wrinkle.
3. In an hour or two remove them from the air-pump and drop them into a vessel of perfectly transparent camphine. This may be quite cold when the horny, tubular polypidoms, as those of the Sertulari?, are used; but should be previously heated to 100° when the calcareous, cellular Polyzoa are the objects to be preserved. The vessel should be covered with a watch-glass and placed under the receiver, the air being exhausted and re-admitted two or three times.
4. The slide which is to receive the specimen should be well cleaned and warmed so as to allow the balsam to flow freely over it. This must be applied in good quantity, and air-bubbles removed with the needle-point. Take the polypidom from the camphine, drain it a little, and with the forceps immerse it fully in the balsam. The glass to be laid upon it should be warmed and its surface covered with a thin layer of balsam, and then lowered gradually upon it, when no bubbles should be imprisoned. A narrow piece of card-board at each end of the object for the cover to rest upon, prevents any danger of crushing the specimen.
67 This mode of mounting polypidoms, &c., seems to give almost the complete beauty of the fresh specimens. They are very beautiful objects when viewed with common light, but much more so when the polarizer is used (in the manner described a little farther on).
To the above instructions there can be little to add; but I may here mention that some young students may not be possessed of the air-pump, and on this account put aside all search for those specimens which need little looking for at the seaside. Many of these, however, though they lose some beauty by the ordinary mode of drying, will by steeping for some time in turpentine not only be freed from the air-bubbles, but suffer so little contraction that they are a worthy addition to the cabinet.
Another class of objects is the spicula met with in sponges, &c. These are often glass-like in appearance and of various shapes; many are found resembling needles (whence their name); some from the synapta are anchor-like, whilst others are star-like and of complex and almost indescribable combinations. As some of these are composed of silex and are consequently not injured by the use of nitric acid, the animal substance may be got rid of by boiling them in it. Those, however, which are calcareous must be treated with a strong solution of potash instead; but whichever way is used, of course they must afterwards be freed from every trace of residue by careful washing.
These spicules may be often found amongst the sand which generally accumulates at the bottom of the jars in which sponges are kept by those who deal in them, and must be picked out with a camel-hair pencil. The specimens obtained by this means will seldom if ever require any cleaning process, as they are quite free from animal matter, &c.
In the former chapter was noticed those insects or parts of them which are usually mounted dry. When they are large and too opaque to admit of the dry treatment, they68 must be preserved in Canada balsam or fluid. The first of these may now be considered.
It may be here mentioned, that with these objects much heat must not be employed, as it would in some instances give rise to a cloudiness, and almost invariably injure them.
In killing the insect it is necessary not to rub or break any part of it. This may be performed by placing it in a small box half filled with fragments of fresh laurel leaves, by immersion in turpentine or strong spirit, as also in solutions of various poisonous salts. After which it may be preserved for sometime in turpentine or other preservative liquid (Chapter IV.) until required. As an assistance to the student, I believe that I can do no better than give him the plan pursued by my friend Mr. Hepworth, whose specimens are in every way satisfactory; but when his method is used, the insects must not have been placed in turpentine for preservation:—
“After destroying the insects in chloroform or sulphuric ether (methylated being cheaper), wash them thoroughly in a wide-necked bottle, half-filled, with two or three waters; the delicate ones requiring great care. Then immerse them in liquid potash (or Brandish’s solution, which is stronger than the usual preparation), and let them remain a longer or shorter time according to their texture. When ready to remove, put one by one into a small saucer of clear water, and with a camel-hair pencil in each hand press them flat to the bottom, holding the head and thorax with the left-hand brush, and apply pressure with the other from above, downwards, giving the brush a rolling motion, which generally expels the contents of the abdomen from the thorax. A minute roller of pith or cork might be used instead of the brush. In larger objects, use the end of the finger to flatten them. Large objects require more frequent washing, as it is desirable to remove the potash thoroughly, or crystals are apt to form after mounting. Having placed them on the slides with thin glass covers, tied down with69 thread,C dry and immerse them in rectified spirits of turpentine; place the vessel under the receiver of an air-pump, and keep it exhausted until the turpentine has taken the place of the air-bubbles: they are then ready for the application of the balsam. Larger objects may often with advantage be transferred to a clean slide, as during the drying there is considerable contracti............