Chapter II. Others are almost always placed in balsam, except where

they are intended to be used with the lieberkuhn and dark background, by which 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-basin 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 POLYCYSTINÆ, 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 of 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 Diatomaceæ, 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 ½ 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.

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, immerse 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 Sertulariæ, 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.

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, they 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 with 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 contraction, and an outline often remains beyond the margin showing this. When closely corked they may remain in the spirits two or three months. As you take them from the bottle, wipe as much turpentine off as possible before removing the thread, and when untied carefully wipe again, placing the finger on one end of the cover whilst you wipe the other, and vice versâ. By this means you remove as much turpentine from under the cover as is necessary; then drop the balsam, thinned with chloroform (see Chapter I.), upon the slide, letting the fluid touch the cover, when it will be taken in between the surfaces by capillary attraction; and after pressing the cover down it may be left to dry, or you may hold the slide over a spirit-lamp for a few seconds before pressing down the cover. If heat is not applied, they are much longer in drying but are more transparent. If made too hot the boiling disarranges the objects, and if carried too far will leave only the resin of the balsam, rendering it so brittle that the cover is apt to fly off by a fall or any jar producing sufficient percussion. Never lift the cover up, if possible, during the operation, as there is danger of admitting air. A few bubbles may appear immediately after mounting, but generally subside after a few hours, being only the chloroform or turpentine in a state of vapour, which becomes condensed.”

[C] This applies to the more delicate ones, which will not bear transferring after being once spread out and dried.

This method of preparing and mounting insects I can strongly recommend as giving first-rate results; but where the specimens are small they seldom need the soaking in caustic potash which larger ones must have. It is only necessary to leave them awhile in turpentine, especially when they have been first dried with gentle pressure between two glasses, and then mount with balsam in the ordinary way.

Amongst the insect tribes there is abundant employment, especially for the lower powers of the microscope. But if the deeper wonders and beauties of the animal economy are to be sought out and studied, it is desirable that the various parts should be set separately, in order that they may receive a more undivided attention, as well as to render them capable of being dealt with under the higher powers. We will, therefore, briefly consider the treatment which the different portions require.

The eyes of the butterflies, and indeed of almost all insects, afford materials for a study which is complete in itself. When examined with a tolerably high power, instead of finding each eye with an unbroken spherical surface, it is seen that many are composed of thousands of hexagonal divisions, each being the outer surface of a separate portion termed the _ocellus_. In others these divisions are square; but in all there is a layer of dark pigment surrounding their lower parts. The ocelli may be partly removed from the eye, which will show how their tapering forms are arranged. But here we have to consider how to place them in balsam for preservation. The eye being removed from the insect, and the dark pigment removed by the use of a camel-hair pencil, must be allowed to remain in turpentine at least for some days. The turpentine should then be renewed and the eye well washed in it just before it is to be mounted. It may then be set in balsam in the same way as any other object;--but here a difficulty is met with. The eye being spherical upon the surface required, must necessarily be “folded” or broken in attempting to flatten it. This difficulty may be often overcome by cutting a number of slits round the edges; but some object to this mode of treatment, and where it is practicable it is much more satisfactory to mount one in the natural rounded form and another flat. Instead, however, of mounting the organ _whole_, four or five slides may be procured from each of the larger ones, as those of the dragon-fly, &c.

The _antennæ_ also are often mounted on separate slides, as being better suited for higher powers and more minute examination than when connected with the insect. These two projecting organs, issuing from the head, are jointed, and moveable at will. They differ very much in form amongst the various species, and are well worth the attention of the microscopist. They are usually mounted with the head attached, and perhaps they are more interesting when thus seen. Some few are very opaque, to prepare which the following method has been advised:--

Bleach the antennæ by soaking in the following solution for a day or two:--

Hydrochloric acid, 10 drops. Chlorate of potash, ½ drachm. Water, 1 oz.

This will render them transparent. Wash well, dry, and mount in Canada balsam. Instead of the above, a weak solution of chloride of lime may be used, by which means the nerves will be well shown. Many, however, are rendered transparent enough by simply soaking in turpentine for a longer or shorter time. Where the antennæ, however, are “plumose,” or feather-like, extreme care is required in mounting, though the difficulty is not so great as some seem to think. If they are first dried with gentle pressure, and then subjected to the action of the air-pump in a small quantity of turpentine until the air is thoroughly expelled, they can be easily finished upon the slide, especially when balsam and chloroform are used.

Insects supply us with another series of beautiful objects, viz., the _feet_.[D] These are sometimes simply dried and mounted without any medium, as before mentioned; but most of them are rendered much more fit for examination by using balsam in their preservation, as it greatly increases their transparency. The smaller kinds may be dried with gentle pressure betwixt blotting-paper, and then immersed for some days in turpentine, without requiring the treatment with liquor potassæ. This immersion will render them beautifully transparent, when they may be mounted in balsam in the usual manner.

[D] See Mr. Hepworth’s interesting articles on the fly’s foot in the second and third volumes of the _Microscopic Journal_.

It is, however, sometimes found difficult to fix the feet when _expanded_, in which state the interest of the object is greatly increased. Mr. Ralph recommends the following mode:--“First wash the feet, while the insect is yet alive, with spirits of wine; then holding it by a pair of forceps close to the edge of a clean piece of glass, the insect will lay hold of the upper surface by its foot, then suddenly drop another small piece of glass over it, so as to retain the foot expanded, and cut it off with a pair of scissors, tie up and soak to get rid of air.” Mr. Hepworth says that he never found any difficulty in expanding the foot on a drop of water or well-wetted slide, and laying a thin glass cover over it, tying with thread, drying, and immersing in turpentine.

The mouth, also, with its organs, is an interesting object in many insects. That of the common fly is often made use of, and is comparatively easy to prepare. By pressing the head, the tongue (as it is commonly termed) will be forced to protrude, when it must be secured by the same means as the foot, and may be subjected to the soaking in turpentine, and mounted as usual. The honey-bee is, however, very different in formation, and is well worth another slide; indeed, even in insects of the same class, the differences are many and interesting.

Another worthy object of study is the _respiration_ of insects, which is effected by tracheæ or hollow tubes, which generally run through the body in one or more large trunks, branching out on every side. These terminate at the surface in openings, which are termed _spiracles_, or breathing organs. The _tracheæ_ often present the appearance of tubes constructed by a twisted thread, somewhat resembling the spiral fibres of some plants. These are very beautiful objects, and are generally mounted in balsam, for which reason they are mentioned here; but as they evidently belong to the “dissecting portion,” they will be fully treated of in another place.

Amongst the parasitic insects a great variety of microscopic subjects will be found. As these are usually small, they may be killed by immersion in spirits of turpentine; and if at all opaque, may be allowed to remain in the liquid until transparent enough, and then mounted in Canada balsam.

The acarida, or _mites_ and _ticks_, are well known; none, perhaps, better than those which are so often found upon cheese. Flour, sugar, figs, and other eatables, are much infested by them; whilst the diseases called the _itch_ in man, and the _mange_ in animals, are produced by creatures belonging to this tribe. These animals are sometimes mounted by simply steeping them in turpentine, and proceeding as with other insects. The “Micrographic Dictionary” gives the following directions as to mounting _parts_ of these:--“The parts of the mouth and the legs, upon which the characters are usually founded, may be best made out by crushing the animals upon a slide with a thin glass cover, and washing away the exuding substance with water; sometimes hot solution of potash is requisite, with the subsequent addition of acetic acid, and further washing. When afterwards dried and immersed in Canada balsam, the various parts become beautifully distinct, and may be permanently preserved.”

Feathers of different kinds of birds are usually mounted in balsam when required to show much of the structure. This is particularly interesting when the feathers are small, as they then show the inner substance, or _pith_, as it may be termed, with the cells, &c. The “pinnæ,” or soft branches of the feathers, will be found of various constructions; some possessing hooks along one side, whereby they fasten themselves to their neighbours; others branching out, with straight points somewhat resembling the hairs from certain caterpillars. But, of course, when the metallic-looking gorgeous colours are all that is required to be shown, and reflected light used (as with the feathers of the hummingbird, peacock, &c.), it is much better that they should be mounted dry, as in Chapter II.

The _seeds_ and _pollen_ of plants are most frequently mounted dry, as mentioned in Chapter II.; but the more transparent of the former, and the darker kinds of the latter, are perhaps better seen in Canada balsam. There is nothing particular to be observed in the manipulation, except that the glass cover must be applied lightly, otherwise the grains may be crushed. There are some objects which cannot be shown in a perfect manner when mounted _dry_, but when immersed in balsam become so very transparent that they are almost useless. To avoid this, it has been recommended to stain the objects any colour that may be convenient, and afterwards mount in balsam in the ordinary manner.

Most objects intended for the polariscope may be mounted in Canada balsam; but there are some exceptions to this. Many of the salts are soluble in this medium, or their forms so injured by it, that glycerine or oil has to be used (see Chapter IV.); others must be left in the dry form, as before mentioned; and some few it is impossible to preserve unchanged for any length of time. _Crystals_, however, are amongst the most beautiful and interesting subjects for polarisation; and it is very probable that, by the aid of the polariscope, new and valuable facts are yet to be made known. For one who finds pleasure in form and colour, there is a field here which will only open wider upon him as he advances; and instead of being in anywise a merely mechanical occupation, it requires deep and careful study. The little here said on the subject will show this in some degree.

With almost every salt the method of _crystallization_ must be modified to obtain the best forms; I may even go further than this, and say that it is possible to change these forms to such a degree that the eye can perceive no relationship to exist betwixt them. If a solution of sulphate of iron is made, a small quantity spread evenly upon a slide, and then suffered to dry whilst in a flat position, the crystals often resemble the fronds of the common fern in shape. But if, whilst the liquid is evaporating, it is kept in motion by stirring with a thin glass rod, the crystals form separately, each rhombic prism having its angles well defined, and giving beautiful colours with the polarized light. Again, pyro-gallic acid, when allowed to flow evenly over the slide in a saturated solution, covers the surface in long “needles,” which are richly coloured by polarized light; but if any small portion of dust or other matter should form a nucleus around which these “needles” may gather, the beauty is wonderfully increased. A form very closely resembling the “eye” of the peacock’s tail, both in form and colour, is then produced, which to one uninitiated in crystallography bears very little resemblance to the original crystal. From these simple facts it will be clearly seen that in this, as in every other department, study and experience are needful to give the best results.

To obtain anything like uniformity in the formation of crystals upon the glass slide, every trace of grease must be removed by cleaning with liquor potassæ or ammonia immediately before using, care also being taken that none of the agent is left upon the slide, otherwise it may interrupt and change their relative position, and even their form.

Amongst those which are generally esteemed, the most beautiful are the crystals of oxalurate of ammonia. The preparation of this salt from uric acid and ammonia is a rather difficult process, and will not on that account be described here; but when possessed, a small quantity of a strong solution in water must be made, and a little placed on the slide, and evaporated slowly. Part of the salt will then be deposited in circles with the needle-like crystals extending from common centres. They should then be mounted in pure Canada balsam; and, when the best colours are wanted, used with the selenite plate. Of this class of crystal salicine is a universal favourite, and can be easily procured of most chemists. The crystals may be produced in two ways:--A small portion of the salt must be placed upon the slide, and a strong heat applied underneath until fusion ensues; the matter should then be evenly and thinly spread over the surface. In a short time the crystals will form, and are generally larger than those procured by the following process; but the uncertainty is increased a little when fusion is used, which, however, is desirable with many salts. Secondly, make a saturated solution of salicine, which in cold water is effected by adding one part of the salt to eighteen parts of water. Lay a little upon the slide, and allow it to evaporate spontaneously, or with the aid of gentle heat. The crystals are generally uniform, and with ordinary powers quite large enough to afford a beautiful object. The circular shape and gorgeous colours of this crystal have made it so great a favourite that there are few cabinets without it.

Many new forms may be procured by uniting two totally different salts in solution in certain proportions. This is a field affording new facts and beauties; but requires some chemical knowledge and much perseverance to obtain very valuable results. One of the most beautiful I have met with has been composed of sulphate of copper and sulphate of magnesia. The flower-like forms and uniformity of crystallization when successful make it well worth a few failures at first; and as I became acquainted with some new facts in my frequent trials, I will give the preparation of the double salt from the beginning.

Make a saturated solution of the two sulphates, combined in the proportion of three parts copper to one part magnesia, and then add to the solution one-tenth of pure water. Dust or other impurities should be guarded against, and the slide made free from all trace of grease by cleaning immediately before use with liquor potassæ or ammonia. A drop of the solution should then be placed upon the slide, and by a thin glass rod spread evenly upon the surface. Heat this whilst in a horizontal position until the salt remains as a viscous transparent substance, which will not be effected until it is raised to a high degree. The slide may now be allowed to cool, and when this is accomplished, the flower-like crystals will be perceived forming here and there upon the plate. When these are at any stage in which it is wished to preserve them, a few seconds’ exposure to the fire, as warm as the hand can comfortably bear, will stop the expansion, when the portion which we wish to mount should be cut off from the mass of salt by simply scratching the film around, and pure Canada balsam with the thin glass used. Breathing upon the film, or allowing the slide to become cold and attract the moisture from the atmosphere, will cause the crystallization to extend, and sometimes greatly rob the effect; so it is necessary to mount quickly when the desired forms are obtained. As the crystals are very uncertain as to the place of their formation, I may here mention that they may be got in _any_ part of the slide by piercing the film with a needle-point; but in some degree this necessarily interferes with the centre. As the cause of this has no need to be entered into here, and has been elsewhere discussed, I shall only give the above directions, and say that there is a great field in this branch of study which the microscope alone has opened.

It would be useless to enter into particulars respecting the various salts and treatment they require, as a great difference is effected even by the strength of the solution. There are some crystals, also, which are called forth in insulated portions, showing no formation upon the ground; but even when mounted in any preserving fluid, and unchanged for a year, a new action seems to arise, and a groundwork is produced which bears little resemblance to the original crystal. Sometimes this new formation adds to the beauty of the slide; in other cases the reverse is the result, the slide being rendered almost worthless. This action, I believe, frequently arises from some liquid being contained in the balsam or other mounting medium used; and this is rendered the more probable by the crystallization being called forth in an hour after the balsam diluted with chloroform is employed, whereas no change would have taken place for months (if at all) had pure balsam been used.

Sections of some of the salts are very interesting objects, but the method of procuring these and their nature will be described in Chapter V.

The scales of various fish have been before mentioned as mounted “dry;” when, however, they are required for polarising objects they are generally mounted in balsam, and some few in liquid. The former method will be considered here.

The eel affords a beautiful object for this purpose. The scales are covered by a thin “_skin_,” which may be slightly raised with a knife and then torn off, in the same manner as the covering of the geranium and other petals, described in Chapter II. The required portion may then be removed; or if a piece of skin can be procured as stripped off in cooking, the scales may be easily taken from the inner surface. They must then be washed and thoroughly cleaned. After drying, soak for a day in turpentine, and mount in the ordinary manner with balsam. This is a good polarising object; but the interest, and I think the beauty, is increased by procuring a piece of eel’s skin with the scales _in sitû_, washing and drying under pressure, and mounting in balsam as before. The arrangement of the scales produces beautiful “waves” of colour, which are quite soothing to the eye after examining some of the very gorgeous salts, &c.

There are many scales of fish which are good subjects for the polariscope when mounted in balsam; but as they require no particular treatment, they need no mention by name.

Among hairs we find some which are beautiful when mounted in balsam and examined by polarized light. Some, when wanted as common objects, are always used dry, as before mentioned; but if they are intended to be shown as _polarizing_ objects, they must be placed in some medium. The “Micrographic Dictionary” mentions a mode of making an interesting object by plaiting two series of white horse-hairs at an angle, mounting in balsam, and using with the polariscope. All hairs, however, must be steeped in turpentine for a short time before mounting, as they will thus be rendered cleaner and more transparent. When this is done, there is no difficulty in mounting them.

Many of the “tongues” of fresh-water and marine mollusca are deeply interesting and most beautiful objects when examined by polarized light. As these are usually mounted in balsam, I mention them in this place; but as they must be removed from the animals by dissection, particulars respecting them will not be entered into until we come to the part in which that operation is described (Chapter V.).

The manner of preparing and mounting many of the Polyzoa and Zoophytes has been before described; but any notice of _polarizing_ objects would be incomplete without some allusion to them. A small piece of the _Flustra avicularis_, well prepared, is beautiful when examined in this manner. No selenite is needed, and yet the colours are truly gorgeous. It is often met with upon shells and zoophytes of a large size, and will well repay the trouble of searching for. Many of the Sertularidæ are very beautiful with polarized light, and, indeed, no ramble upon the seaside need be fruitless in this direction.

The different _starches_ are quite a study in themselves, and are peculiarly connected with polarized light. They are found in the cellular tissue of almost every plant in small white grains which vary considerably in size; that from the potato averages one-three-hundredth of an inch in diameter, and that from arrow-root about one-six-hundredth. To procure starch from any plant, the texture must first be broken up or ground coarsely; the mass of matter must be then well washed in gently-flowing water, and, as all starch is totally insoluble in cold water, the grains are carried off by the current and deposited where this is stayed. In procuring it from the potato, as well as many other vegetables, it is but necessary to reduce the substance to a coarse pulp by the aid of a culinary “grater;” the pulp should then be well agitated in water, and allowed to rest a short time, when the starch will be found at the bottom, its lighter colour rendering it easily distinguishable from the pulp. It should, however, be washed through two or three waters to render it perfectly clean.

These grains have no crystalline structure, but present a very peculiar appearance when examined with polarized light. Each grain shows a dark cross whose lines meet at the point where it was attached to the plant, called the _hilum_. Round the grain, also, a series of lines are seen, as though it were put together in plates. This is more distinctly visible in some kinds than others.

As to the mounting of these starches there is little to be said. If the grains are laid upon the slide, and as small a portion as possible of the balsam diluted with turpentine, as before mentioned, be applied, they will cling to the glass and allow the pure balsam to flow readily over them without being so liable to imprison air-bubbles when the thin glass is put upon them.

The raphides, which were fully described in Chapter II., when required for use with polarized light, must be mounted in balsam, and many are found which give beautiful colours. They require no peculiar treatment, but must be washed quite clean before putting up.

There is one class of objects for the polariscope which differs in preparation from any we have yet considered, and affords very beautiful specimens. Some of the plants, including many of the grasses and the Equisetaceæ (_i. e._ horsetails), contain so large a quantity of silica, that when the vegetable and other perishable parts are removed, a skeleton of wonderful perfection remains. This skeleton must be mounted in balsam, the method of performing which will now be considered.

Sometimes the cuticle of the equisetum is removed from the plant, others dry the stem under pressure, whilst the grasses, of course, require no preparation. The vegetable should be immersed in strong nitric acid and boiled for a short time; an effervescence will go on as the alkalies are being removed, and when this has ceased more acid should be added. At this point the modes of treatment differ; some remove the object from the acid and wash, and having dried, burn it upon thin glass until all appears _white_, when it must be carefully mounted in balsam. I think, however, it is better to leave it in strong acid until all the substance, except the required portion, is removed; but this will take a length of time, varying according to the mass, &c., of the plant. Of course, when this latter method is used, the skeleton must be washed from the acid, &c., before being mounted in balsam.

These _siliceous cuticles_ are readily found. The _straws_ of most of the cereals, wheat, oat, &c.; the _husks_, also, of some of these; many _canes_; the equisetum, as before described; and some of the grasses. Many of these are everywhere procurable, so that the student can never want material for a splendid object for the polariscope.

In Chapter II. the _scales_ (or hairs) which are often found upon the leaves of plants were mentioned as beautiful objects when mounted dry; but some of these when detached from the leaf--which is easily done by gently scraping it, when dried, with a knife--present brilliant starlike and other forms, if mounted in balsam and used with the polariscope. There is a little danger, when placing the thin glass upon the balsam, of forcing out the scales in the wave of matter which is always ejected; this may be overcome by applying to the slide, previously to placing the objects upon it, an extremely thin covering of the balsam diluted with turpentine as before mentioned, and thus giving them every chance of adherence; or by using the balsam with chloroform, as before noticed. These scales are much more abundant than was formerly supposed, and daily new specimens are discovered; so that the student should always be on the lookout for them in his researches in the vegetable world.

Most classes of objects, and the treatment they require when mounting them in balsam, have now been considered. The next chapter will be devoted to the preservative liquids, and the best methods of using them.