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.