The preparation of diatom-skeletons as permanent objects is easy. Consisting, as they do, of pure silex, or flint,—i.e., practically glass,—they resist long boiling in acids, so that there is little difficulty in isolating them from any organic matter with which they are mingled. It is generally recommended to treat them with strong nitric acid. This is a mistake. The acid acts much more powerfully and less violently when diluted with an equal bulk of water, and it is in an acid so diluted that portions of water-plants, or other diatomaceous material, should be boiled in a glass beaker until all the organic matter is dissolved. The beaker should be covered with a glass plate, to prevent dissipation of the acid fumes. When the process is complete, usually in about half an hour, the contents of the beaker are thoroughly stirred with a glass rod, poured rapidly off into a larger bulk of cold water, and allowed to settle for another half-hour. The process is then repeated with a smaller bulk of water, several times, to allow the removal of the last traces of acid, and finally with distilled water. The separation of the diatoms into grades is effected by settlement. The final result is poured into a tall glass vessel, and allowed to settle for, at first, a minute, the supernatant fluid again poured off, and allowed to settle for two minutes, and so on, the period being gradually increased, and each sediment preserved apart. The first will probably only be sand, but the proportion of diatoms will increase with each separation, though there will always be a certain proportion of sand of such a size as to settle at the same rate as the diatoms. Marine plants especially will furnish a rich harvest by treatment as described.
Solid diatomaceous deposits, such as kiesel-guhr, mountain-meal, and especially the famous Oamaru deposit from New Zealand, demand different treatment, and perhaps the best way is to disintegrate the mass, either by boiling with Sunlight soap (though the alkali attacks the flint to some extent) or to mix the mass with a super-saturated solution of acetate of soda (made by saturating water with the crystals whilst boiling), and by successive coolings, heatings, and stirrings to cause the process of crystallisation to break up the mass, which it will do very thoroughly. The diatoms are then separated by sedimentation, as above described.
A small portion of the deposit may now be spread thinly on a glass slide, allowed to dry thoroughly, be treated with balsam, and covered.
If it be desired to select individual diatoms, this must be done under the microscope, by means of a bristle fixed in a handle either with glue or sealing-wax. The diatom selected will adhere to the bristle if the latter be slightly greasy, and should then be transferred to a slightly adhesive slide, coated either with thin solution of white shellac, or with thin gum nearly dry. When the forms desired are mounted, the preparation should be covered in balsam. The process is by no means as easily effected as described, however.
The preparation of insects, or parts of insects, as microscopic objects is a tedious and difficult task. The main point is the trouble of softening the integument and eliminating the colour.
The latter can, in any case, be only partially effected. The beginner would do well to begin with a fairly easy form, such as the worker-ant. A good supply of these insects may be placed in a bottle of liquor potassæ, and left there for at least some days until they begin to become clear and limp. From time to time a specimen may be taken, well washed with several waters, then with acetic acid and water of a strength of about 10 per cent., then with weak spirit, about 50 per cent. An attempt may then be made to arrange the insect upon a slide, spreading out the legs so as to exhibit them to the best advantage, and when this has been done a cover-glass may be put on, supported in such a way as to prevent absolute pressure. The spirit is then withdrawn by means of a piece of filtering-paper cut to a point, and strong spirit added. This is again succeeded by absolute alcohol, then by a mixture of turpentine and crystal carbolic acid in equal proportions, and finally the cover-glass is carefully lifted, and some thick balsam solution dropped on, the limbs finally arranged by means of warm needles, and the cover-glass carefully replaced and pressed gently down by means of a clip, which may be obtained for a few pence. The whole is then set aside to harden, the deficiency caused by evaporation made good, the balsam allowed to dry, and the preparation finally painted round.
The contents of the body, in large insects, must be removed, and this is effected during the washing in water by gentle pressure with a camel-hair brush, the process being aided, if necessary, by a small incision made through the integument at the root of the tail. Sections of insects require very special methods, which will hardly fall within the scope of this work.