The glycerine jelly of Mr. Lawrance, before mentioned, requires almost a similar treatment. “The objects to be mounted in this medium should be immersed for some time in a mixture of equal parts of glycerine and dilute alcohol (six of water to one of alcohol). The bottle of glycerine jelly must be placed in a cup of hot water until liquefied, when it must be used like Canada balsam, except that it requires less heat. A ring of asphaltum varnish round the thin glass cover completes the mounting.”
The Infusoria (see [Chapter III.]) are sometimes preserved in liquid; but present many difficulties to the student. Different kinds require different treatment, and consequently it is well, when practicable, to mount similar objects in two or more liquids. Some are best preserved in a strong solution of chloride of calcium, others in Thwaites’ liquid, whilst a few keep their colour most perfectly when in glycerine alone. Many of them, however, are so very transparent that they present but faint objects for ordinary observance. The Desmidiaceæ require somewhat similar treatment, and may be mentioned here. The solution of chloride of calcium has been strongly recommended; but no preservative liquid seems to be without some action upon them. Both of the above classes of objects should be mounted in shallow cells, so as to allow as high a microscopic power as possible to be used with them.
Many of the Zoophytes which are obtained on our sea-coasts are well preserved by mounting in cells, in the manner before mentioned, with Goadby’s fluid, or distilled water with one of the additions noticed amongst the preservative liquids. For examination by polarized light, however, they are usually mounted in balsam (see [Chapter III.]), whilst those in cells present a more natural appearance as to position, &c., for common study.
As to the use of preservative liquids with the Diatomaceæ there are various opinions. Some experienced microscopists say that there is little or no satisfaction in mounting them in this way. Dr. Carpenter, however, explains this difference by his instructions as to what method should be used when certain ends are desired. He says: “If they can be obtained quite fresh, and it be desired that they should exhibit as closely as possible the appearance presented by the living plants, they should be put up in distilled water within cement-cells; but if they are not thus mounted within a short time after they have been gathered, about a sixth part of alcohol should be added to the water. If it be desired to exhibit the stipitate forms in their natural parasitism upon other aquatic plants, the entire mass may be mounted in Deane’s gelatine in a deeper cell; and such a preparation is a very beautiful object for the black-ground illumination. If, on the other hand, the minute structure of the siliceous envelopes is the feature to be brought into view, the fresh diatoms must be boiled in nitric or hydrochloric acid” (which process is fully described in [Chapter II.]). It is very convenient to have many of these objects mounted by two or more of the above methods; and if they are to be studied, this is indispensable. Mr. Hepworth once showed me about one hundred slides which he had mounted in various ways, for no other purpose than the study of the fly’s foot.
My friend, Mr. Rylands, successfully mounts the diatoms in the state in which he finds them, and gave me the following method as that which he always employs. He says that he has had no failures, and hitherto has found his specimens unchanged. Take a shallow ring cell of asphalt or black varnish (which must be at least three weeks old), and on the cell, whilst revolving, add a ring of benzole and gold-size mixed in equal proportions. In a minute or two pure distilled water is put in the cell until the surface is slightly convex. The object having been already floated on to the cover (the vessel used for this purpose being an ordinary indian-ink palette), is now inverted and laid carefully upon the water in the cell. By these means the object may be laid down without being removed. The superfluous moisture must not be ejected by pressure, but a wetted camel-hair pencil, the size made in an ordinary quill, being partially dried by drawing through the lips, must be used repeatedly to absorb it, which the pencil will draw by capillary attraction as it is very slowly turned round. When the cover comes in contact with the benzole and gold-size ring, there is no longer any fear of the object being removed, and a slight pressure with the end of the cedar stick of the pencil will render the adhesion complete, and cement the cover closely and firmly to the cell. When dry, an outer ring of asphalt makes the mounting neat and complete.
The Fungi have been before mentioned; but it may be here stated that some few of the minute forms are best preserved in a very shallow cell of liquid. For this purpose creosote-water may be advantageously used.
The antennæ of insects have been before noticed as being very beautiful when mounted in balsam. This is readily accomplished when they are large; but those of the most minute insects are much more difficult to deal with, and are less liable to injury when put up in fluid. Goadby’s Fluid serves this purpose very well; but, of course, the object must be thoroughly steeped in the liquid before it is mounted, for a longer or shorter time according to the thickness.
The eggs of insects afford some worthy objects for the microscope, amongst which may be mentioned those of the common cabbage butterflies (small and great), the meadow-brown, the puss-moth, the tortoiseshell butterfly, the bug, the cow-dung fly, &c. These, however, shrivel up on becoming dry, and must, therefore, be preserved in some of the fluids before mentioned. To accomplish this no particular directions are required; but the soaking in the liquid about to be employed, &c., must be attended to as with other objects.
Glycerine may be advantageously used for the preservation of various insects. These should first be cleaned with alcohol to get rid of all extraneous matter, and then, after soaking in glycerine, be mounted with it like other objects. This liquid may also be used for the Entomostraca, which offer a wide field for study. They are to be found abundantly in ponds of stagnant water, &c. Some insects, such as May-flies, &c., are, however, often preserved by immersion in a solution of one part of chloride of calcium in three or four parts of water; but this has not been recommended amongst the “preservative liquids,” as the colour, which is often an attractive quality of this class of objects, is thereby destroyed.
We have now noticed the treatment which must be applied to those objects which are to be preserved in liquids and cells. We may here state that all slides of this kind should be examined at short intervals, as they will be found now and then to require another coating of varnish round the edge of the thin glass cover to prevent all danger of leakage. The use of the air-pump, in the first instance (as before recommended), and this precaution as to the varnish, will render the slides less liable to leakage and air-bubbles, which so very frequently render them almost worthless.