The particular methods used for certain objects may be now entered upon. Many of the Diatomaceæ 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 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.