In mounting these objects, some are so delicate that they are almost invisible when balsam is used with them; they are therefore usually mounted _dry_. Others, however, are much coarser, and may be mounted in balsam like the Diatomaceæ mentioned in Chapter III.
The common Infusoria cannot be mounted dry with any great success, though a few may be placed upon the glass slide and allowed to dry naturally, when their characters will be very well shown. To obtain anything like a natural appearance, they must be put up in fluid as in Chapter IV.
Next to the Diatomaceæ, no class of microscopic objects has been more looked into of late than the Foraminifera. These animals are almost all marine, having a jelly-like body enclosed in one or more chambers of shell generally composed of carbonate of lime. The shells are made with minute orifices, through which the pseudopodia (false feet) are extended by which the animal is enabled to lay hold of anything and draw itself along. From the possession of these orifices they derive their name, as _foramen_ means a door or opening. They have been found in every depth of sea hitherto sounded, each depth being abundant in certain species; the lowest beds containing the greatest number of specimens, though with less variation of kinds. In chalk they are found in a fossil state, and may readily be shown (see Chapter III.); in limestone and other hard stones they are abundant, and some mountains are composed principally of these shells.
The methods of obtaining Foraminifera are various. Many may be found upon seaweeds, which should always be examined as soon as possible after gathering. They are found in masses upon some coasts where the waves have carried and left them; but they are to be found the most abundantly in sand or mud dredged from the bottom of the sea. They must, however, be cleansed and separated from the mass of impurity with which they are usually mixed. This may be done in various ways, according to the nature of the accompanying matter. If sand alone, as is frequently the case, the whole mass must be _thoroughly_ dried, and then stirred up in clean water. The sand will soon subside by its own weight, but the chambers of the Foraminifera, being filled with air, will float upon the surface, and may be skimmed off. There is, however, one objection to this mode of proceeding--some of these objects are so minute, the chambers containing comparatively so small a quantity of air, that they sink and are cast away with the refuse sand. On this account it is preferable to take the trouble of searching certain soundings under the microscope, using the camel-hair pencil, or some other contrivance before mentioned, to extract those objects which are required. To clean the Foraminifera, Professor Williamson advises to transfer the specimens to an evaporating dish containing a weak solution of caustic potash. This must be boiled for “some moments,” when the organic matter will be entirely dissolved, and the calcareous shells left free from impurity. They must now be well washed in water, so that all alkaline matter may be entirely got rid of.