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.
If the specimens are in mud, we must proceed in a different way:—Stir up the whole mass in water, and allow it to stand until the heavier portion has sunk to the bottom; the water may then be poured off and examined to see if there are any objects contained in it. This process must be repeated until the water comes off quite clear, when (if the search is for Foraminifera only) the solution of caustic potash may be used as before mentioned. However the soundings, &c., are cleaned, it is necessary to assort them under the microscope with the camel-hair pencil or other contrivance, as it is impossible to obtain them fit for mounting without undergoing this process.
The sea soundings taken by order of Government are drawn from the bottom in a kind of apparatus ingeniously made for the purpose, and the sand, mud, &c., are brought up in their original state. Common soundings, however, are taken by lowering a heavy piece of lead coated with tallow, which consequently brings up a small portion of the matter from the bottom. Mr. George Mosley, the late Secretary of the Manchester Microscopic Society, obtained numbers of the “scrapings” from the sounding leads. To make any use of these it is, of course, necessary to free them from all traces of the tallow. Mr. Dancer places the sounding in a basin and pours boiling water upon it, which causes the melted grease to rise to the surface. When cold, this may be removed, and the water carefully decanted. The operation may be repeated until no grease appears, when the water may be withdrawn and liquor ammoniæ used, which will form a soapy solution with any remaining grease. This must be treated with hot water for the final washing. Care must be taken lest the finer forms be carried away in decanting the washing liquid. Should it be wished to make certain as to this point, each washing should be examined under the microscope. In some cases the process of Mr. Dancer will prove sufficient. Mr. Dale, however, gives a method of accomplishing the same result, which is much more readily completed; and as the results cannot be found fault with, I will here give it in full:—It is now well known that one of the products obtained from the naphtha of coal-tar is a volatile, oily substance, termed benzole (or, by French chemists, benzine), whose boiling-point, when pure, is about 180° Fahrenheit, which is a perfect solvent for fatty substances. In a capsule, previously warmed on a sand-bath, Mr. Dale mixes with the tallow soundings benzole, whose boiling-point may be about 200°, until sufficiently diluted so as to run freely, pressing the lumps with a glass rod until thoroughly mingled; the solution and its contents are then poured into a paper filter, placed in a glass funnel; the capsule is again washed with benzole, until the whole of the gritty particles are removed into the filter. A washing-bottle is then supplied with benzole, and the contents of the filter washed to the bottom until that liquid passes off pure, which may be tested by placing a drop from the point of the funnel on a warm slip of glass or bright platinum, when, if pure, the benzole will evaporate without residue or tarnish; if grease be present, the washings must be continued until they are free from it. After rinsing through weak acid, or alcohol, for final purification, the calcareous forms will be ready for mounting.
The filter and its contents may be left to dry spontaneously, when the latter can be examined by the microscope. Should time be an object, rapid drying may be effected by any of the usual methods; one of which, recommended by Mr. Dale, is to blow a stream of hot air through a glass tube held in the flame of a Bunsen’s burner. The lower the boiling-point of the benzole, the more readily can the specimens be freed from it. A commoner quality may be used, but it is more difficult to dry afterwards.
Pure benzole being costly, this may appear an expensive process; but, with the exception of a trifling loss by evaporation, the whole may be recovered by simple distillation. The mixture of tallow and benzole being placed in a retort in a hot-water, a steam, or a sand bath, the benzole will pass into the receiver, and the tallow or other impurities will remain in the retort. When the whole of the benzole has distilled over, which is ascertained by its ceasing to drop from the condenser, the heat is withdrawn and the retort allowed to cool before the addition of fresh material. Half a dozen to a dozen filters, each with its specimen, can be in process at the same time; and the distillation of the recovered benzole progresses as quickly as the filtration, which was practically proved on the occasion named. Great caution in the use of benzole is to be taken in the approach of lights to the inflammable vapour.
After the Foraminifera and calcareous forms have been removed, the residue may be treated with acids and levigation in the usual manner, to obtain siliceous forms and discs, if there are any present; but to facilitate their deposition, and to avoid the loss of any minute atoms suspended in the washings, I would suggest the use of filtration. The conical filter is unsuitable, as the particles would spread over too great a surface of paper; but glass tubes open at both ends (such as broken test-tubes) will be found to answer, the broad end covered with filtering paper, and over that a slip of muslin tied on with a thread to facilitate the passage of the water and prevent the risk of breaking the paper. Suspend the tube over a suitable vessel through a hole cut in thin wood or cardboard, pour in the washings which can be thus filtered and then dried. The cloth must be carefully removed, the paper cut round the edges of the tube, and the diatoms on the paper disc may be removed by a camel-hair pencil or otherwise, ready for mounting. Thus many objects may be preserved which would be either washed away or only be obtained by a more tedious process.
Such is Mr. Dale’s method of cleaning the soundings from the tallow, and as it thoroughly accomplishes its end, and is alike effective and not injurious to Foraminifera and diatoms, it may be safely recommended. The weak solution of caustic potash before advised for Foraminifera, must not be used where it is desired to preserve the diatoms, as they would certainly be injured, or destroyed altogether, if this agent were employed.
In fixing the Foraminifera upon the slide, no better plan can be followed than the “dry cells” and gum recommended in the early parts of this chapter. Owing to their thickness and composition, most of them are opaque objects only; but they are exquisitely beautiful, and require no particular care, except in allowing the cell, &c., to be perfectly dry, when the cover is placed upon it, or the damp will certainly become condensed upon the inner side, and the examination seriously interfered with.
Many of the Foraminifera require cutting into sections if it is wished to examine the internal structure, &c.,—“decalcifying” is also desirable in some cases;—both of these processes will be found described at length in the chapter on Sections and Dissection.