Page images

Epicauta and Meloé, is not conspicuous, and in this respect, as well as in the final transformations taking place within the two unrent skins, the insect approaches Sitaris. In the hairless and unarmed surface of the second larva, and of the third larva and pupa, as shown by careful examination of their shrunken exuviae, the insect also resembles that genus.

[merged small][ocr errors]



T is often supposed by those who have never prepared sections of rocks or fossils that the process is associated with great difficulties and expenditure of time, but this in reality is not the case. A small amount of practice is needed for the beginner and he may soon be able to manufacture sections which are fit for the microscope. The time spent in the mechanical operation of grinding rocks is not only remunerated by interesting and instructive preparations, but adds also to a complete knowledge of their physical structure, gained by a close observation of the various features they present during the process of preparation. In cases where large numbers of sections are to be prepared it is not possible for the lithologist or palaeontologist for want of time to perform all the work himself: he may be assisted by any person of ordinary ability in the preliminary part of preparation. Those who do not personally wish to prepare sections can be referred to Mr. Fues, of Berlin, or to Mr. A. Julien, School of Mines, New York city, who prepare and deal in sections, which, on account of their perfection, deserve commendation. In the present article I wish to describe the process of preparing rocks or fossils for microscopical study. They are mainly intended for those who design to make sections themselves, and to whom a detailed description of a mechanical operation may be of some assistance in their first attempts. Selection of Rock Material—It is quite important in order to make a complete microlithological analysis of rocks to select material which is characteristic. Most rocks are in various states of alteration; it is therefore necessary to choose fragments both from the fresh and decomposed parts, as thin sections of the latter ones are just as important and instructive as those taken from parts in which no visible change has occurred. Usually such decomposed rocks are very fragile and crumbling, because their crystalline structure or their cementing medium has been destroyed by the process of decomposition. Before undertaking to grind them their firmness must be restored. The fragments of such decomposed rocks are at first well dried and then placed in liquid Canada balsam, which is exposed on an iron plate to a gentle heat till it has filled and penetrated the pores and fissures of the rock. They are then separated from the Canada balsam and laid upon the warm iron plate till the balsam proves to be hard when cooled down, and are then capable of undergoing the process of slicing or grinding.

vol. xii.-No, Iw. 16

Apparatus.--It consists in its most simple form of some plates of iron and glass about eighteen inches square, to allow a full stroke with the arm during the grinding process. They have to be perfectly even and flat. Emery of various degrees of fineness is required to be used on the plates with an addition of water. Fragments of rocks are ground by hand and treated as will be described in the following lines:

In the New York State Museum of Natural History at Albany, where a large number of rocks and fossils have to be cut and prepared for thin sections for illustrating the Palaeontology of New York, the slow process of grinding by hand had to be abandoned, and a small boiler and steam engine of two and a-half horse power were purchased for the purpose of using steam-power for the grinding and slicing operations. The grinding apparatus is formed by two circular iron plates (A and B) of eighteen inches diameter and one-half inch thickness, which rotate horizontally with a velocity of about 300 revolutions a minute. The slicer which is used to cut the rock in certain directions, or to separate from it small slices to be ground afterwards, is a thin copper or steel disk of twelve inches diameter, and turns also horizontally with the same velocity as the grinding plates. For rocks above hardness 5, another slicer of steel is used, which turns vertically, and which is fed with diamond powder moistened with oil.

The whole apparatus may be run at the same time, and is arranged in such a manner that no one operation interferes with another.

The grinding and slicing is performed by the aid of emery, which is constantly applied to the plates and slicer with a brush held in the left hand. The plates are kept wet by a small current of water. It is important to use emery of different but uniform degrees of fineness as the work progresses. A mistake usually made by beginners is to choose a too coarse emery, in order to hasten the work of grinding, but they soon discover that all their sections break and wear off long before being half finished. It is advisable to use for the coarse grinding on plate A an emery which can be bought in any drug store as emery No. 90 (A). Plate B is charged with flour of emery (B); it serves also to prepare one or two sorts of finer emery from it, by treating a certain quantity of the flour with water in a vessel which may be about three feet high and six inches in diameter. In stirring the water, the emery becomes suspended in it; after a lapse of fifteen minutes the water with emery which has not settled down during . this time is decanted in a similar vessel and left there for twenty minutes more, when the water may be drained off. A sediment of fine emery will thus form in the latter vessel; and after having repeated the process for some time the emery sediment (emery C) is taken from the vessel, dried and bottled. Its degree of fineness is expressed by the number of minutes, which is in this case fifteen minutes.

Canada balsam serves as a cementing medium, it also is used to increase the transparency of various sections. It should be entirely clear and not of yellowish tint. Usually a solution of Canada balsam in turpentine is employed to mount such rocks as will not undergo any change in heating them gently over a warm plate of iron. For rocks which cannot be exposed to any heat, a solution of pure Canada balsam in chloroform is used. Both solutions of Canada balsam should be well bottled to prevent evaporation.

Oóject Glasses.—The slides of glass which are intended to bear the section should be entirely clear, free of any color, air bubbles or any other enclosures, as those which are used in other branches of microscopy. Their size should be uniform. Prof. Zirkel, of Leipzig, the distinguished lithologist, has proposed and introduced the following size, 45 mm. by 25 mm., which is preferable to the

ordinary 3 by i inch, as a slide 45 mm. by 25 mm. can be rotated on the table of the microscope; besides they are less liable to break in dropping them, and also take up less room.

In the New York State Museum of Natural History the slide measuring 45 mm. by 25 mm. is adopted. As it was found that this glass slide was too small for a number of sections of fossils, which exceed the usual size, two other standard sizes were introduced which are in proportion to the first. Size B is twice the size of first 50 mm. by 45 mm. Size C is twice the size of second – 50 mm. by 90 mm. It would be of great advantage if a uniform size was introduced in the various collections of those who prepare sections. The slide 45 mm. by 25 inm, is generally adopted with lithologists and palæontologists both in America and Europe.

Process of Slicing and Grinding.The fossil or rock is at first marked with a pencil and afterwards with a file in the direction in which it is designed to be cut. It is then held with the right hand without any further apparatus, in such a direction that the revolving slicer will cut the specimen according to the marked line. The slicer is constantly supplied during the operation with flour of emery and water. If there is plenty of material and no special direction needed, small slices can be separated from the rock by a heavy blow with a hammer. The rock fragments should as a rule not be beyond the size of a twenty-five cent piece; of course it is of great advantage to prepare sections which offer a large field for observation, and the rock specimens should not be chosen too small, as they lose in size during the grinding process.

The slice of rock which has been separated either by the slicer or by the heavy blow with a hammer, is gently pressed with the right hand against the turning plate A, while the left hand supplies plate A with emery A and water. It is necessary to change constantly the position of the rock-slice in the right hand in order to grind it to an even and plain surface. The slice is then well rinsed and cleaned of any particle of emery A, and then transferred to plate B (emery B) where the grinding proceeds till its surface is free of any scratches. Being cleaned of the emery B, it is brought upon glass plate C (emery C) and ground till its surface is entirely smooth. The rock section is finally well rinsed and brushed to clean off all impurities and allowed to dry.

Mounting Process.-A slab of iron two and a half feet long. four inches broad and half an inch thick is laid horizontally upon a tripod (of iron) and heated with a Bunsen burner. The glass slide on which the rock fragment is to be mounted is well cleaned and laid upon a part of the slab which is of a constant and moderate heat; after being warmed a few drops of Canada balsam are placed upon the glass and left with it on the slab till the balsam by evaporation has assumed a tenacious condition, in which its films will not adhere much to the fingers and yet be pliable enough to be bent readily without breaking. The warmed mineral slice is now laid upon the Canada balsam with its ground face down, and allowed to remain on the slab till all air bubbles have disappeared below the slide in the balsam. It is then gently pressed on the glass and then taken off and allowed to cool. It is of great importance to use Canada balsam in its above described condition, as the rock section during the process of grinding thin unavoidably breaks to pieces in case the balsam proves to be too soft, and if on the contrary it has been too long exposed on the slab it will become brittle and the section will, during the grinding, be liable to break off. The loosened mineral slice must in such a case be laid into a dish with turpentine to dissolve the balsam, and after being well cleaned is mounted again as described. There is no considerable difference in the process of mounting with balsam in solution with chloroform or with turpentine. The balsam in chloroform is allowed to harden in a warm room: it usually needs one or two days, while the solution in turpentine by application of heat hardens in from five to ten minutes. In the latter case care should be taken not to heat the balsam too quickly, as it turns yellowish on account of a partial carbonizing

Grinding Operation. After having used all necessary precaution in cementing the rock fragments to the glass, the grinding manipulation now begins on the reverse side of the section. It is ground at first on plate A (emery A), then on plate B (emery B) till the section commences to be translucent, or till it is so thin that it is not advisable to continue the process on the rotating wheel. It is, of course, only a matter of experience to know how long to use the various wheels, as much depends on the consistency of the rock material. It is advisable to discontinue the use of plate B before the slice is too much reduced in thickness, so

« EelmineJätka »