Chalk.--The following method is that recommended by M. Ehrenberg, Place a drop of water upon a plate of thin glass, and put into it as much scraped chalk as will cover the fine point of a knife, spreading it out, and leaving it to rest a few seconds; then withdraw the finest particles which are suspended in the water, together with most of the liquid, and let the remainder become perfectly dry. Cover this dried spot of chalk with Canada balsam (the turpentine of _Abies balsamea_), and hold the plate of glass over the flame of a lamp, until the balsam becomes slightly fluid, without froth or air-bubbles; it should be maintained in this position (the glass being kept as hot as the finger will bear) for a few minutes, until the balsam is found to have thoroughly permeated the substance to be examined. It is preferable to place a piece of very thin glass upon the balsam, and gently press it down, and allow it to remain. The best flatted crown-glass should be used for placing the chalk or other objects on. It is convenient to have the slips of glass of one size, or the specimens will require different boxes for their reception; three inches by one inch is that usually employed. These objects require to be viewed with a power magnifying three hundred times linear, that is, in diameter; and if the process has been properly conducted, it will be seen that the chalk is chiefly composed of well-preserved organisms. In these preparations the cells of the foraminifera appear at first black, with a white central spot; this is caused by the air contained in those cavities, for air-bubbles always appear as black annular bodies; by degrees, the balsam penetrates into all the single cells, the black rings of the air vesicles disappear, and the structure of the original is beautifully displayed.[347] [347] Specimens of chalk, flint, and other rocks for examination; or specimens prepared on slides, may be obtained of the microscopic artists, named in the Appendix. Soft part of Rotaliæ in chalk.--The manner in which I obtained the unmineralized soft bodies of foraminifera from chalk has already been mentioned (_ante_, p. 360); but it may be useful to offer a few additional suggestions; for such fossil remains are not easily extracted. Many experienced microscopical observers have not succeeded in obtaining one good specimen; but others have been more fortunate, or persevering.