A Method of Determining Specific Gravity

Abstract

THE specific gravity of a single Foraminifer, such as a Globigerina, of the scales from a butterfly's wing, or of a drop of its blood, might seem a difficult task to ascertain, as indeed by the ordinary gravimetrical methods it would be plainly impossible. Yet nothing can be easier, given the following method. And to conduce to brevity we shall describe its application in a particular case, say to the spicules of the common shore sponge (Halichondria panicea). A quantity of one of the well-known heavy fluids, such as cadmium-boro-tungstate, or potassium-mercury-iodide solution, or methylene iodide, is diluted down to a density of about 2˙25 (which is known to be above that of the spicules), and introduced into a small glass tube, about one-quarter of an inch in diameter, and with two opposite flattened faces. This is cemented by one of its flat faces to an ordinary microscope slide, the axis of the tube being set at right angles to the length of the slide.¹ The tube being about half-filled with heavy fluid, water (or in the case of methylene iodide, benzole) is poured in, and this not too carefully, since partial admixture will serve to expedite the process. The vessel is now left to stand for a few hours or over-night. In the morning the change in the specific gravity of the column of fluid will be found to increase uniformly on passing from the top downwards. To make certain of this three small indexes of different specific gravities, say 2˙15, 2˙03, and 1˙98, are thrown in: naturally they sink till they each reach that level in the column where the specific gravity is identical with their own-“their own level,” as we may briefly term it. The distances between them may be determined by bringing them successively into the focus of the microscope, and then reading off the position of the edge of the microscope slide by means of two parallel scales attached to the stage of the instrument.