The properties of colloidal systems. IV. —Reversible gelation in living protoplasm

Protoplasm itself, using the name to express living substance in its simplest, undifferentiated form, is generally recognised as having the properties of a somewhat viscous liquid. This fact was realised as long ago as 1864 by Kühne (1864). As usually seen under the microscope, it contains suspended in it a number of granules of a great variety of dimensions and properties. But in the pseudopodia of an amoeba or of a leucocyte, when examined by the ordinary method of illumination with transmitted light, it appears completely devoid of contents or structure. As Hardy (1899) showed, the various networks and similar arrangements seen in fixed preparations are produced by the action of the reagents used, although they indicate that protoplasm contains matter in the colloidal state. The use of the method of brilliant lateral illumination on a dark ground (so-called “ultra-microscope”) has led to the detection of particles in protoplasm which are too minute to be visible by ordinary illumination. Of course, these particles, being comparable in dimensions with the mean wave-length of light, are not seen in their true dimensions or form, but by their diffraction discs. Remembering that the late Lord Rayleigh showed that the more intense the illumination, the more minute are the particles that it is possible to detect, I tested the result of increasing the intensity of the dark-ground illumination applied to the apparently clear and structureless pseudopodia of large amoebae. The broad flat pseudopodia of a species, which appeared to correspond toAmœba princeps (Leidy), were found to be most appropriate for the purpose. A paraboloid condenser, made by Zeiss, was used in most cases. The source of light was the positive crater of a small arc lamp with carbons at right angles to one another. The rays were made parallel by a condenser, and passed through a cell with parallel sides, about 5 cm. apart, before falling on the mirror of the microscope. The water-cell was found to be necessary on account of the heat otherwise transmitted being sufficient to kill the organisms. In order to obviate the injurious effect of any ultra-violet rays which might be transmitted through the system, quinine sulphate was added to the water. The objective used for the majority of the observations was an excellent ⅙-inch dry lens made by Swift, which was found to admit of magnification by fairly high-power oculars, such as No. 12 compensating of Zeiss. Other methods, such as that of an objective as sub-stage condenser, with a central stop in the observing objective to cut out direct rays, were tried; but the paraboloid was found to be the best on the whole.