find ourselves in a position to deal with some simple examples of organic forms.
When we melt a stick of sealing-wax in the flame, surface tension (which was ineffectively present in the solid but finds play in the now fluid mass), rounds off its sharp edges into curves, so striving towards a surface of minimal area; and in like manner, by melting the tip of a thin rod of glass, Leeuwenhoek made the little spherical beads which served him for a microscope11. When any drop of protoplasm, either over all its surface or at some free end, as at the extremity of the pseudopodium of an amoeba, is {210} seen likewise to ãround itself off,ã that is not an effect of ãvital contractility,ã but (as Hofmeister shewed so long ago as 1867) a simple consequence of surface tension; and almost immediately afterwards Engelmann12 argued on the same lines, that the forces which cause the contraction of protoplasm in general may ãbe just the same as those which tend to make every non-spherical drop of fluid become spherical!ã We are not concerned here with the many theories and speculations which would connect the phenomena of surface tension with contractility, muscular movement or other special physiological functions, but we find ample room to trace the operation of the same cause in producing, under conditions of rest and equiôÙlibôÙrium, certain definite and inevitable forms of surface.
It is however of great importance to observe that the living cell is one of those cases where the phenomena of surface tension are by no means limited to the outer surface; for within the heterogeneous substance of the cell, between the protoplasm and its nuclear and other contents, and in the alveolar network of the cytoplasm itself (so far as that ãalveolar structureã is actually present in life), we have a multitude of interior surfaces; and, especially among plants, we may have a large inner surface of ãinterfacialã contact, where the protoplasm contains cavities or ãvacuolesã filled with a different and more fluid material, the ãcell-sap.ã Here we have a great field for the development of surface tension phenomena: and so long ago as 1865, NûÊgeli and Schwendener shewed that the streaming currents of plant cells might be very plausibly explained by this phenomenon. Even ten years earlier, Weber had remarked upon the resemblance between these protoplasmic streamings and the streamings to be observed in certain inanimate drops, for which no cause but surface tension could be assigned13.
The case of amoeba, though it is an elementary case, is at the same time a complicated one. While it remains ãamoeboid,ã it is never at rest or in equiôÙlibôÙrium; it is always moving, from one to another of its protean changes of configuration; its surface tension is constantly varying from point to point. Where the {211} surface tension is greater, that portion of the surface will contract into spherical or spheroidal forms; where it is less the surface will correspondingly extend. While generally speaking the surface energy has a minimal value, it is not necessarily constant. It may