total surface-area is capable of being satisfied. Thus the phenomenon of ãliquid cryôÙstalôÙliôÙsaôÙtionã does not destroy the distinction between crystalline and colloidal forms, but gives added unity and continuity to the whole series of phenomena7. Lehmann has also demonstrated phenomena within the crystal, known for instance as transcryôÙstalôÙliôÙsaôÙtion, which shew us that we must not speak unguardedly of the growth of crystals as limited to deposition upon a surface, and Bû¥tschli has already pointed out the possible great importance to the biologist of the various phenomena which Lehmann has described8.
So far then, as growth goes on, unaffected by pressure or other external force, the fluidity of protoplasm, its mobility internal and external, and the manner in which particles move with comparative freedom from place to place within, all manifestly tend to the production of swelling, rounded surfaces, and to their great predominance over plane surfaces in the contour of the organism. These rounded contours will tend to be preserved, for a while, in the case of naked protoplasm by its viscosity, and in the presence of a cell-wall by its very lack of fluidity. In a general way, the presence of curved boundary surfaces will be especially obvious in the unicellular organisms, and still more generally in the external forms of all organisms; and wherever mutual pressure between adjacent cells, or other adjacent parts, has not come into play to flatten the rounded surfaces into planes.
But the rounded contours that are assumed and exhibited by {205} a piece of hard glue, when we throw it into water and see it expand as it sucks the water up, are not nearly so regular or so beautiful as are those which appear when we blow a bubble, or form a drop, or pour water into a more or less elastic bag. For these curving contours depend upon the properties of the bag itself, of the film or membrane that contains the mobile gas, or that contains or bounds the mobile liquid mass. And hereby, in the case of the fluid or semifluid mass, we are introduced to the subject of surface tension: of which indeed we have spoken in the preceding chapter, but which we must now examine with greater care.
Among the forces which determine the forms of cells, whether they be solitary or arranged in contact with one another, this force of surface-tension is certainly of great, and is probably of paramount importance. But while we shall try to separate out the phenomena which are directly due to it, we must not forget that, in each particular case, the actual conformation which we study may be, and usually is, the more or less complex resultant of surface tension acting together with gravity, mechanical pressure, osmosis, or other physical forces.
Surface tension is that force by which we explain the form of a drop or of a bubble, of the surfaces external and internal of a ãfrothã or collocation of bubbles, and of many other things of like nature and in like circumstances9. It is a property of liquids (in the