We now pass from the consideration of the solitary cell to that of cells in contact with one another,—to what we may call in the first instance “cell-aggregates,”—through which we shall be led ultimately to the study of complex tissues. In this part of our subject, as in the preceding chapters, we shall have to give some consideration to the effects of various forces; but, as in the case of the conformation of the solitary cell, we shall probably find, and we may at least begin by assuming, that the agency of surface tension is especially manifest and important. The effect of this surface tension will chiefly manifest itself in the production of surfaces _minimae areae_: where, as Plateau was always careful to point out, we must understand by this expression not an absolute, but a relative minimum, an area, that is to say, which approximates to an absolute minimum as nearly as circumstances and the conditions of the case permit. There are certain fundamental principles, or fundamental equations, besides those which we have already considered, which we shall need in our enquiry. For instance the case which we briefly touched upon (on p. 265) of the angle of contact between the protoplasm and the axial filament in a Heliozoan we shall now find to be but a particular case of a general and elementary theorem. Let us re-state as follows, in terms of _Energy_, the general principle which underlies the theory of surface tension or capillarity. When a fluid is in contact with another fluid, or with a solid or a gas, a portion of the energy of the system (that, namely, which we call surface energy), is proportional to the area of the surface of contact: it is also proportional to a coefficient which is specific for each particular pair of substances, and which is constant for these, save only in so far as it may be modified by {294} changes of temperature or of electric charge. The condition of _minimum potential energy_ in the system, which is the condition of equilibrium, will accordingly be obtained by the utmost possible diminution in the area of the surfaces in contact. When we have _three_ bodies in contact, the case becomes a little more complex. Suppose for instance we have a drop of some fluid, _A_, floating on another fluid, _B_, and exposed to air, _C_. The whole surface energy of the system may now be considered as divided into two parts, one at the surface of the drop, and the other outside of the same; the latter portion is inherent in the surface _BC_, between the mass of fluid _B_ and the superincumbent air, _C_; but the former portion consists of two parts, for it is divided between the two surfaces _AB_ and _AC_, that namely which separates the drop from the surrounding fluid and that which separates it from the atmosphere. So far as [Illustration: Fig. 99.]