where the medium is more accessible to us, there is more or less suggestion of an act of dissociation by the system in which the parts become associated. Thus an atmosphere overcharged with moisture literally expels water from itself, first into fine vesicles, and later, by cumulative aggregation of these, into the drops which constitute rain; so when the vapor of water issues from the spout of a kettle, the air which it traverses condenses it into the droplets visible as steam. Water thrown upon a dusty surface; molten lead let fall from a tower in the process of shot-making; melted glass dropped into water—all these assume a more or less spherical shape under circumstances which suggest, not only cohesion of the parts, but also repulsion by the medium. The spherical shape assumed by oil dropped on water is not wholly due to cohesion of the parts of oil, but is also due in very large measure to the repulsive action of the water system itself. A still more striking illustration of these acts of simultaneous association and dissociation is yielded by mixing small quantities of water with large quantities of oil: here the water, in descending, breaks up into spherelike globules, each of which exemplifies at once cohesion of the intruding parts and repulsion by the receiving system. There is also to be added the evidence of smoke and vapor rings, the forms of which are largely determined by the action of the atmospheric system in which they are produced.
That assimilative action takes place within material systems is also to be noted. Such action is of several kinds, and includes (1) assimilation of movement, (2) assimilation of substance by diffusion, and (3) assimilation of mass or structure, the latter being divisible into (a) assimilation by change of form in the case of gross aggregates rudely associated, and (b) assimilation by changes of arrangement in the case of minute parts closely associated. The simplest form of assimilation is seen when volumes of two different gases are brought together within a closed receptacle; for, though the molecules of the two gases may originally possess different "kinetic" energy, they undergo in association a change by which the molecules of both gases come to have a like degree of the energy of movement. What is true of gases is true also of matter in each of its states: heat communicated to an aggregate is more or less rapidly distributed through it until all the parts possess, roughly speaking, like degrees or amounts of movement; a mass of metal heated in a furnace becomes gradually assimilated to the character of its surroundings; by a precisely similar process, the overheated earth radiates energy into the atmosphere. There is also the assimilative distribution of heat through liquids by means of convection currents: as the surface of the sea becomes cooled in winter, the cooler layers, grown heavier, sink, and are constantly being replaced by warmer water
