IMITATION AMONG ATOMS AND ORGANISMS. 507 



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 sur- 

 face ; 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 descend- 

 ing, breaks up into spherelike globules, each of which exemplifies 

 at once cohesion of the intruding parts and repulsion by the re- 

 ceiving 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 (6) assimilation by 

 changes of arrangement in the case of minute parts closely asso- 

 ciated. The simplest form of assimilation is seen when volumes 

 of two different gases are brought together within a closed recep- 

 tacle ; 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 ag- 

 gregate is more or less rapidly distributed through it until all the 

 parts possess, roughly speaking, like degrees or amounts of move- 

 ment; 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 



