THE PHYSICAL BASIS OF LIFE i8r 



that no single unit, no cell, that is, can increase to more than 

 microscopical dimensions. When it reaches the limit of size it 

 becomes unstable, a field of force of a peculiar and special nature 

 is formed within it, and by this field of force the cell is presently 

 rent in twain. 



The basis of this curious limitation of size is not far to seek. 

 Living matter is composed of very large molecules, and sub- 

 stances so built possess certain special properties which mark 

 them off from simpler substances. To them the name of colloids 

 is given, after the type of the class the jellies. Now, a jelly is a 

 curious half-way house between the solid and the liquid states. 

 Like a solid, it is capable of retaining differences of state, it 

 is rarely of uniform character throughout. The rate of relaxation, 

 as Clerk Maxwell called it, of jellies is slow, much slower than 

 that of simple liquids, much faster than that of true solids. 

 Combined with this characteristic inertia, however, is a degree 

 of molecular mobility sufficient for chemical changes of great 

 velocity. A jelly is in this way a meeting-place of extremes, 

 and this it is which enables the colloidal state to manifest life. 



Consider now a small free cell, an infusorian swimming 

 in a wayside pool. It displays many activities, it digests in 

 this region of its living body, it maintains a store of starch 

 in that region, in the movements of its parts there is diversity. 

 Both its chemical and physical characters betoken a complexity 

 which show it to be not a homogeneous droplet, but, in spite 

 of its minute size (less than T ^th inch), to be heterogeneous. 

 It has a structure, an architecture, the coarser features of which 

 we can decipher with the aid of the microscope. 



It is only in the colloidal state that we could have within so 

 small a space so great a diversity of matter, and such differences 

 of chemical potential as must exist to support the multifarious 

 activities of the living cell, combined with the molecular mobility 

 necessary to give chemical change free play. At the same time 

 this capacity for maintaining differences of state imposes limita- 

 tions, one of which is that of size. Large molecules can move in the 

 substance of the cell scarcely at all. Therefore, when the size 

 exceeds a certain critical limit, the dynamical balance fails, and 

 internal strains appear of a magnitude great enough to tear the 

 cell apart. On this blending of opposites, on the curious com- 

 bination of inertia and chemical mobility in the colloidal state, 

 is reared the whole fabric of the dynamics of living matter. 



