COLLOIDS, OR THE MATERIAL OF LIFE 123 



has been measured, and found to be exactly that which would 

 be possessed by molecules of similar mass, or produced by 

 collision with the invisible molecules of the liquid. It is one of 

 the triumphs of colloid chemistry that, in this way, it has been 

 successful in demonstrating what, hitherto, even chemists have 

 sometimes doubted, the real existence of molecules. 



One of the most important properties of colloid particles is 

 the low rate of diffusion that results from their larger size. 

 For this reason such particles are retained by colloidal mem- 

 branes, like parchment and gold-beaters' skin, while crystal- 

 loids can pass through into the water in which the membrane 

 is immersed. Graham utilised this property for the separation 

 of colloids and crystalloids, and his method of " dialysis " 

 is still one of the fundamental operations of colloid chemistry. 

 This slowness of diffusion is responsible, also, for the curious 

 hysteresis of colloid phenomena that is so suggestive of an 

 analogy with vital processes. Indeed, a slow rate of diffusion 

 appears to be essential for the existence of living matter. 

 The cells of organisms are surrounded by precipitation 

 membranes, through which the colloid contents of the cell 

 are unable to pass. The substances which can permeate 

 the cell walls, and cause physical or chemical changes in the 

 material within, vary according to the nature of the mem- 

 brane. Thus the properties of cells are differentiable and each 

 cell becomes a diminutive chemical factory. 



The cell walls are formed by a process, also characteristic of 

 colloid solutions, i.e. the tendency to form skins on the surface. 

 Such membranes may be produced by contact with a precipi- 

 tating reagent, or may be evolved spontaneously. The latter 

 case is particularly interesting as resulting from the diminished 

 internal pressure in the surface layer of a liquid. Water 

 molecules appear to attract one another more than the colloid 

 particles, so that the latter tend to be squeezed out into the 

 external layer in spite of their kinetic energy that continually 

 returns many of them to the bulk of the liquid. This collection 

 of solid in liquid, or solid, surface layers is called ** adsorp- 

 tion." If sufficient colloid collects at the surface it may be 

 thrown out of solution and form a solid skin. The effect is 

 familiar in the case of hot milk. Such precipitation membranes 

 have remarkable properties. When produced around a crystal 

 suspended in a solution with which, when dissolved, it forms 

 a colloidal precipitate, the membrane grows like a plant, sending 

 down roots into the liquid, and stems and branches which 

 sometimes grow out of the solution into the air. Moreover, 

 such plant-like forms exhibit the properties of youth, maturity, 

 age, and decay, just like living organisms. By merely dropping 

 gelatin solution into a solution producing quick coagulation, 



