THE COLLOIDAL STATE 109 



there are boundary surfaces of contact between the phases, although these phases 

 cannot be readily separated by mechanical means. 



Most of the characteristic properties of this state depend on the enormous 

 development of surface in proportion to the total mass. 



The chief factor in the stability of such systems, except those of two solid 

 phases, is the Brownian movement of the particles ; this movement is essentially 

 identical with the molecular movement of the medium in which the particles are 

 suspended. 



There is reason to believe that, by appropriate means, any substance could be 

 obtained in the colloidal state, and that substances usually met with in the colloidal 

 state might be made crystalline. 



The two great classes of colloids, emulsoid or lyophile, and suspensoid, or 

 lyophobe, which are of the most importance in physiology, differ in the state of 

 the internal phase, which is liquid in the former, solid in the latter. Other 

 properties go along with these, and the names lyophile and lyophobe call attention 

 to the relation of the dispersed phase to the liquid surrounding it. The internal 

 phase in emulsoids frequently consists of a solid substance associated with varying 

 amounts of the solvent, a fact which confers on it the properties of a liquid to a 

 greater or less degree. The relative proportion of water, etc., in the two phases 

 can be changed reversibly by various agencies, especially electrolytes. 



The existence of finite particles in many cases can be demonstrated by the 

 ultra-microscope, in which diffraction discs of light, sent off by the illuminated 

 surfaces of the particles, are observed. 



Owing to the dimensions of these particles, they are unable to pass through a 

 membrane of colloidal substance, such as parchment paper or collodion ; whereas 

 crystalloids pass rapidly through these. The process is known as dialysis and is 

 of frequent use to separate colloids from crystalloids. By the application of 

 pressure, which must be greater than the osmotic pressure of the solution 

 concerned, water also can be forced through, so that this process, known as 

 " ultra-filtration," can be used to concentrate colloidal solutions. 



When the internal phase consists of a substance capable of electrolytic 

 dissociation in water, one ion being freely soluble and diffusible, it is found that 

 the surface of the particles is dissociated in this way ; the soluble ions move off as 

 far as electrostatic forces permit, leaving the opposite ions concentrated on the 

 surface of the particle, and giving it their combined electric charges. The giant 

 multivalent ion so formed is called by Hardy a colloidal ion. 



The possibility of a source of electrification akin to frictional electricity cannot 

 as yet be definitely excluded as another source of the electric charge, usually found 

 on the contact surface between phases. 



The possession of this electric charge renders colloidal particles sensitive to the 

 presence of ions of opposite charge. These neutralise the charges on the particles 

 and cause precipitation, themselves being carried down with the precipitate. In 

 this process, the effect of valency is out of all proportion to the increased number 

 of charges. t 



In the case of emulsoids, which are less sensitive than suspensoids to this 

 purely electrical effect, neutral salts have a further action, shown in its most 

 marked form as "salting out"; but in lower concentration than necessary for this 

 purpose, they have an action due to their effect on the general properties of water, 

 altering its distribution in the two phases of the system, and therewith other 

 properties, such as surface tension, viscosity, compressibility, coagulation time, 

 etc. This phenomenon may be brought into relation with the association of part 

 of the water with the ions of the electrolytes. 



There is also evidence of adsorption of salts in the case of proteins; but 

 whether any true chemical combination occurs is doubtful. 



Certain emulsoids, such as gelatine, have the property of forming semi-solid 



