30 A TEXTBOOK OF PHYSIOLOGY 



suspended in water in such a manner as to be evenly distributed 

 throughout the fluid with but little -tendency to settle out or aggre- 

 gate together. Such substances form suspensions or emulsions. They 

 are non-diffusible, refract light, exert no osmotic pressure, do not 

 conduct electricity, and contain particles visible under the microscope. 

 Between these two extremes comes the group of bodies classed at the 

 present time as colloids, some approaching more nearly the crystalloids, 

 some more nearly the suspensions. But for the most part colloids 

 possess characteristics which clearly differentiate them from crystal- 

 loids. These characteristics may be enumerated as follows: 



They are generally amorphous in form; some, however, can be 

 made to crystallize under appropriate conditions. Although giving 

 a homogeneous solution when seen beneath the microscope with 

 ordinary illumination, yet if a beam of light be passed through the 

 solution particles become visible, or. rather, halos surrounding these, 

 owing to the dispersion of light waves from the surfaces of the particles 

 suspended in the solution, just the same as a ray of light becomes 

 visible on passing into a dusty room. This is known as " Tyndall's 

 phenomenon." The particles are too small, but the halos surrounding 

 them are large enough, to be seen under the microscope. Since colloids 

 are not far removed from suspensions, relatively slight changes suffice 

 to aggregate the particles and throw them out of solution. If the 

 colloid, thus thrown out, can again be dissolved in the solvent, it is 

 said to be precipitated; often, however, it cannot be redissolved, and 

 it is then said to be coagulated. Agencies which produce aggregation 

 or agglutination are a rise of temperature, and the adding of large 

 quantities of neutral salts, a process known as " salting out." 



The suspension of the colloid particles in the solvent depends on the 

 particles carrying an electrical charge and their mutual repulsion. 

 Any factor which reduces this charge tends to aggregate the particles. 

 Colloidal suspensions, like those of colloidal gold, are at once thrown 

 out by the electrical discharge of the particles labile colloids. In the 

 case of colloidal emulsions there is a relation between the molecules 

 and the solvent, and the particles are less easily thrown out stabile 

 colloids. 



In colloidal solutions the size of the particles, roughly, is between 

 the limits of microscopical vision (0-1 /u) and ultra-microscopical vision 

 (0-001 n). Above the limit we have suspensions, and below it we 

 approach the true molecular solutions. The surface of the particles 

 plays a great part in the chemistry of the colloids. The minute sub- 

 division causes an enormous increase in surface. Suppose a cubic 

 centimetre of gold be subdivided into particles with a side of 001 ytt 

 the little cubes (10 18 in number) would have a total surface of 600 

 square metres, roughly equal to a surface measuring 25 yards by 

 25 yards. All surfaces have the power of adsorption e.g., charcoal 

 adsorbs gases, colouring matters; fire-clay adsorbs coal-gas in such 

 a way that intense incandescence with very perfect combustion is 

 brought about in the surface of the brick when coal-gas is forced 

 through it and lighted; platinum black adsorbs and brings about the 



