46 



ILLINOIS ACADEMY OF SCIENCE 



solution, has two thin quartz windows, a and a. By means of 

 a slit and a series of lenses, rays of light from an arc light en- 

 ter window a in a plane a few thousandths of a millimeter 

 thick. This light impinges on a few of the colloidal particles 

 in the solution and causes them to be centers of light emission. 

 The effect can then be observed by means of an ordinary mi' 

 croscope, M against a black background through window a. 

 The same principle is, of course, involved when a ray of sun- 

 light enters a darkened room and makes brilliant spots of light 

 of the motes in the beam. 



A property common to colloidal solutions is the slow dif- 

 fusion of the particles through gelatinous substances and 

 through membranes. Upon this property is based the easiest 

 method of determining whether a solution is colloidal or not. 

 A test of this kind can be made by pouring some gelatin in 

 the bottom of test tubes, and on top of the gelatin solutions 

 of, say, colloidal gold and of potassium dichromate. After an 

 hour or so it will be observed that the dichromate has diffused 

 several millimeters down into the gelatin, whereas the boun- 

 dary between the gelatin and the colloidal solution is just as 

 sharp as at the beginning of the experiment. 



The surface exposed by the colloidal particles to the sol- 

 vent is, as a simple calculation will show, enormous, so that 

 surface effects that are very slight with ordinary solids may be 

 greatly increased by getting the solids into the colloidal con- 

 dition. One such surface effect is adsorbtion. Willard Gibbs's 

 reasoning led him to the conclusion that any substance the ad- 

 dition of which lowers the surface tension at a solid-liquid 

 or gas-liquid surface is in greater concentration at that sur- 

 face than in the bulk of the liquid, in other words, the sub- 

 stance is adsorbed at the surface. This has been amply veri- 

 fied by experiment. Adsorption phenomena play a large role 

 in the theory of colloids. It can readily be shown that adsorp- 

 tion has a large part in the catalytic effect of colloidal platinum 

 on the decomposition of hydrogen peroxide. 



Under most conditions the colloidal particles are found to 

 be charged electrically and will migrate to one pole or the 

 other when an electric current is passed through the solution. 

 The origin of this charge is not entirely clear, and its magni- 

 tude and even its sign may be changed by the addition of small 

 quantities of electrolytes. If by some means the electric charge 

 is neutralized the colloidal material is rendered unstable and 

 may be readily coagulated or precipitated. 



