94 PROTOPLASM 



fication of dirt (from cloth) jvith the aid of peptizing agents such 

 as soap, proteins, and gums, and the emulsification of oil in water 

 with the aid of soap and proteins are all dispersion methods by 

 peptization. 



The second general method of preparing colloidal solutions 

 involves the aggregation of dispersed molecules. (For this 

 Graham introduced the term pedization; it is, however, not much 

 used.) Such condensation methods include the replacement 

 of the solvent (as when a few drops of an alcohol solution of gum 

 are dispersed in a large quantity of water) ; reduction of a metallic 

 salt to its metal (2AgOH + H2 -^ 2Ag + 2H2O) ; oxidation 

 (2H2S + O2 = 2S + 2H2O) decomposition; (AS2O3 + 3H2S = 

 AS2S3 + 3H2O); and hydrolysis (FeCl3 + 3H20 = Fe(0H3) + 

 3HC1). All involve the aggregation of molecules of soluble sub- 

 stances into colloidal particles of insoluble substances. The 

 preparation of colloidal iron oxide (or hydroxide) is readily carried 

 out. Several small pieces of ferric chloride are dropped into 

 boiling water. The salt is immediately converted into the oxide 

 or the hydroxide, which is insoluble in water and therefore remains 

 suspended in colloidal form. A rich brick-red dispersion results. 

 If oxygen is passed through an aqueous solution of hydrogen 

 sulphide, colloidal sulphur results. (The oxygen of the air is 

 usually sufficient.) Stannic chloride often reacts similarly; a 

 solution of it may become cloudy, the salt having hydrolyzed 

 instead of ionizing. 



Tyndall Effect. — The optical properties of colloidal systems 

 particularly distinguish them from molecular dispersions. 

 Among these properties, two are very characteristic — the 

 Faraday-Tyndall effect, commonly known as the Tyndall cone; 

 and structural color. The first of these ties all the diverse types 

 of colloidal systems together — the solid and liquid suspensions 

 and the gluelike colloids. All show a Tyndall cone. The 

 Tyndall phenomenon may be illustrated by a commonplace 

 event, which, however, is not truly colloidal. When a ray of 

 sunlight creeps through the crack of a shutter into a darkened 

 room, the particles of dust suspended in the air, which are not 

 visible when the room is well lighted, become bright spots of 

 light because they are illuminated laterally against the dark 

 background of the room. If, now, we perform a similar experi- 

 ment but with truly colloidal "dust," by allowing tobacco smoke 



