II] THE COLLOIDAL STATE 18 



of soap ill distilled water. It is opalescent and froths strongly, (c) Chloruphijll . 

 [See Expt. 32.] 



The above sols should l)e kept for further experiment [see Exjjt. ij]. 



Expt. 5. Dialysis of sUtrch and salt solution. Make a 2"/o solution of starch in 

 water, as in Expt. 2, and mix it with an equal volume of a 2 "/„ solution of sodium 

 chloride in water. Pour the mixture into a parchment dialyzer and dialyze in a 

 beaker of distilled water. (The dialyzer should first be carefully tested to ascertain 

 if there be a leak.) Test the liquid in the beaker with solutions of both iodine and 

 silver nitrate. Some precipitate of chloride will be given, but no blue colour with 

 iodine. After 24 hours, test the liquid again. There will be an increased amount of 

 silver chloride formed, but a negative result with iodine. On addition of iodine to 

 the liquid in the dialyzer, a blue colour is obtained. Hence we may assume that the 

 colloidal starch does not pass through the membrane. 



Some substances, such as gelatine (animal) and agar (vegetable), are 

 only in the eniulsoid condition at a raised temperature. When cold 

 they set to form a " gel," in which the particles of the dispersed phase are 

 no longer separate but united to make a solid. Silicic acid, the best known 

 inorganic emulsoid, also sets to a gel on standing, either spontaneously 

 or on addition of electrolytes. It is of classical interest since it was the 

 substance largely used by Graham, the first worker on colloids. 



Expt. 6. Preparation of gels, (a) Agar. Weigh out 2 gms. of agar and put it 

 to soak in 100 c.c. of distilled water for an hour or two. Then boil : the agar gives a 

 thick opalescent solution (sol) which sets to a gel on cooling. On warming, the gel 

 again becomes a sol, and, on cooling, again sets to a gel. Thus the change is a 

 reversible one. Agar is a mucilage which is obtained from certain genera of the 

 Rhodophyceae (see p. 49). (6) Silicic acid. Weigh out 20 gms. of commercial "water- 

 glass" syrup (a concentrated solution of sodium silicate) and dilute with 100 c.c. of 

 freshly boiled distilled water (free from carbon dioxide). Pour 75 c.c. of this solution 

 into a mixture of 25 c.c. of concentrated hydrochloric acid and 75 c.c. of water. 

 Dialyze the mixture in a parchment dialyzer against running water for 3-4 hours. 

 If to the dialyzed liquid a little very dilute ammonia is added, a gel will be formed 

 in the course of a few hours. In this case, however, the process is irreversible, that 

 is the gel cannot be reconverted again into the sol. 



An interesting point in connexion with the colloidal state is that 

 emphasized by Ostwald, i.e. that this condition is a state, not a type, 

 of matter. Further, substances in the colloidal state do not constitute 

 a definite class. It is reasonable to suppose that all substances which 

 exist in the colloidal state can, under suitable conditions, also exist in 

 the crystalline state, and vice versa. Further, the continuous phase is 

 not always water. Sodium chloride, which is a very definite crystalloid, 

 can be obtained in the colloidal state in petroleum ether. Most metals, 

 even the alkali metals, have been obtained in colloidal solution : also a 

 great many metallic oxides, hydroxides and sulphides. 



