08 



since absorption effects are often observed, due to the action of the 

 paper on the dispersed phase, and often lead to clogging of the pores 

 of the filter. The results are, however, interesting, smce they show 

 that typical colloids, with particles having a diameter less than • 1 jU,, 

 are easily able to pass through all the filters tested. 



Size of Pores in Filters. 



Average Size of Pores 

 Filter. (Permeability to Water). 



Ordinary thick filter paper - - - 3 • 3 /x 



Filter paper, No. 556. (Schleicher and 



Schiill) l-l II 



Filter paper. No. 602. (Extra hard, 



Schleicher and Schiill) .... 0-89-l-3/.i 



The degree of solubility is also of importance in this question of 

 filtration, chiefly because the solubility of a substance is dependent 

 upon its specific surface, or, in other words, the solubiHty rises greatly 

 with the extreme subdivision. The jelly concentration of silicic acid 

 was found by Graham {Jour. Chem. Soc, 1864), in the very early days of 

 colloid chemistry, to influence the maximum molecular solubility 

 in excess of water. Thus he found that only two parts of I per cent, 

 silicic acid jelly formed a molecular disperse solution in 10,000 parts 

 of water, one part of a 5 per cent, jelly, and less of greater concentra- 

 tions of jelly in the same amount of water. Concentrated jelHes, 

 therefore, are less disperse than the more dilute, and so have a lower 

 molecular solubility. The low solubility of gelatinised starch will be 

 appreciated when the figures shown in Tables II. and III. are compared. 



The viscosity, of starch solutions and separation of starch paste 

 into two phases has already been pointed out in the First Report 

 (p. 49), but, before passing to other points, it is as well to consider 

 another aspect of true soluble starch. Some description " of soluble 

 starch has already been given in a previous Report (No. I., p. 51), 

 but there is little doubt that confusion has arisen owing to its variable 

 nature according to the method of its preparation. 



Soluble Starch. — Soluble starch paper, prepared b}^ the action of 

 diastase or acids on starch paste or by heating dry starch in a suitable 

 manner (Zalkowski, Chem. Zeit., 1888, 1060), is soluble only to a 

 very small extent>— about 2 • 3 per cent. — in cold water, yet it is 

 possible to obtain a j)reparation, by the action of sodium peroxide 

 (Syniewski, Ber., 1897, XXX., 2415) on starch suspended in water, 

 that is soluble to the extent of some 12 per cent, in the cold. The 

 latter seems quite a different compound to that prepared by Lintner's 

 method with diastase. 



The varied descriptions of the behaviour of soluble starch when 

 placed in water, dissolved and subsequently cooled, would add weight 

 to the conclusion that it was not always the same kind of soluble 

 starch that "was under consideration. It may, and usually does, form 

 a thin opalescent paste, remaining fluid on cooling, however prepared, 

 but it does not always revert to the insoluble form in time. Foxiard 

 (Compt. Bend., 1908, 97, 931-3) used a soluble starch that reverted 



