TRANSACTIONS OJ? SECTION B. 367 



the ultramicroseopie dimensions of their particles, are termed ' Suspension- 

 colloids ' or 'Lyophobic Sols.' These include colloidal metals, sulphides, and 

 many hydroxides. 'Emulsion-colloids,' or 'Lyophilic sols,' which include 

 albumin, gelatin, starch, &c, approach more nearly to the true solutions. The 

 members of this latter class are far more irregular in their behaviour than those 

 of the former. 



The question whether the particles of colloidal solutions are necessarily 

 amorphous, is not easily answered, and it is net yet known how far a crystal- 

 line solid may be sub-divided and yet retain its crystalline properties. The 

 particles of many colloidal solutions approach molecular dimensions, and it cannot 

 be assumed without proof that they retain the properties of the corresponding 

 phases in larger mass. In many cases the particles are certainly amorphous. 



Adsorption is chiefly manifested by -amorphous substances, and the sub- 

 stances adsorbed are frequently of high molecular weight. It remains uncer- 

 tain whether adsorption is an effect of surface condensation, solid solution, or 

 chemical combination, although the author inclines towards the first of these 

 views. Owing to the great adsorbing power of amorphous substances, it is 

 almost impossible to obtain a pure colloidal solution. The tendency of small 

 particles to become larger, and to be transformed into crystals, is greatly 

 lessened by the presence of absorbed impurities. Thus the amorphous condition 

 of the particles favours adsorption, and this again favours the maintenance of 

 the colloidal state. Other factors affecting the stability are the internal friction 

 of the liquid and the electrical charge on the particles. 



The electrical charge is of importance in the coagulation of suspension colloids, 

 the addition of an electrolyte, by discharging the particles, facilitating coagula- 

 tion. When the sol is positively charged, as in colloidal metallic hydroxides, 

 the order of influence of coagulating agents is the reverse of that which is found 

 with negatively charged sols, such as colloidal metals and sulphides. A series 

 of experiments by Elisafoff and the author on the influence of electrolytes in 

 diminishing the electroendosmosis of a solvent shows that a complete parallelism 

 exists between this effect and the discharging effect of the same electrolytes on 

 a hydrosol. 



In the case of emulsion-colloids, electrical conditions are of far less import- 

 ance, and the determining factors are chiefly the individual characters of the 

 substances concerned. 



In conclusion, attention was drawn to the importance of colloid chemistry or, 

 in a wider sense, of capillary chemistry, in its bearings on physiological problems. 



(ii) Colloids in Pharmacology. By Dr. George Bakger. 



(iii) The Adsorption of Iodine by the Glucoside Saponarin. 

 By Dr. George Barger. 



Saponarin is a glucoside occurring in Saponaria officinalis and other plants ; 

 it has the composition C 31 H 21 12 (MWt = 468), and is obtained pure by crystal- 

 lisation from pyridine. It has been fully characterised chemically, 1 and has 

 been shown to yield on hydrolysis glucose and the colouring matter vitexin.* 

 Saponarin was first known to botanists as ' soluble starch,' on account of the blue 

 coloration given by its solutions on addition of iodine dissolved in potassium 

 iodide. When the pure crystalline substance is shaken with water at room tern 

 perature, a solution is formed containing about one part in 7,000, which is not 

 coloured blue by iodine. The solubility in boiling water is about 1 : 1000; on 

 cooling, the glucoside does not crystallise out for some days, but remains dis- 

 solved as a colloidal solution, and this solution closely resembles a solution of 

 soluble starch as regards its behaviour towards iodine. A more concentrated 

 hydrosol can be produced by acidifying a solution of the glucoside in alkali, in 

 which it is readily soluble; but this, of course, introduces a salt. In the cell- 

 sap of Saponaria the substance is held in solution by saponin. In order to 



1 Journ. Chem. Soc, 1906, 89, 1210. 



1 First isolated by A. G. Perkin, Journ. Chun. Sec, 1898, 73, 1030. 



