1909.] 



The Properties of Colloidal Systems. 



277 



uniform in size. It appears that the action of an electrolyte is, so to 

 speak, selective, leaving some molecules free, while causing others to aggregate 

 into particles, consisting themselves of very different numbers of molecules. 



Faraday showed, more than 50 years ago,* that the ruby-red solutions of 

 gold which he prepared by reduction of gold chloride were suspensions of 

 minute particles of metallic gold. He also noticed that the colour of the 

 solution became blue under the action of sodium chloride in dilute solution, 

 and was precipitated by stronger solutions. Both these effects were absent if 

 a small amount of "jelly " had previously been added to the gold solution. 

 This latter " protecting " action of " stable " colloids is now well known, 

 forming the basis of Zsigmondy's " gold number " as a characteristic of proteins 

 Congo-red behaves, as regards this protection from precipitation by electro- 

 lytes when a stable colloid is present, in the same way as the inorganic 

 colloids. It may be noted, in passing, that this is a phenomenon usually 

 ascribed to surface properties. The dye also is protected from adsorption 

 by paper, under the influence of electrolytes, when a trace of gelatin is 

 present.f It was, therefore, of interest to examine the influence of stable 

 colloids on the reduction of osmotic pressure produced by electrolytes. 



Since my experiments on adsorption above referred to indicated that this 

 protective action was greater when the stable colloid had an electric charge of 

 the same sign as that of the dye, or of opposite sign to that of the precipi- 

 tating ion, I chose for the present experiments a dialysed solution of Grubler's 

 serum-albumin, to which a trace of ammonium hydroxide was added in the 

 first experiment. The solution in the osmometer contained about 018 per 

 cent, of the dye and - 25 per cent, of serum-albumin. When dialysed against 

 distilled water, the osmotic pressure rose to 42 mm. of mercury. The water 

 was then displaced by a decinormal solution of sodium chloride. To my 

 surprise, the pressure fell to zero in about 14 hours. Moreover, as in the experi- 

 ments without stable colloid, it was found that after 14 days' changes of 

 distilled water the pressure could only be brought back to three-quarters of 

 its original value. Measurements of the electrical conductivity of the water 

 after interchange with the colloidal solution showed that the latter parted 

 with its electrolytes very slowly. They were probably held in a state of 

 adsorption by the protein as well as by the dye. 



In another experiment I first tested the particular solution of serum- 

 albumin used and found that in the proportion of 5 c.c. to 50 c.c. of one- 

 thousandth normal congo-red solution, precipitation by one-hundredth 



* ' Phil. Trans.,' vol. 147, 1858 : As to the Nature of the Solutions, see pp. 160 and 172 ; 

 Precipitation by Salt, p. 165 ; Protection from Action of Salt by " Jelly," p. 175. 

 t 'Biocheiu. Journ.,' vol. 1, p. 201, 1906. 



