1909.] 



The Properties of Colloidal Systems. 



273 



account for a very small part of the total quantity of the dye present in the 

 solution, as can easily be shown as follows : The blue colloidal free acid of 

 congo-red, even in extreme dilution, shows the track of the beam of light 

 filled with shining points of a beautiful copper colour and of nearly equal 

 size, so far as their diffraction images enable one to judge. If a drop 

 of dilute alkali be added to this solution, the track of the beam suddenly 

 vanishes, occasionally a bright point moves into the field and back again. 

 These few particles seem to be slightly larger than those of the acid. When 

 the illumination is carefully adjusted and made as brilliant as possible, close 

 attention shows that the track of the beam is very faintly visible as a bluish 

 grey haze, not resolvable into separate points, at all events not with the 

 means at my disposal, viz., arc light, Zeiss D* objective. As I am inclined 

 to interpret the phenomena, the faint haze is the optical expression of the 

 part of the dye present in the molecular state, and the rare bright points are 

 due to aggregates of a number of molecules, produced by the action of traces 

 of electrolytes, to which congo-red is enormously sensitive, as will be shown 

 below. The' solutions described by previous observers as being resolvable 

 into particles by the ultra-microscope were, in all probability, not sufficiently 

 free from electrolytes. 



The ultra-microscope, then, does not throw much light on the nature of 

 solutions of congo-red, since, although it does not contain particles large 

 enough to be visible by means of this instrument, other undoubted colloids, 

 such as ferric hydroxide, are similarly non-resolvable, but show a faint haze.* 

 Moreover, the phenomena described above in the case of congo-red are very 

 like those seen by Michaelisf in certain protein solutions, namely, a part 

 visible as granules and the rest not so resolvable. 



The property of carrying an electric charge, not as an ion, but on 

 undissociated molecules, is shared by congo-red with matter in mass. In 

 Whetham's boundary apparatus the dye moves as a whole towards the anode 

 and is, therefore, negatively charged. The origin of the charge is obscure, 

 but is, perhaps, derived from electrolytic dissociation. % In accordance with 

 its nature as an electro-negative colloid, congo-red is aggregated or pre- 

 cipitated by cations, especially powerfully by bi- and tri-valent ions. It is 

 also precipitated by an electro-positive colloid, such as toluidine-blue or 

 ferric hydroxide. The precipitate has the properties of an adsorption- 



* Zsigmondy, ' Zur Erkeimtniss der Kolloide,' Jena, 1905, p. 148. 

 t ' Virchow's Archiv,' vol. 179, pp. 205—208, 1905. 



\ The phenomena seen in the boundary apparatus are of some complexity, being accom- 

 panied by slight electrolysis. These will more properly form the subject of a separate 

 paper. 



