1909,] The Properties of Colloidal Systems. 279 



Kesults similar to the above have been obtained in the case of arsenious 

 sulphide, aggregation without precipitation also occurs here. In order that 

 the albumin shall be efficient as a protecting colloid it is necessary that it be 

 of the same sign as regards its electric charge as the arsenious sulphide, 

 that is, electro-negative ; if electro-positive, it tends to aid the action of 

 the electrolyte. 



I am unable, at present, to state definitely whether the same con- 

 siderations apply to the action of electrolytes and stable colloids on gold 

 hydrosols, the " gold number " in fact. I cannot make out any difference in 

 the appearances under the ultra-microscope of mixtures of gold hydrosols 

 and serum-albumin, with and without calcium sulphate. To the naked eye, 

 there is perhaps a slight tendency to a more purple colour in the case of the 

 former, but nothing approaching the blue of the mixture of gold and 

 electrolyte in the absence of the albumin. The ultra-microscope shows also 

 that the particles in the blue solution are larger and less numerous than in 

 the ruby-coloured ones. It is possible that complexes of gold particles and 

 protective colloid, of the kind described by Zsigmondy,* may be formed 

 without change of colour of the gold. The whole series of phenomena in 

 these cases of protection are of much complexity and in need of further 

 investigation. 



It is somewhat remarkable that congo-red is the only dye which I have 

 been able to make use of which shows this combination of non-diffusibility 

 through parchment-paper with existence of single molecules in solution. 

 The greater number pass through parchment-paper with more or less 

 rapidity, although showing many colloidal properties, so that no permanent 

 osmotic pressure can be obtained. Aniline-blue, with a molecular weight of 

 734, does not pass through. A solution containing - 266 per cent, gave an 

 osmotic pressure of 30"4 mm. Hg instead of the theoretical one of 

 68 - 8 mm. Hg, indicating a mean " solution-aggregate " of two molecules. It 

 must be stated, however, that this solution was apparently not free from 

 electrolyte, since, although when the experiment ceased no further permanent 

 rise in osmotic pressure was produced by change of water, there was still 

 a certain amount of "initial osmosis." The displaced water, also, showed 

 a fairly high conductivity, 40 gemmhos, which did not diminish during the 

 last five days. This outer solution was always stained faintly blue, due, as I 

 shall show in another paper, to dissociation of the dye and permeability of 

 the membrane to the products of this dissociation. In a similar way, the 

 dialysate of congo-red is very faintly red. In the ultra-microscope the 

 appearance of the solution removed from the osmometer was very like that 

 * ' Zur Erkenntnis der Kolloide, ; p. 116, Jena, 1905. 



