204 BIO-CHEMICAL JOURNAL 



the salt. In this respect they differ from the globulin of Hardy and 

 the serum-albumin of Pauli, which in acid or alkaline solutions assume 

 the sign of the charge of the H + or OH- ions respectively. They 

 are in fact more like certain complex ions, such as the ferrocyan ion, 

 Fe (CN) 8) which, although containing an iron atom is, nevertheless, 

 electro-negative. 



1 have been unable, for want of time, to test the behaviour of 

 more than a few typical dyes of the two classes, but it appears that 

 there is considerable difference of degree in the sensitiveness to 

 electrolytes. This property indeed seems to depend on the degree of 

 colloidality of their solution and the amount of their electric charge. 

 Each individual dye requires separate investigation. 



As to the explanation of the action of electrolytes, 1 think the 

 clue is given by the following considerations. V. Henri and Larguier 

 des Bancels, 1 in their work oh colloids, observed that gelatin, as 

 hydrogel, when immersed in a solution of anilin-blue or congo-red 

 in distilled water took up little or none of the pigment, and give as 

 the reason for this that both the bodies are electro-negative colloids 

 and therefore mutually repel one another. If, on the contrary, a 

 solution of a bivalent metal, such as barium nitrate, is added the 

 gelatin becomes deeply stained. The negative charge of the anilin- 

 blue is neutralized by the positive barium ions so that it can now freely 

 attach itself to the gelatin. This case then resolves itself into one of 

 the mutual precipitation of colloids, and the only doubtful point about 

 the explanation is whether the negatively-charged particles of the 

 gelatin hydrosol may be regarded as retaining their charge when 

 in the form of gelatine. There seems no a priori reason why this 

 should not be so, and there is also experimental evidence in favour of it. 

 Picton and Linder* in referring to the adsorption affinity of the 

 hydrogels of ferric hydroxide and arsenious sulphide for anilin dyes 

 state that these hydrogels ( retain the same selective affinity for the 

 dyes which will coagulate them ' as they possessed as hydrosols, viz., 

 ' the hydrogel of ferric hydroxide for anilin-blue, that of arsenious 



1. C.R. Soc. de Biologie, LIX, p. 132, 1905. 



2. Journ. Chem. Sot., 88, 1905, p. 1934. 



