ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 5D 
to show that the electrolyte-coagulation theory is not quite adequate 
to explain all dyeing phenomena. When one colloid so unites with the 
other it forms a ‘ protective colloid’ and shows the utmost reluctance 
to coagulate. This is connected with P. P. von Weimarn’s ‘ Grundgesetz 
der Dispersoidologie’ : ‘When a substance suffers physical or chemical 
division it strives with all possible means for a diminution of its 
free surface energy.’ This diminution of surface energy may take 
place by coagulation, which, however, is impossible so long as all 
the particles have the same charge, and only takes place in presence of 
an electrolyte or under influence of a current or by deposition upon a 
surface whose charge is opposite to their own, giving adsorption according 
to Freundlich’s rules. Perrin’s"® and Pelet-Jolivet’s researches into 
the contact charges which appear on various solid substances, including 
textile fibres, when brought into contact with liquids show, not only 
a definite ‘latent heat of adsorption,’ but also a charge (measured by 
the electrosmosis of the liquid through the diaphragm) in the case of 
ionising solvents, but not of non-ionising solvents like chloroform, petro- 
leum, and benzol. All solid substances take on a positive charge in an 
acid liquid, a negative charge in an alkaline liquid; the difference of 
potential is greater when the number of H ions is greater, and smaller 
in presence of a greater number of OH ions. These phenomena show 
a remarkable parallelism to the results observed in the coagulation of 
colloids and point to some deep-seated analogy. Wool, cotton, silk, 
&e., follow the general charge-law in acids and alkalies, but they all 
take a negative charge already in pure distilled water; this charge is 
increased in contact with alkaline solutions and decreased in acid 
solutions—with silk and wool it is possible to show an actual change 
in sign of charge. 
The work of Linder and Picton '! may be mentioned here. They stated 
that dyeing is connected with the electrical charges which substances carry 
- when existing in colloidal solution. They experimented on colloidal ferric 
hydroxide and basic dyes which carry positive charges and arsenious 
sulphide and acid dyes which are negatively charged. Hence—as two 
oppositely-charged colloidal solutions precipitate one another—soluble 
blue (acid) precipitates ferric hydroxide, but methyl violet does not. 
Exactly the reverse process occurs with arsenious sulphide. Ammonium 
sulphate also precipitates colloidal ferric hydroxide, but they found a 
distinction between its action and that of the dye. A definite quantity only 
of the salt is required to precipitate the colloid, and more added remains in 
solution. However, with the dye, after the colloid had been precipitated the 
dye continued to be adsorbed by the ferric hydroxide as a whole. They 
explained this by assuming that the precipitated colloid still retained a 
portion of its original charge, and that by virtue of this it still continued to 
take up more dye. The same kind of action was supposed to take place in 
dyeing—the fibre taking the place of ferric hydroxide or arsenious sulphide. 
These facts will account for the phenomena of dyeing wool with a basic 
dyestufi of the methylene-blue type: it is an evident case of mutual 
discharge and consequent precipitation by two oppositely-charged colloids, 
etl Loc. cit. 
4 Journ. Chem. Soc. 1905, pp. 1931-1935. See also Journ. Chem. Soc. 1892, 
61, 114, 137, 148; ibid., 1895, 67, 63; ibid., 1897, 71, 568. 
