56 REPORTS ON THE STATE OF SCIENCE.—1917. 6 
and the effect of adding either acid, base, or salt to the bath can also bé 
demonstrated. But the colloid-precipitation theory can by no means be 
directly applied to dyeing with acid dyes upon animal fibres in the neutral 
bath: the fibre charges itself negatively, the dye also has a negative charge, 
and no two colloids of the same sign will precipitate each other. This leads 
Pelet-Jolivet"* to revert to the adsorption theory, assuming that the 
dyestuffs act as unhydrolysed electrolytes in their solutions, and are 
adsorbed according to the valency of their combining ion, e.g.—the sodium 
salt of the colour acid should dye less strongly than the calcium or 
magnesium salt, and this less strongly than the alumiziium salt or than the 
free acid with its hydrogen ion; similarly the hydrochloride of a basic 
dyestuff should give weaker colours than the sulphate or than the phosphate, 
or finally than the hydrate with its OHion. This theory Pelet was able to 
demonstrate clearly by experiments both with acid and basic dyes upon 
wool ; he concludes that the dyes, as von Georgievics, Walker Appleyard, 
and Vignon had already stated, act as dissociable electrolytes in the bath 
and dye according to valency—the H and OH ions always having dis- 
proportionate action to their valency. 
These results also accord with his experiments (following Goppels- 
roder™) in capillary-ascent of various dyes (in water-solution) in strips of 
linen, cotton, flannel, and silk cloth, as well as filter-paper. Positive 
dyestuffs (colloid) cease to ascend at the actual surface of the liquid ; 
added acids cause them to ascend, and decrease the ascent of acid dyes ; 
alkalies will not cause acid dyes to ascend further, and actually decrease the 
ascent of basic dyes. 
The dyeing process by adsorption plus dissociation Pelet ingeniously 
explains by assuming an accompanying dissociation of the water ; the fibre 
becomes covered in the case of basic dyes by a ‘ double layer ’ of H and OH 
1ons, which, like those of the sodium, &c., base in acid dyes and the SQ,, 
&c., ion in basic dyes, are very small and mobile in contrast with the large 
organic ion, and therelore reach the fibre first and deposit themselves as a 
film upon it, giving it thus their charge. The organic ion of the dyestuff has 
an opposite charge to these, and fixes itself, by a process analogous to 
coagulation, upon them. The actual existence of this ‘ double layer,’ which 
by selection results in a fixation of the dye accompanied by a quantitative 
survival of the acid in the bath, has been demonstrated by Pelet, von 
Helmholtz, G. Quincke, and A. Pellot to exist on the surface of solid bodies 
immersed in water. Similar cases of chemical action subsequent to 
adsorption have been noticed by H. Freundlich and W. Neumann.4 
It would appear that in this theory—which takes account of the 
electrical properties of solutions colloidal and crystalloidal, of the 
phenomena of contact-electrification and surface-tension expressed in 
terms of capillary attraction, of the effect of temperature upon all these, 
without denying the possibility of chemical reaction between the fibre and 
the dye presented to 1t—lines are laid down upon which an adequate theory 
of the dyeing process may develop. 
2 Theorie des Farbeprozesses, p. 105, Pelet-Jolivet and N. Anderson, Koll. 
Zeitschr. 1908, 8, 206. 
43“ Verhandlungen der Naturforschenden Gesellschaft zu Basel,’ Koll. Zeitschr. 
1909-10, 4, 5, 6. 
ila Zeitschr. f. phys. Chem. 67, 538. 
