ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 59d 
Pieces of such colourless and transparent silicic-acid-gel thrown into 
colloidal dye solutions Zsigmondy found to behave as ‘ ultra-filters,’ the 
dispersion-medium being absorbed as by a sponge, and the dye-particles 
held on the surface of the gel-particles in a semiliquid state. Silk, cotton, 
and wool fibres show a similar hygroscopy and were described by Dreaper. 
as ‘ colloidal substances dry to the touch,’ like solid gelatine. Like it, they 
swell up in water and other solvents but do not dissolve; they may be 
looked on as irreversible gels, like silicic acid. Their ‘ pores,’ however, are 
not of the extreme minuteness of those of the latter substance : they do not 
show anything like the same optical homogeneity ; and therefore they are 
not ‘ ultra-filters ’ to the same degree. Moreover, the fact that they show no 
readiness to enter into solution, in spite of their ‘ swelling,’ causes Justin- 
Mueller 1% to suggest the name ‘ turgoids,’ not ‘ colloids,’ for them, and to 
call the process turgescence and not colloidal solution. He considers this 
turgescence of the fibre to be a necessary part of the process of adsorption of 
the dye ; colours that rub off do so because adhesion, not adsorption, has 
taken place, and the dye has been ‘salted out’ or coagulated on the 
fibre. Both the deposition and the adsorption are reversible according 
as the adsorption-coeflicient and solubility-coefticient approach each 
other. 
Some such laws appear to be followed as were noticed by Pelet-Jolivet 
and his assistants in recording the capillary attraction of filter-paper on 
various kinds of solutions; the height to which the coloured layer, or the 
layer that gives a direct reaction acid or basic, appeared to rise, was found 
to agree in a remarkable way with the observed electrical and colloid- 
precipitating effects of the same solutions. Schoenbein, W. Ostbald, 
Goppelsroder, and Fichter and Sahlbom have recorded the foilowing 
results: Alkali and acid solutions give reaction up to seven-tenths of the 
wetted paper; calcium carbonate only about one-tenth, and barium 
hydrate about three-tenths. Positively-charged colloids are found to be 
deposited on the paper at the surtace of the liquid, while negative colloids 
mount with the water. (Thus Fichter and Sahlbom refer to the currents 
set up in the capillaries of the paper: even in glass capillaries water rises 
much higher than basic dyestutf solutions or positive colloids in general.) 
Thus any influence which causes colloidal coagulation, or decreases solu- 
bility, limits the capillary rise and favours dyeing ;_ the influences which 
keep the particles small and disperse act in the other direction and are 
against rapidity of colouring. From this the actual value of ‘ assistants ’ 
in the dye-bath may be deduced, without postulating a liberation of the 
free dye-acid, which has been diversely shown to be little, if any, more 
effective pure than combined to form a dye salt. 
The work of Alexander? is of interest as showing the influence of 
protective colloids on dye solutions. He has previously pointed out 
that, after the addition of protective colloids (gelatine &c.) to solutions 
of benzopurpurin, dilute acids produce colour changes analogous to 
those adsorbed in the dyed animal fibres. In the case of a dilute solution 
of benzopurpurin, addition of dilute mineral acid quickly changes the 
bright red colour to dark blue, and stronger acid coagulates the dye 
which settles out of solution. If gelatine is added to the benzopurpurin 
solution, dilute mineral acids give. a claret-red solution, and stronger 
102 Chemikerzeitung, p. 845, 1914. 
103 Journ, Soc. Chem. Ind. 1911, p. 517. 
