ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 47 
by F. Krafft ‘’ to the theory of dyeing. His summary of the facts then 
known is this: a colloid solution or system contains the solute in mole- 
cular form; the molecules are large in mass and volume, and tend to 
form systems rotating round one another. When the gel coagulates 
these spheroid forms become rigid—the solid is not ‘amorphous’ but 
* globomorphous ’ (this is not true unless the disperse phase occupies no 
more than 74 per cent. of the total volume of the sol ; 48 above 74 per 
cent. the globules become flattened to dodecahedra, with walls, of in- 
creasing tenuity, consisting of the continuous phase—but such fine walls 
are rare Save in the soaps). Krafft goes on to state that in dilute solutions 
the soaps are hydrolytically dissociated, but the constant interchange 
of +ve and —ve ions causes the neutral reaction to persist and the solution 
to remain ‘clear.’ His proof of dissociation is that the melting-point 
is that of stearic acid, not of sodium stearate. Hence Krafft’s theory 
of the dyeing process is that it consists in the deposition of colloids in 
or on the fibre in the form of globules or membranes, very plastic, which 
(like the soaps) have the power of clinging to solid bodies. Dyes which 
have a small molecular weight must be presented to the cotton fibre in 
the form of colloidal compounds with a mordant, which is in itself always 
a colloid, to form colour lakes. Many dyes are colloids in water and not 
in alcohol; the substance is hydrated and forms immense molecules. 
Colloid solutions of iron hydroxide, aluminium chloride, 7.e., in water, 
will all form colour lakes which ‘ fall out of solution’ (z.e., coagulate) 
at 0° C. (see Zsigmondy’s experiments in freezing sols). Their tough, 
plastic, clinging nature all makes for good dyeing, e.g., alizarin red in 
presence of a fatty acid of low melting-point forms a colloidal membranous 
deposit, Turkey red. Direct dyes are mostly colloids of more or less slight 
solubility, ¢.g., benzopurpurin. The direct cotton colours were supposed 
to exist in the colloidal condition to a much greater extent than the dyes 
of the acid and basic groups, and this would explain them being taken 
up direct by the cotton fibre. Wool and silk enter into combination with 
dyes, forming membranous colloidal salts; leather in tanning forms a 
similar surface. Hence his theory is that the dyer ‘imitates Nature’ 
in ‘forming a protective insoluble colloid membrane on the outside of 
the fibre.’ Biltz*® also showed that colloidal solutions of inorganic sub- 
stances like selenium, tellurium, gold or molybdenum blue would dye 
wool or silk, and that analogously with organic dyestufis an electrolyte 
(salt) hastened, while a protective colloid retarded, the process. 
Later research has confirmed much of Krafit’s work, if it has also 
served to point out his errors, most glaring of which is of course that of 
the * protective exterior membrane.’ Certainly the adsorption process 
is by its very nature largely confined to the surfaces, but the entire struc- 
ture of a sol (or gel) may be permeated by another substance, colloid or 
crystalloid, and then, according to McBain and Zsigmondy, the term 
‘ sorption ’ is more descriptive of the phenomenon. Sisley takes exception 
to the use of the word ‘ adsorption’ which is now largely used in colloid 
chemistry to indicate the extraction of a dissolved body by a solid. He 
submits that the word is no improvement on absorption and that so-called 
adsorption compounds are in no way distinguishable from chemical 
47 1896-9, Berichte, 27, 28, 29, 30, 32. 
‘® Zsigmondy, p. 67, 157 et seq. 
49 Journ, Soc. Dyers and Col, 1904, p, 145; 1905, p. 276, 
