ON COLLOID CHEMISTRY AND 1's INDUSTRIAL APPLICATIONS. 33 
the particles are electrically charged, and which combine with the fibre 
in the same way as the tannin particles may be supposed to do, though 
probably with an opposite charge (v.s.). 
The oil squeezed out and known as moellon or dégras is a natural 
emulsion, and finds wide use in leather-dressing for the ‘ stuffing’ of 
light leathers. This stuffing, the primary object of which is to lubricate 
the fibres and make the leather supple and water-resisting, may in many 
cases be also regarded as a supplementary and partial oil-tannage. The 
fats are applied to the moist leather either by hand as a pasty mixture 
of oils and harder fats, or in a melted state in a heated rotating drum. 
In the first method the main effect of the harder fats is to retain the mixture 
on the surface until the oils are absorbed. The water in the leather lowers 
the surface tension between oil and leather at the interface, and as the 
water dries out the oil replaces it by capillarity, leaving the harder fats 
outside. The surface tension of the various fats with regard to water 
and their consequent easy emulsification is thus of great practical im- 
portance. 
A third way of applying fatty matters to leather much used for chrome 
and other light leathers, and called ‘ fat liquoring,’ consists in drumming 
the skins with a prepared emulsion, which at first was the alkaline liquor 
from the washing of oil-leathers, but is now usually an artificial mixture 
of oils and soaps, though occasionally acid emulsions are employed. It 
has been found that sulphonated oils, especially castor and fish oils, have 
extraordinary emulsifying powers even on hydrocarbon oils, and the 
writer has examined a commercial product containing 80 per cent. of 
mineral oil, which yet was perfectly and spontaneously emulsifiable when 
poured into water. The question of surface tension at interfaces and 
against solid surfaces is one of much technical importance, and probably 
its effect on adsorption is greater than that of the Willard Gibbs law that 
“substances which lower surface tension accumulate on that surface.’ 
The action of protective colloids on metallic sols has been explained 
- as due to the fact that the surface tension of the medium at the metallic 
surface is greater than the sum of the tensions of the medium and the 
metal with regard to the protective colloid, which therefore spreads in 
a thin film between them. This coating of the metal by the colloid is 
of course an adsorption; and a similar action may account for many 
cases of the latter which are called ‘ anomalous,’ that is, to which_the 
Willard Gibbs law does not apply. 
BIBLIOGRAPHY. 
General Physical Chemistry of Gelatine, &c. 
1912. 
TRUNKEL, ‘ Pharmaceut. Centralhalle,’ No. 38, 52 Jahrg. (Abst., ‘ Coll.,’ 1912, 11, 
146.) 
“Ueber Leim und Tannin.’ 
Volumetric determination of the proportions of gelatine and tannin giving com- 
plete precipitation. Sensitiveness of indicators. 
*Pantker, M. A. K., and Strasny, Epmunp, ‘Trans. C.S.,’ 99, 1911, 1819, and 
*Coll.,’ 1912, 11, 9. (Abst., «J.A.L.C.A.,’ 1912, 7.) : 
“An investigation into the acid character of gallotannic acid.’ Determination 
by catalysis of diazo-acetic ester. Acidity very small, but more than can be attri- 
buted to hydroxyl. The most carefully purified gallotannin is probably not a single 
substance. 
1917. D 
