32 REPORTS ON THE STATE OF SCIENCE.—1917. 
to form colloidal, rather than true ionic solutions, and the particles are 
negatively charged, going to the anode in electrophoresis. Whether 
the change is due to ionisation or to the fixation of an electrolyte ion 
is immaterial for our purpose. The gelatinous fibres, as we have seen, 
take a positive charge in acid, and a negative one in alkaline solutions 
(v.s.). Hence in faintly acid solution, which produces the strongest 
positive charge, they attract and precipitate the tannin particles, while 
in alkaline solution no tannage takes place, and in those too strongly acid, 
the tannins themselves are precipitated.'* In fact, such colloid pre- 
cipitations due to electric charges do not seem to differ in principle 
from ionic reactions, though owing to the varying size of the particles 
and of their charges they are less definite and quantitative. Whether 
ultimately any closer combination with the fibre ensues, as is suggested 
in the case of mineral tannages, remains for the present uncertain, but 
in long-continued tannage there is a further deposition of difficultly 
soluble matters on and between the fibres by forces generally called 
‘ adsorption.” We may thus divide vegetable tannage into two stages, 
in the first of which the tannins combine electrically or chemically 
with the fibre and render it msoluble, and in the second matters are 
deposited upon it which add to the weight and solidity of the leather ; 
but of course the two stages overlap in time, and the different qualities 
of leather produced by different tannages are largely due to their relative 
proportion, and the amount of precipitable matter which the tanning 
materials contain. It does not appear that the same affinities are saturated 
in mineral and vegetable tannages—chromed leather will fix as much vege- 
table tannage as raw hide, and vice versa; and corresponding differences 
occur in their behaviour to dyestuffs. 
Besides the mineral and vegetable leathers there is a third class which 
demands consideration. If raw skins are fulled with oxidisable oils, their 
water is gradually expelled and replaced by the oil, and if the skins are 
now allowed to oxidise (which they do with considerable liberation of 
heat and of acryl aldehyde and other volatile products), and are then 
freed from unfixed oil by pressing and subsequent washing with alkaline 
solutions, such leathers as ‘ chamois,’ ‘ wash-leather,’ ‘ buckskin,’ and 
‘ puff-leather’ are the result. Oil leathers, like chrome leathers, are 
very resistant to hot water, and also to hot soap or alkaline solutions, 
and may even be shrunk or ‘ tucked’ to increase their thickness and 
solidity by dipping in these liquids at boiling temperature. Their re- 
sistance to hot alkaline solutions, in which all oxidised oil products are 
soluble, proves that something more has occurred than a mere coating 
of the fibres with oils, but a full explanation has not yet been given. 
Since aldehydes are known to produce insoluble conjugated products 
with hide fibre, the explanation that acryl aldehyde (derived from the 
glycerine by dehydration) was the active agent was a plausible one, 
but is negatived by the recent knowledge that equally good leathers 
can be made with the free fatty acids alone. This, however, does not 
altogether disprove the aldehyde theory, since the unsaturated oils 
which alone will chamois are apt on oxidation to break at a double 
linkage with the production of higher aldehydes. Another possibility 
is that these oils, which are more or less colloid, form emulsions of which 
18 7',C.8., 1916, 109, 1329. 
