ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 95 
future, and several patents have been taken out in this connection for 
separating ions from colloids, for removal of water from peat, for removal 
of impurities from clay, &c. 
Reverting again to the action of chromium compounds on gelatin, the 
effect of chromic compounds is also important since it has developed into an 
industry, that of ‘chrome tannage.’ According to Lumiére and Seyewetz 
( Bull. Soc. Chim.’ 1903, 1077), 100 grams of gelatin fix from 3-2 to 3-5 
grams Cr,Q, irrespective of the chromium salt employed. On the other 
hand, J. T. Wood (* Jour. Soc. Chem. Ind.’ 1908, 384) found that the amount 
of chromic oxide taken up varied with the concentration of the salt; in 
dilute solution it amounted to 5 per cent. and in a strong solution 18-6 per 
cent., showing that it is an adsorption phenomenon. 
Formaldehyde added to a gelatin sol raises the viscosity and finally 
solidifies it to an irreversible gel. This reaction is made use of in water- 
proofing, preparation of artificial silk, &. 
The maximum amount of formalin absorbed by gelatin is between 
4-0 and 48 grams per 100 grams of dry gelatin (A. L. Lumiére and A. 
Seyewetz, ‘ Bull. Soc. Chim.’ 1906, 35, 872-879). 
Compositions containing gelatin and glucose, invert sugar (treacle), 
or glycerine are employed in the manufacture of printers’ rollers, ‘ jelly- 
graphs,’ and for moulding purposes. 
The permanent softening effect which these products have upon 
gelatin is probably due to their OH groups. 
The gelatinising temperature or setting-point of gelatin is raised by 
partial purification by washing out some of the salts with water (K. 
Wenkiblech, ‘ Zeitschr. angew. Chem.’ 1906, 19, 1260-1262). 
Alum and aluminium salts also raise the setting-point or ‘ harden’ 
gelatin ; in this respect the alkali aluminates have no action. In the 
case of alum or aluminium sulphate, the salt is first adsorbed, then by 
washing with water the SO, is removed, finally leaving Al,O,. The 
amount of Al,O, adsorbed increases with increase in concentration up 
toa maximum of 3-6 per cent. Al,O, (A. L. Lumiére, ‘ Brit. Jour. Phot.’ 
1906, 53, 573-4). 
The most important reaction of gelatin especially in connection with 
tanning is that with tannic acid, the two being mutually precipitated 
in neutral or acid solutions, the gelatin then having a positive charge and 
the tannic acid a negative one. In a paper by J. T. Wood (‘ Jour. Soe. 
Chem. Ind.’ 1908, 384) it is mentioned that Humphry Davy (‘ Proc. Roy. 
Soc.’ Feb. 4, 1803) found the amount of precipitate to increase with the 
concentration, 100 parts of the coagulum containing 54 of gelatin and 
46 of tannin. Lipovitz (‘ Jahres. Forts. Chemie,’ 1861, p. 624) states that 
100 parts of dry isinglass precipitates 65 of tannin. Rideal (* Glue and 
Glue Testing,’ 1900, p. 111) found 42-7 gelatin and 57-3 tannin, Mulder 
(Allen, “Com. Org. An.’ iv. 463) says that 100 parts of dry gelatin precipi- 
tates 135 of gallotannic acid. R. Williams (Allen, p. 484), using 1 per cent. 
solutions of glue and tannic acid, and estimating the excess of tannic acid 
in the filtrate, arrived at the figures 77:5, 77-9, and 78-6 parts of tannin for 
100 of gelatin. Boéttinger, quoted by Procter (‘ Principles,’ p. 63) found 
66 per cent. of gelatin in the precipitate, which equals 50 parts of tannin 
to 100 parts of gelatin. These figures vary to such a degree as to oe 
that the reaction is not truly a chemical one. J. T. Wood (ibid.) found 
the amount of tannin precipitated by 1 part of gelatin to vary from 0-91 
