ON COLLOID CHEMISTRY AND ITs iNDUSTRIAL APPLICATIONS. 93 
which is moderately impure, a portion may diffuse from the swollen gel 
into the water. 
Gelatin is used for food purposes, as an adhesive, in sizing paper and 
cloth, in the manufacture of photographic plates, printers’ rollers and 
similar compositions, and as a size for painters’ use. 
In addition to having considerable viscosity, gelatin or glue solutions 
show in the most marked manner the properties of adhesiveness or 
~ stickiness,’ which renders it so useful in many different trades. It also 
forms continuous films on drying, these being perfectly transparent when 
the material is pure, and having considerable toughness, except when the 
natural moisture is dried out, when it becomes brittle. 
On long-continued heating, gelatin solutions lose viscosity ; heating 
with acids and alkalies has a more pronounced effect, in some cases the 
product remaining fluid even at the ordinary temperature (basis of liquid 
glues) ; these products, however, have less viscosity and adhesiveness than 
the original material. Impurities, both organic and inorganic, tend to 
produce a similar effect, thus fish glue is often quite fluid and does not set 
to a gel, even when concentrated. 
Moore and Roaf (‘ Biochem. Jour.’ vol. 2, 39) found the osmotic 
pressure of a 10 per cent. gelatin, using a parchment paper membrane, to be 
90 mm. at 40°, and 158 mm. at 91°. When injected into the blood, 
gelatin has the remarkable property of increasing its coagulability (H. 
Grau, ‘ Deutsche medizinische Wochenschrift,’ 1910, 27, also Schultz, as 
quoted by Umber, ‘ Zeitschr. fiir arztliche Fortbildung,’ 1912, No. 20). 
Gelatin swells in glacial acetic acid and on heating forms a fluid sol, used as a 
liquid glue ; it is readily dissolved by crude carbolic and cresylic acids. 
It dissolves to the extent of 40 per cent. in a saturated solution of urea, the 
sol reverting to a gel on dialysing. 
Gelatin is powerfully affected by even weak solutions of NaHO, KHO, 
and LiHO, which cause solution even in the cold ; in presence of ammonia it 
swells considerably but does not pass beyond this stage. Alkaline car- 
bonates and tribasic phosphates have the opposite effect, retarding solution, 
and with concentrated solutions entirely preventing solution even at 100° 
(A. L. Lumiére and A. Seyewetz, ‘ Bull. Soc. Frang. Photo.’ 1912, 3, 
159-163). 
The effect of dilute acids, alkalies and salts upon gelatin has been closely 
studied by H. R. Procter (‘ Kolloidchem. Beihefte,’ 1911, 2, 243-284), who 
found that the swelling of gelatin in highly ionised acids is considerably 
greater than in water, but weak acids, e.g., H,BO;, CO; and SH», have not 
the same effect. Some of the acid is fixed either as a salt or by adsorption, 
the whole reacting to phenolphthalein but not to methyl orange. The 
' swelling is partly due to the hydrogen-ion concentration. Swelling is 
retarded by salts, especially when concentrated. Alkalies also cause the 
swelling of gelatin, but in this case the swelling is not inhibited by neutral 
salts, and therefore appears to be due to the H-O ions and not to the 
cation. See also M. H. Fischer and A. Sykes (* Les Matiéres Grasses,’ 1914, 
4202-4204). The last-named authors state that with non-electrolytes the 
maximum dehydrating effect is observed only with high concentration, 
whereas, with electrolytes it is attained at a lower concentration, there being 
only slight increase in action for each successive increase in concentration. 
According to J. L, de Bancels (‘ Compt. Rend.’ 1908, 146, 290-291), 
gelatin dissolves“in water in presence of certain salts at the ordinary 
