ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 77 
phenol-phthalein as indicator (T. von Fellenberg, ‘ Mitt. Lebensmittel, 
unters. Hyg.’ 1914, 5, 56-259). 
Tragacanth gives with borax a siightly more viscous mass, a reaction 
which is sometimes made use of in detecting adulteration (W. L. Scoville, 
‘ Drugg. Cir.’ 1909, 53, 116-117; also H. C. Fuller, ‘ Amer. Jour. Pharm.’ 
1912, 84, 155-158). It can be distinguished by not containing oxydates. 
Tragacanth yields a much more viscous solution than gum arabic, 
but does not form a continuous film on drying, breaking up into large 
curved flakes. The viscosity is lowered by heating under pressure. Traga- 
canth more nearly resembles starch than gum arabic in its properties. 
It shows only a slight osmotic pressure, 7.e., a 0-72 per cent. solution gave 
a pressure of 5 m.m. at 17° C. with a parchment paper membrane (Moore 
and Roaf, ‘ Biochem. Jour.’ 2, 39). 
The commoner qualities of gum tragacanth, known in the market as 
‘gum trag’ or ‘ hog gum,’ is in larger and thicker pieces, white or yellow 
in colour, and often quite opaque. It yields a solution which is more 
pasty than real tragacanth and also contains more insoluble matter. The 
moisture in this gum is about 11 to 12 per cent., mineral matter 3-16, 
and potash neutralised after heating 8-14 per cent. 
For certain purposes mixtures of two or more colloids are sometimes 
preferable to a ghd one (F. Beckmann, Ger. Pats. 219,651 and 223,709, 
1908.) 
In a general paper on thickenings for calico-printing from a colloid 
point of view, E. Austin-Miiller (‘ Chem. Zeit.’ 1910, 34, 598-599) divides 
them into three classes: 
(1) Homogeneous, 7.e., those which dissolve to a clear sol which can 
be filtered through paper—examples, gum arabic and better qualities 
of gum senegal. 
(2) Heterogeneous. Those which form two phases, a sol and a gel, 
the latter retained by a filter, ¢.g., tragacanth. 
(3) Heterogeneous. Those forming micella-sols, which are entirely 
retained by a filter, ¢.g., starch paste. The latter can be rendered more 
homogeneous by acetic acid. The products obtained from starch by roast- 
ing, 2.e., soluble starches and dextrins, fall between classes 2 and 3 or 
1 and 2, according to their properties. By addition of NaHO they form 
homogeneous sols. : 
“ese 
Insoluble Gums. 
The gums which are insoluble in water exude naturally from cherry 
and peach trees, &c. ; there are also certain gums found in India, known 
as Indian gum or Bombay gum, the products of species of Sterculia, and 
the gum of Cochlospermum gossypium, which have the remarkable property 
of spontaneously evolving acetic acid. H. H. Robinson (‘ Jour. Chem. 
Soc.’ 1906, 89, 1496) regards the latter as an acetyl derivative, and found 
that on distillation it yielded 14 per cent. of acetic acid. 
Bombay gum and the gum of Cochlospermum gossypium are very similar 
im composition. They lose 19 to 214 per cent. on heating at 100° C. (water 
and acetic acid), and contain 7 to 9 per cent. of mineral matter. On 
heating with alkali, the amount of KHO neutralised equals 13 to 16 
per cent. They contain galactan and araban, but less of the former 
than in gum arabic; there is probably a third carbohydrate present. 
On treatment with water they swell up very quickly to form nearly 
