ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 75 
only because of the remarkable results which have been obtained, but also 
in view of the difficulties which are encountered in many industries because 
of their presence. Generally speaking, colloids are more or less protective 
in their effect; that is to say, in the case of two solutions which in the ordin- 
ary way react to form aninsoluble precipitate which settles easily, in presence 
of a colloid either no precipitate is formed or the precipitate is much more 
finely divided, and refuses to settle, or it settles much more slowly. 
Problems of this kind often occur in chemical works, sewage works, and 
in water filtration, &c., the remedies being either application of heat or 
addition of some chemical which in itself is colloidal, or which forms a 
colloidal precipitate of opposite sign, as, for instance, alum, ferrous sulphate, 
potassium permanganate, &c.; in the case of sewage also the colloidal 
matter may be destroyed or flocculated by bacterial action. On the other 
hand, it may be advantageous to form a highly flocculent precipitate 
instead of a crystalline one, as, for instance, in steam boilers, the deposit 
then being easier to deal with—a result which is brought about by the 
addition of colloids, e.y., tannin and soda, infusion of straw, &c. The 
precipitation of mineral matter by slow diffusion of salts through colloidal 
jellies leads to the production either of spheroidal or nodular aggregates 
(Rainey, ‘On the Mode of Formation of Shells of Animals, Bone, &e.,” 
published in 1858 ; also H. B. Stocks, ‘ Precipitation of Carbonate of Lime 
under Varying Conditions,’ ‘ Jour. Soc. Chem. Ind.’ 1912, 21, 527) or to 
banded structure, which appears to explain the formation of mineral lodes 
(Hatschek, ‘ Jour. Soc. Chem. Ind.’ 1911, 256; F. 8. Shannon, ‘ Jour. Ind. 
and Eng. Chem.’ 1912, 526-528, and HE. Marriage, ‘ Z. Chem. Ind. Kolloide,’ 
1912, 11, 1-5). 
Other Soluble Gums. 
There are two other types of soluble gums differing from gum arabic in 
their properties which are worth considering: the one the ghatti-gums, 
because of their high viscosity, the other the wattle-gums, because of their 
abundance. 
Ghatti-gum is the product of Anogeissus latifolia, but the commercial 
varieties are mixed products containing the gums from several species, and 
therefore are very variable. The solutions of ghatti-gums are very much 
more viscous than those of gum arabic, and also contain more or less of a 
~ product which swells to a gel in water, but does not pass into the state of 
solution (metarabin). (‘ Colonial Reports,’ No. 63, Imperial Institute, 
‘Report, Gums and Resins,’ p. 160.) 
The moisture in these gums ranges from 4 to 7 per cent., mineral matter 
2 to 3 per cent., and potash neutralised on heating 0-22 to 3-99 per cent. 
(Rideal and Youle, ‘ Jour. Soc. Chem. Ind.’ 1891, 160). Ghatti-gum 
contains more araban and less galactan than arabic gums. 
The wattle-gums of Australia and South Africa are of two types— 
although there are many gradations—the one entirely soluble in water, the 
other leaving more or less swollen but insoluble gum. To the former group 
belong the gums of Acacia farnesiana, A. ferruginea, A. leucophlea, &c., 
and to the latter gums from A. decurrens, A. mollissima, A. vestita, &c. 
For a full account of the wattle-gums see J. H. Maiden (* Pharm. Jour. 
20, 869-980). Several of the wattle-gums contain a low mineral content, 
t.e., 1 per cent. or less, and they are all very low in viscosity. The wattle- 
gums are distinguished by containing a much greater proportion of galactan 
