ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 8&3 
viscosity, immediately after mixing ; upon standing, however, the viscosity 
progressively increases until after a few hours the product forms a soft 
gel which takes the form of the containing vessel; after a still further 
period the material separates into a stiffer gel and a clear supernatant 
liquid, about equal parts. On removing from the liquid, the gel pro- 
gressively dehydrates until it forms a tough material like leather, and 
on fully drying it forms a hard solid. This product is an adsorption 
compound containing C,,H,)O, and C,H,,0, molecules in the proportion 
1:2. Owing to the special properties of this material, it is employed for 
the purpose of tanning, the colloidal carbohydrate having a restraining 
influence on the tannic acid, so that concentrated tanning liquors can 
be employed at the commencement and tanning completed in a few days 
(C. V. Greenwood, Eng. Pat. 5,018, 1910, and 7,635, 1915; also C. F. 
Cross, “ Lectures on Cellulose,’ p. 38). On treating the material with 
water, tannic acid first dissoives, but on further dilution the complex 
becomes dispersed. Alcohol dissolves the whole of the tannic acid, pre- 
_ ¢cipitating.the carbohydrate in the form of a flocculent gel. Some salts 
such as alum, Fe,Cl,, FeSO,,and ZnCl,, which yield precipitates with tannic 
— acid, do not coagulate the hexosan-tannin complex, while others such 
as lead acetate, tartar emetic, SnCl,, Na,WO,, and ammoniacal copper 
solution coagulate it. 
The turbid complex becomes clear and much more viscous, é.e., more 
dispersed at a temperature of about 42° C., reverting to the original con- 
dition on cooling ; this change is strictly reversible. Alkalies and certain 
salts, e.g., sodium benzoate and KCNS, also strong formaldehyde, sugar, 
_ glucose, and glycerine, cause a similar dispersion, the effect of hydroxyl, 
ions and groups, being most marked in this respect. Acids appear to have 
very little action. 
’ The nuts of vegetable ivory (Phytelephas macrocar‘pa) and also probably 
_ of the dum dum palm (‘ Bull. Imp. Inst.’ 1911, 105-109), coffee, date 
_ Stones, &c., contain the carbohydrate mannan, which on hydrolysis with 
; acid yields mannose. Vegetable ivory consists of almost pure mannan, 
which has been carefully studied by 8. W. Johnson (‘ J. Amer. Soc.’ 1896, 
F 214-222). The nuts at present used in turnery work in place of real ivory 
__are very large, extremely hard and tough white seeds. On treatment with 
water it is not dissolved, but it is readily soluble in 70 per cent. sulphuric 
acid, from which it is precipitated unchanged by addition of alcohol and 
_ ether. The mannan thus formed swells in water and partially dissolves 3 1b 
_ is more readily soluble in alkalies. This would be a very useful product if a 
_ colloidal solution in water could be prepared. 
There is also contained in woods generally a gummy prod uct known as 
_ Xylan, which yields xylose on hydrolysis. This dissolves in solutions of the 
_ alkalies, but is insoluble in water. It is contained in large quantity in the 
_ alkaline liquors of paper-pulp manufactories, but has not yet been utilised 
for technical purposes. 
7 
Seaweed Jellies. 
Several algie yield colloidal products when boiled with water, but only a 
few have been utilised for technical purposes. They differ in composition 
and properties from the colloids previously described, containing less 
carbon and more oxygen, and in addition 0-15 to 1-0 per cent. of nitrogen, 
but in what state of combination is not known. They contain galactan and 
G2 
