VEGETABLE DYES 103 
are all built upon a basis of proteins. That is, their 
chemical constituents, while very complex, are yet 
relatively reactive. They contain groups, analogous to 
ammonia and carbonic acid, arranged alternately on 
immense chains, many hundreds of atoms long. Each 
group in the chain still retains some of its original 
qualities ; so that the proteins are capable of acting 
either as weak bases or as weak acids, as occasion re- 
quires. This is very important from the dyer’s point 
of view, for it means that either acid or basic dyes (from 
neutral or slightly acid solution) will be absorbed and 
held by animal fibres. 
The vegetable fabrics, cotton, artificial silk, linen, 
jute, hemp, etc., are all composed of cellulose, an 
extremely inert carbohydrate which is insoluble in 
almost everything and has practically no chemical 
affinities. Dyeing upon vegetable fibres is consequently 
a far more difficult process, and there are very few dyes 
which can be applied to them direct. The most 
general special treatment for rendering vegetable fibres 
amenable to dyeing is to cause them first to absorb 
tannic acid (found in galls, sumach bark, tea leaves, 
etc.), and then, upon the tannic acid groundwork, basic 
dyes may be applied from a solution made faintly 
alkaline by soap, lime, or soda. Acid dyes cannot be 
used for vegetable fibres. 
The dyes themselves may be roughly classified as : 
(a) Direct or substantive, when they are applied with- 
out further treatment; (6) Pigment, or vat dyes, when 
the colour is developed upon the fibre ; and (c) Mor- 
dant, or adjective, dyes when they are ‘fixed by means 
of metallic bases. 
