330 CHEMICAL DISCOVERY AND INVENTION 



separation of the water in such cases very great pressures are 

 produced. When immersed in the dye-bath the deposition of 

 the colouring matter takes place in a somewhat irregular manner, 

 some of it forming a layer external to the fibre and some pene- 

 trating into the interior. This can be seen under the microscope. 

 The probability therefore is that the attachment of the dye- 

 stuff to the fibre is due in the first instance to mere surface 

 attraction or adsorption, and that the colouring matter is then 

 retained, partly, at least, by chemical combination. Some dyes 

 can be removed from dyed fabrics by ordinary neutral solvents 

 such as alcohol. This does not prove that chemical combination 

 has not taken place between the dye-stuff and the fibre, and 

 that the union is comparable with mere surface adhesion ; it 

 only seems to indicate that the saline combination formed is 

 weak in character. The picric acid combination with the basic 

 elements in wool fibre, for example, may in fact be similar to 

 the combinations formed by weak bases or acids which are more 

 or less completely decomposed by water. Thus urea combines 

 with nearly all acids forming crystalline compounds which, 

 however, possess a definite composition only when deposited in 

 the presence of excess of acid. Any attempt to recrystallise 

 from pure water leads to the reproduction of the base, while the 

 acid passes into the liquid. 



From these facts it appears that the process by which colour- 

 ing matters become attached to vegetable or animal fibres and 

 to mordants is more complicated than was supposed, and the 

 phenomena of the dye-bath are for the present not fully under- 

 stood. 



Linen and cotton consist of cellulose, a -compound or mixture of 

 compounds which possesses neither acid nor basic character, 

 and accordingly dyes do not attach themselves to the fibre in 

 consequence of chemical combination of the kind just referred 

 to. Cotton, however, is dyed by utilising the action of mordants 

 as already explained in connection with alizarin, and there are 

 substantive cotton dyes which work without the application of 

 a mordant. 



Safflower (Carthamus tinctorius) yields a natural red colour 

 which is one of these, but many artificial dyes made from coal- 

 tar are now used as direct cotton dyes. The first to be discovered 

 was Congo red. 



In many cases of this kind the retention of the colour by the 



