16 COTTON 



When cotton is immersed in a strong solution of 

 caustic soda it undergoes certain remarkable changes, 

 the chief of which is the production of a silky lustre. 

 This phenomenon was discovered by John Mercer in 

 1844, and has led to results of great commercial 

 importance. The process is now carried out on a very 

 extensive scale under the name of " mercerisation," 

 and is sometimes applied to the yarn and sometimes 

 to the woven fabric. The material is either kept in a 

 state of tension during the operation, or is allowed to 

 shrink in the alkaline liquid and afterwards stretched 

 to its original length before being washed. The 

 process not only produces an exceedingly high lustre 

 on the cotton, but also increases its power of absorbing 

 dyes. The structure of the cotton fibre undergoes 

 alteration ; the flattened, twisted tube characteristic 

 of ordinary cotton swells out, and is thus converted 

 into a hollow, cylindrical form. This change in form 

 is accompanied by a decrease in length and also by an 

 increase in tensile strength, mercerised cotton being 

 in some cases as much as 50 per cent, stronger than 

 ordinary cotton. Egyptian cotton generally mer- 

 cerises better than ordinary American cotton. 



When cotton is treated with a mixture of nitric 

 and sulphuric acids, it is converted into nitrocellulose 

 or gun-cotton. This product has the general appear- 

 ance of ordinary cotton, but is rougher to the touch, 

 grating when rubbed between the fingers, and is in- 

 elastic. It explodes on percussion, and burns with a 

 flash. The combustion of gun-cotton is accompanied 

 with the liberation of a large volume of gas, to which 

 the force of the explosion is due. On dissolving nitro- 

 cellulose in ether, or a mixture of ether and alcohol, the 

 product known as collodion is obtained, which, when 

 painted on a surface, dries to a thin film. Collodion 

 is employed for closing wounds, for the preparation 

 of photographic plates, for the manufacture of cellu- 

 loid and other purposes. Gun-cotton is also em- 

 ployed in the manufacture of celluloid. 



