32O THE FORAGE AND FIBER CROPS IN AMERICA 



This table shows that in proportion to its weight, cotton has 

 three times and silk four times the tensile strength of wool. 

 The value of hemp and manila hemp for cordage is also em- 

 phasized. The full tensile strength of fibers is not utilized when 

 made into cordage or fabrics. In the case of cotton only 20 

 to 25 per cent, of the total breaking strain is realized when 

 spun into yarn. 



Vegetable fibers in general are less elastic and less hydroscopic 

 than wool or silk. In this respect raw cotton is no exception, 

 although the moisture content, normally seven to eight per cent., 

 does vary with the atmospheric conditions, and hence is a factor 

 in the purchase as well as in the spinning of cotton. Although 

 raw cotton is rather non-absorbent to water, it becomes ex- 

 tremely absorbent when the waxy and fatty matters are re- 

 moved. 



409. Structure of Fiber. Aside from its natural impurities 

 consisting chiefly of pectic acid, coloring matter, wax, oil, and 

 proteid matter, the cotton fiber may be divided into three 

 parts: (i) the main cell wall, probably pure cellulose and con- 

 stituting 85 per cent, of the fiber; (2) an outer membrane or 

 skin; and (3) an inner membrane or wall of the central canal. 

 Both of these membranes are less soluble in a solution of am-- 

 monio-copper oxide than the cell wall. The former is probably 

 modified cellulose, while the latter is a thin layer of dried proto- 

 plasm which was contained in the living fiber. Some authorities 

 recognize a secretion within the canal itself corresponding to the 

 pith of a quill. 



When a fiber is treated with a solution of ammonio-copper 

 oxide the fiber swells, but not throughout its whole length. 

 According to some authorities the ligatures which prevent the 

 swelling of the cell walls at intervals are distinct from the outer 

 membrane or skin, while according to others they are believed 

 to be sections of the outer skin which have resisted the action 

 of the reagent. 



