FIBER 



213 



usually from 1.50 to 2.00 inches in length. The tensile strength of 

 the fiber is estimated by the weight required to break a single strand. 

 This is usually 6 to 8 grams, but extreme breaking weights of 4 to 

 14 grams have been found. A fiber of cotton is about three times as 

 strong as a strand of wool in proportion to size. 



Fia. 80. The fiber of an upland short-staple variety. 

 FIG. 81. The fiber of an upland long-staple variety. 



Classification of Fibers. In every lot of cotton there are three 

 classes of fibers (Fig. 82) (1) unripe, (2) half -ripe, (3) ripe. 

 These may readily be distinguished by observing with a microscope 

 the extent to which they are twisted. The unripe fiber is cylindrical 

 and tubular in form for most of its length. It is transparent, some- 

 what turgid and shows little or no twist. As the fiber ripens its 

 tubular form collapses and contracts until finally it is much like a 

 twisted ribbon with somewhat thickened and corrugated edges. 



