Microscopical Examination of Fibers. 33 



5. Behaviour with reagents. 



6, Relations to polarized light. 



Animal fibers are either silk, feathers or hair. The smooth, 

 solid, cylindrical form of silk fiber is too well known to require 

 much description. The length is indefinite, and the diameter 

 uniform or nearly so, which is a marked peculiarity. The ends 

 are square, and, as seen in manufactured goods, there are usu- 

 ally particles of stiffening, etc., adherent to the fibers, which in 

 certain forms seem to be regarded by some authors as perma- 

 nent, and there is entire absence of cell structure. 



Feathers, either in whole or part, have at different times been 

 employed as fibers. They are usually covered with sharp barbs 

 arranged at uniform distances, and may often be sharply differ- 

 entiated from cotton, with which they are generally mixed in 

 woven fabrics, by polarized light, in which they are quenched 

 while the cotton glows brilliantly. 



Feathers, in structure, are modified hairs, and display a some- 

 what similar arrangement of cells. 



But most textile fabrics of animal origin are composed of 

 hair, which varies from the rigid spines of the porcupine to the 

 softest and most delicate fur or wool, without changing its type 

 of structure. All hairs are composed of short overlapping scales 

 forming a kind of tube, more or less serrated on the surface, 

 and inclosing one or more rows of medullary cells arranged in 

 symmetrical and characteristic modes, enabling the microscop- 

 ist to assert with considerable certainty the animal from which 

 they are derived. The shape is usually tapering ; often the 

 same animal wears two or more distinct kinds, as the fox, seal, 

 or cashmere goat, coarse long hair forming the outer coat, and 

 fine curly wool the inner. The duck bill {O rnithorhynchus) of 

 Australia, and the common water mole {Scalops agiiaticus)^ have 

 hairs very long, slender, and with the ends flattened out like a 

 trowel.* 



All fibers of animal origin when burned give a disagreeable 

 odor, and leave a crispy coal, while those of vegetables con- 

 sume more perfectly without smell. 



Both silk and wool are soluble in strong hydrochloric acid, 

 the solution being hastened by heat, but in dilute acid silk is 

 soluble and wool is not. Vegetable fibers in the same reagent 

 are disintegrated but not dissolved. 



* See Micrographic Dictionary for figures of a variety of hairs, feathers, etc. 



