COMMERCIAL FIBRES. 117 



The fibres of silk are long, slender and rod-like, with occasion- 

 ally one having a flattened side. When broken the ends separate 

 with a straight or a smooth fracture. They are solid, having very 

 much the appearance of glass rods, no cells, no central opening, no 

 structure whatever. Averaging 1-1600 of an inch in diameter, 

 though some are even 1-800 of an inch in diameter. Their average 

 size is the smallest of the commercial fibres. They have a clear, 

 white color when unstained, and are semi-transparent, and highly 

 refractive. 



Wool fibres are either cylindrical or oval. The surface of the 

 fibre is covered by minute cells, lying one upon another like shingles 

 on a roof, or like scales on a fish, though each scale is bordered by 

 a waving line. The value of wool for felting depends on the pro- 

 portion and size of these epidermal cells or scales. Wool fibres are 

 remarkable for their softness, flexibility and wavyness. These cells 

 are most beautifully seen in white hairs that have been thoroughly 

 soaked in oil of turpentine, and mounted in Canada balsam. Soak- 

 ing wool fibres in a solution of soda will separate the epidermal 

 cells or scales from the rest of the fibre. Hairs of some animals 

 polarize light. An interesting object of this kind may be made by 

 placing two series of white hairs of a horse in Canada balsam so as 

 to cross each other at an angle and viewing them by polarized light. 



The fibres of cotton and linen are not affected by water, alco- 

 hol, ether, benzol or any weak solution of the acids or the alkalies, 

 not even when raised to the boiling point. But strong solutions of 

 either acids or alkalies, when applied with gentle heat, will slowly 

 destroy the fibres. 



A simple test is the brilliancy of the coloring matter taken by 

 the different fibres, for the aniline dyes give a strong permanent 

 color to silk and wool, while in cotton it is merely surface color and 

 easily washed out. 



If you are called' upon to examine a piece of dress goods, or 

 any material in which the presence of foreign fibre is suspected, 

 take a small piece of the goods and boil it for a few minutes in a 

 solution of soda (ten parts of soda to ninety parts of water). It 

 dissolves the silk or wool fibres and leaves the cotton or linen unaf- 

 fected. It is possible to estimate very nearly the proportion of the 

 mixture of the animal and the vegetable fibres, by filtering carefully 

 the residue and comparing with the known amount taken. If the 



