THE MICROSCOPE. 41 



The fibres of cotton and linen are not effected by water, alcohol, 

 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 desolves the 

 silk or wool fibres and leaves the cotton or linen uneffected. 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 alkaline filtrate be 

 treated with the acetate of lead the silk will give a white percipitate 

 and the wool a black percipitate. 



Wool contains a certain amount of sulphur — silk being entirely 

 free from it. This gives us another means of detection. In a solu- 

 tion of plumbate of soda, wool becomes black while silk is not 

 effected. 



