72 MICROSCOPIC OBJECTS. 



Cotton. — The fibres of cotton are readily distinguished 

 under the microscope from those of hnen, wool, &c. 

 Cotton fibres are tubular, and are formed of pure cellular 

 tissue. They grow in the fruit-pod surrounding the seeds. 

 These tubes, from the thinness of their sides, often col- 

 lapse ; hence they appear like flat ribbons or bands ; but 

 the fact that they are hollow may be proved by immers- 

 ing them in a drop of Canada balsam and heating it over 

 the flame of a candle, placing the slide on the stage of 

 the microscope while warm, when portions of the fibre 

 containing air will be seen dark, while those filled with 

 balsam are clear and transparent ; as the shde cools the 

 air in the tubular fibres contracts, and the balsam is seen 

 in the act of filling them. The reason assigned for the 

 preference given to hnen (flax) over cotton for lint, for 

 surgical purposes, is that the fibres of the former are 

 sohd cylinders, and those of the latter flattened bands, 

 the edges of which are supposed to irritate the wounds. 

 Cotton, which is almost entirely carbon, does not afiect 

 polarized light. The newly discovered explosive gun- 

 cotton transmits a hne of light when polarized. The 

 commercial value of cotton fibres, usually called cotton 

 wool, depends upon the length of the staple or fibre, and 

 their colour. The finer kinds of muslin are made from the 

 unripe pod fibre. 



Membrane without fibre. — If the seeds mentioned under 

 this heading be immersed for a short time in water, they 

 become covered with a white fiocculent matter, which on 

 examining it under the microscope will be found to be 



