SMITH S INTERMEDIATE CHEMISTRY 



The same conclusion could have been reached in other ways. 

 For example, some threads could have been taken from the 

 edges of the sample and placed under a microscope. In this 

 case, they would have been seen to be made up of long, hollow 

 fibers or tubes (Fig. 2). But the cotton fibers (A) are smooth on 

 the surface, while the woolen fibers (B) are covered with scales^ 

 By this difference in properties, the presence of both kinds, or of 

 only one of the kinds, could quickly be found out. Still again, the 

 chemist knows that wool will " take " almost any dye, while 



cotton remains uncolored by 

 the greater number of dyes. 

 He could, therefore, boil a piece 

 of the cloth with a solution of a 

 soluble dye such as red ink 

 (eosin) for a few minutes, and 

 1 1 t| ^ jSjjl then wash the sample thor- 



oughly in clean water. On 

 examining the cloth with a 

 microscope he could then ob- 

 serve whether any fibers were still white. Here wool has the 

 property of uniting with the dye, while cotton has not. The last 

 plan could be used only with white goods, or goods not already 

 strongly dyed. 



The first method is the one which the chemist would probably 

 employ in practice, because by its means he can ascertain and 

 report, not only the presence of cotton, but the proportion of 

 cotton present. To do this, he weighs the dry piece of cloth 

 before placing it in the test-tube. Then, after the boiling with 

 caustic soda solution, he washes what is left of the sample very 

 thoroughly in running water, dries it, and weighs it again. The 

 weight of this " residue " is that of the cottoi:. The difference 

 between this and the original weight is the weight of the wool. 

 The chemist is then able to state the percentages by weight of 

 cotton and of wool in the original material. 



FIG. 2 



