CHEMISTRY OF INDUSTRIAL SUBSTANCES 163 



contact with the color and impresses the colored design 

 on the fabric. A strip of steel, called a doctor, removes the 

 color from every part of the roller except that covered by 

 the design. 



Discharge printing consists in removing color by means 

 of a chemical from goods already dyed in the piece, thus 

 leaving a white pattern. 



Resist printing consists in stamping a chemical on a plain 

 white cloth, and dyeing the cloth afterwards. The chemical 

 makes the dye ineffective on the pattern. 



191. Sizing. Sizing is a process of applying a thickening 

 agent or mixture to cloth, paper, etc. The change brought 

 about is distinctly physical. The object of sizing is to add 

 weight, strength, and smoothness (luster) to the material. 

 A considerable variety of substances are used in size mixtures, 

 the more important of which are included in the following 

 list: 



(a) Substances possessing adhesive properties to strength- 

 en the material and fix other ingredients. This class includes' 

 flours and starches of wheat, sago, rice, maize, and potatoes. 



(6) Substances to render the material soft, pliable, and 

 smooth. This class includes tallow, grease, oils, wax, gly- 

 cerin, and soap. 



(c) Substances to make the material heavier. This class 

 includes French chalk and salts of barium and sodium. 

 * (d) Substances to destroy or prevent the growth of germs 

 that cause mildew. Zinc chloride is almost exclusively 

 employed for this -purpose. 



(e) Deliquescent substances to attract moisture to the 

 material, whereby it may retain its pliability, and to prevent 

 powdery substances from being rubbed off. This class 



