L. TECHNOLOGY. 431 



green. This principle has long been applied in the so-called 

 etch printing, in which the aniline colors are extracted in par- 

 ticular parts of the pattern by means of the hydrogen. This 

 is done by laying on a sheet of metallic zinc, with water and 

 the proper sizing. Water consists of hydrogen and oxygen ; 

 the metallic zinc takes the oxygen from the water, and the 

 hydrogen developed renders the aniline tints colorless. Aft- 

 er this it is only necessary to rinse out the fabric in order to 

 extract the colorless combination. 



Again, by saturating the substance to be deprived of its 

 aniline dye with a feeble acid, such as vinegar or much di- 

 luted hydrochloric acid, and sprinkling the whole with pow- 

 dered zinc, the color will be removed, especially if the fabric 

 be slightly heated. This process is, however, much too com- 

 plicated ; and instead of it, we can better use liquids which 

 will give off hydrogen, and thus have a reducing influence. 

 Such a liquid we find in the solution of chloride of tin, usu- 

 ally known as the salt of tin. This must be of the very best 

 quality to be efficacious, and in external appearance should 

 be of a white color, and composed of clear, dry, and tolerably 

 transparent crystals. A solution of such a salt of tin should 

 be placed in a stone vessel and diluted until it can not injure 

 the fabric (about one to two degrees B. of strength), and some 

 leaves of tin-foil placed at the bottom of the vessel. The fab- 

 rics, previously rendered perfectly free from dirt or grease, 

 are to be placed in the solution and the vessel covered, the 

 whole being then heated by immersion in boiling water. 



As soon as the decolorization has been accomplished, the 

 cloth is to be taken out and rinsed in clean water previously 

 warmed. Generally a new fabric should be left in the hot 

 solution from a quarter to half an hour, and the vessel then 

 be set aside to cool, after which the color will be found to 

 have vanished completely. 



There still remain instances, however, in which even this 

 efficient method does not entirely accomplish its object, and 

 the last resort, which is absolutely certain and never-failing, 

 is to the cyanide of potassium. This, however, is a deadly 

 poison even in a very small quantity, and the utmost precau- 

 tion must be adopted in using it. The operator must be cer- 

 tain that he has no sore or cut on the hand, as contact with 

 the liquid in that case would be extremely dangerous, al- 



