[ 4'6 ] 



Rawson, who by a proper method of oxidising has obtained 

 an increased yield of 25 per cent. 



631. Pure Indigo Tests. Whether a fabric has been 

 dyed with pure indigo or with some inferior dye can be judged 

 by the following tests, (i) Put 2 or 3 drops of ordinary 

 commercial nitric acid on some portion of the fabric. A 

 yellow spot with a green rim quickly appears if the dye is 

 pure indigo. (2) Make a mixture of i part Sulphuric acid, and 

 9 parts of water and in it boil quickly for 10 minutes a piece 

 of the cloth to be tested, say \\ inches square. Care must be 

 taken always to pour the acid gently into the water and not 

 the water into the acid. If the cloth has been dyed in pure 

 indigo the solution will remain colourless. (3) Dissolve about 

 i oz. of common washing soda in half a pint of water and 

 gently boilin it for 15 minutes a \\ inches square pieceof cloth. 

 If the dye used is pure indigo the liquid will remain 

 colourless. 



632. The artificial indigo of commerce manufactured in 

 the Badische aniline factory in Germany, is almost pure in- 

 digotin, containing no indigo red, no indigo brown and no 

 indigo glue, which is a disadvantage, as these substances have 

 some beneficial effects in dyeing. But artificial indigo is 

 likely to supplant natural indigo in the long run when the 

 defects of thejartificial product will be supplied by artificial 

 means. Woollen fabrics dyed with natural indigo may be 

 distinguished from those dyed with Badische indigo by hold- 

 ing the two fabrics over steaming water. The one dyed with 

 vegetable indigo will emit an agreeable odour, while the 

 chemical indigo will give out a tarry smell. 



