68 SCIENCE PROGRESS 



values were followed throughout Behar. The disturbances 

 induced by the irregularity of the process in Assam, however, 

 led Davis to devise a simple method whereby the termination 

 of the mahai for optimum yield was regulated by a colori- 

 metric test. Ten cubic centimetres of vat-liquor were with- 

 drawn, and, after adding a few drops of ammonia, shaken up 

 with air to oxidise to indigo blue. The whole was then diluted 

 to 250 cubic centimetres and its colour compared with a sample 

 taken half an hour later. When the colour no longer increases 

 with time the process is stopped by drawing off the water for 

 beating. 



As previously mentioned, when the bacterial processes 

 have not proceeded in quite the normal fashion, a green vat is 

 obtained after the indigo has been formed. Owing to the 

 large volume of water used in proportion to the yield of dye, 

 namely, by weight, 3,000 : i, as little ascoi per cent, of indigo 

 in the seet-water represents a loss of 30 per cent, of the yield, 

 At Pusa up to 0-015 per cent, of indigo has been found by 

 analysis in the seet-water of green vats. A much increased 

 yield may be obtained by the use of a small quantity of dhak 

 gum as an aid to flocculation and precipitation. This causes 

 but little lowering in the quality of the indigo cake, whereas 

 other substances, such as alum, tried for this purpose caused 

 the subsidence of much extraneous material and resulted in a 

 cake of lower purity. The gum is a ruby-coloured exudate 

 from the dhak-tree {Butea frondosa), and is an article of com- 

 merce in parts of India. 



Final Product. — The issue in this lies between cake indigo 

 and paste indigo, each of which has certain advantages. The 

 German synthetic product, indigo blue, was exported as a 20 

 per cent, paste. It was thus of uniform quality and purity, 

 and easily mixed for use. The natural indigo of commerce is 

 usually produced as cake, which contains from 70 per cent, 

 to about 50 per cent, or less of indigo blue, together with 

 small quantities of indirubin and other colouring matters 

 which collectively produce (especially on wool) a dyeing effect 

 slightly greater than that due to its indigo blue content. The 

 balance of the cake is made up of organic material derived 

 from the plant (indigo gluten) or from destructive transforma- 

 tion of indoxyl, and a small amount of ash. The cake is 

 valued by its general appearance and colour or by analysis. 

 For use the cake has to be ground up and got into the form 

 of soluble indigo-white by reduction, nevertheless it is a 

 handy form for transport and needs no special container. The 

 English market is used to cake indigo, but the very large i 

 amount of natural indigo produced as a i per cent, watery ! 

 paste in China, and utilised locally, was in part displaced by | 



