Valuation of Indigo. 287 



quantity of water required to be added to reduce the colour 

 of the solution to a certain shade. This process, which is 

 simple and convenient, is in every respect similar to that 

 long since applied by Ure,* though Persoz t ascribes it to 

 Houton-Labillardiere. 



The advantages and disadvantages of these processes have 

 been so fully discussed by Bolley in his paper, before referred 

 to, that it is unnecessary, I conceive, to make any further 

 comment on their respective merits. 



The method I have now to propose, is based upon the cir- 

 cumstance that indigo-blue in presence of hydrochloric acid, 

 is decolorised by bichromate of potash. This salt has long 

 been used for discharging indigo-blue and other colours in 

 the printing of textile fabrics, as well as for bleaching oils, 

 fats, and several other substances. In employing it for esti- 

 mating the comparative value of commercial indigo, the 

 necessary manipulations are extremely simple. 



Ten grains of the sample, in very fine powder, are carefully 

 triturated with two drachms by measure of fuming sulphuric 

 acid, and the mixture being excluded from the air is allowed 

 to digest with occasional stirring for twelve or fourteen hours. 

 A small flat-bottomed flask with a tightly fitting cork, is a 

 very convenient vessel for this operation. Some pieces of 

 broken glass should however be thrown in to facilitate the 

 contact of the indigo and acid during the agitation, and thus 

 to prevent the aggregation of the former into small clots, 

 which the acid by itself cannot penetrate. If a small capsule 

 or test-glass be used, it should be covered, during the diges- 

 tion, with an air-tight gas-jar. It will also be found advan- 

 tageous to place the mixture in a warm situation, say be- 

 tween 70° and 80° F., that the action of the acid may be fully 

 developed ; a higher temperature than this must be avoided, 

 as sulphurous acid is liable to be produced, and the trial in 

 consequence completely vitiated. Great care must be taken 



i to insure the perfect solution of the indigo-blue in the acid. 

 This result being accomplished, the solution is poured slowly, 



j with constant stirring, into a pint of water contained in a 



* J. Roy. Inst., 1830. t Traite des Tissus, i. 434. 



u 2 



