CHEMICAL SCIENCE. 263 



indigo-blue and other colors in the printing of textile fabrics, as well 

 as for bleaching oils, fats, and several other substances. In employing 

 it for estimating the comparative value of commercial indigo, the ne- 

 cessary manipulations are extremely simple. 



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

 rated with two drachms by measure of fuming sulphuric acid, and the 

 mixture being excluded from the air is allowed to digest with occa- 

 sional stirring for twelve or fourteen hours. A small flat-bottomed 

 flask with a tightly fitting cock, 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 digestion, with an air- 

 tight glass jar. It will also be found advantageous to place the mix- 

 ture in a warm situation, say between 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 

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

 sult being accomplished, the solution is poured slowly, with constant 

 stirring, into a pint of water contained in a basin, and f of a volume 

 ounce of strong hydrochloric acid immediately added, the flask or cap- 

 sule being rinsed clean with water. 



An alkalimeter of 100 equal measures, is now made up in the usual 

 way with 7^- grains of dry and pure bichromate of potash, and the so- 

 lution added in small successive portions to the diluted sulphate of in- 

 digo in the basin, until a drop of the mixture, on being let fall on a 

 white slab or slip of bibulous paper, presents a distinct light brown or 

 ochre shade, unmixed with any blue or green. The process is then 

 finished ; the number of measures of bichromate used is read off, and 

 this number shows the comparative value of the indigo subjected to 

 the trial. 



In applying the test-drop to the bibulous paper, the best results are 

 obtained by brinjnno; the end of a glass rod into contact with the in- 



v C3 ^J ~ 



digo-solution, and then gently pressing it against the surface of the 

 paper. The stain thus produced will be circular and conveniently 

 localized to a small space. By using bibulous paper it will also be 

 found much easier to recognize the last traces of the blue color than 

 when a slab is employed, and the results, when dry, may be preserved 

 unchanged for reference or comparison. 



It is advisable to keep the indigo-solution gently heated while the 

 chrome-liquor is being added ; and it is essentially necessary that the 

 mixture should be well stirred after each addition. Several measures 

 of the chrome-solution may at first be poured in without risk of error, 

 but towards the conclusion, the liquor must be added very slowly and 

 with great care, as one or two drops will then be found to produce a 

 very decided effect. The characteristic changes of color which the 

 mixture undergoes during the addition of the chrome-solution, will 



