288 Dr Frederick Penny on the 



basin, and j of a volume ounce of strong hydrochloric acid 

 immediately added, the flask or capsule being rinsed clean 

 with water. 



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

 the usual way with 1\ grains of dry and pure bichromate of 

 potash, and the solution added in small successive portions 

 to the diluted sulphate of indigo in the basin, until a drop of 

 the mixture, on being let fall on a white slab or slip of bi- 

 bulous 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 shews the comparative value of the indigo sub- 

 jected to the trial. 



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

 results are obtained by bringing the end of a glass rod into 

 contact with the indigo- solution, and then gently pressing it 

 against the surface of the paper. The stain thus produced 

 will be circular, and conveniently localised to a small space. 

 By using bibulous paper, it will also be found much easier to 

 recognise the last traces of the blue colour 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 con- 

 clusion, 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 colour which 

 the mixture undergoes during the addition of the chrome-so- 

 lution, will distinctly indicate the approach of the process to- 

 wards conclusion. The blue colour of the solution gradually 

 diminishes in intensity, becoming perceptibly lighter and 

 lighter, and after a time it acquires a greenish shade, which 

 soon changes to greenish-brown, and almost immediately to 



light ochre-brown. 



I have tried this process very carefully upon pure indigo, 

 prepared according to Fritzche's method. The mean of three 





