264 ANNUAL OF SCIENTIFIC DISCOVERY. 



distinctly indicate the approach of the process towards conclusion. 

 The blue color of the solution gradually diminishes in intensity, be- 

 coming perceptibly lighter and lighter, and after a time it acquires a 

 greenish shade, which soon changes to greenish-brown, and almost im- 

 mediately to light ochre-brown. 



I have tried this process very carefully upon pure indigo, prepared 

 according to Fritzche's method. The mean of three experiments, 

 which gave results almost identically the same, showed that 10 grains 

 of pure indigo require very nearly 7 grains of bichromate of potash; 

 and I have accordingly taken this quantity of the salt for solution in 

 the alkalimeter. 



The method here proposed is open, T am well aware, to some of the 

 many objections that have been advanced against the well-known chlo- 

 rine process. It is quite obvious, for instance, that unless particular 

 care is taken in dissolving the indigo in the sulphuric acid, not only is 

 a part of it liable to escape solution and proper estimation, but in the 

 case of inferior indigo, sulphurous acid may be produced, which would 

 of course involve a larger consumption of the bichromate of potash 

 than the indigo-blue itself would require. It may also be objected 

 that bichromate of potash, in the presence of hydrochloric acid will 

 act upon the other constituents of ordinary indigo; but so far as I 

 have been able to judge from a very extensive course of experiments 

 upon a great variety of specimens, the amount of these influences is 

 extremely slight, and altogether inappreciable when the process is ex- 

 ecuted with proper care. The same opinion has been expressed by 

 Berzelius and Schlumberger regarding the chlorine process ; and it is 

 further supported by the fact, that indigos containing a large propor- 

 tion of brown and other coloring matters, consume a very small quan- 

 tity only of the bichromate. While, therefore, this process has no 

 pretensions to supply scientific men with the means of determining 

 the actual amount of pure indigo-blue in samples of commercial indigo, 

 it is in my opinion admirably adapted for ascertaining their relative 

 values, being in many respects superior to those which have hitherto 

 been proposed. The bichromate of potash possesses in an eminent 

 degree all the, qualities requisite for a trustworthy agent of valuation, 

 being easy of purification, unchangeable by keeping, and of uniform 

 composition. 



ESTIMATION OF MINUTE QUANTITIES OF IRON. 



The author having been much occupied with the anlysis of water 

 containing minute quantities of iron, and finding the ordinary mode 

 of estimation by precipitation and weighing very troublesome and 

 inaccurate, when applied to such quantities, was led to avail himself 

 of the following centigrade method, founded on the reaction of per- 

 salts of iron with sulphocyanide of potassium. A standard solution 

 of per-chloride of iron containing a little less than -^i-g- of a grain 

 of metallic iron per cent, was prepared by dissolving 1 grain of iron- 

 in hydrochloric acid with the addition of a little nitric acid, evaporat- 



