164 Dr. Ure on Indigo. 



my researches on this azotized product of vegetation, but the 

 subject having been lately taken up by my pupil and friend, 

 Mr. Walter Crum, I was induced to leave it in his hands." 

 I then thought it likely that some slight modification might 

 require to be made in the weights of the constituents given by 

 me, for ** I did not (then) resume the subject of indigo, after 

 I had become most familiar with the manipulations." I have 

 found since that my mode of analysis was not in fault, but the 

 revived indigo-blue, which I employed, had not been entirely 

 purged of the red principle, by sufficient ebullitions with alco- 

 hol ; for it adheres very tenaciously. Hence that resinous 

 matter introduced a httle oxygen and hydrogen, more than 

 absolute indigo-blue contains. But the error will appear in- 

 considerable, if we compare the result with the analysis pre- 

 viously published by Dr. Thomson. The following is a view 

 of the ultimate constituents of indigo-blue^ as given by dif- 

 ferent chemists : — 



100.000 100.00 100.00 100.00 



That pure indigo contains hydrogen, I have recently placed 

 beyond a doubt, by heating a mixture of it and calomel in a 

 green glass tube, the open end of which terminated in an in- 

 verted tube, filled with nitrate of silver. Copious fumes of 

 muriatic acid were evolved, and chloride of silver was precipi- 

 tated in its characteristic curd. 



The liquor of the dyer's vat (for calico-printing) contains 

 indigo deoxidized by protoxide of iron, and dissolved in lime- 

 water. This solution, in its average state of richness, has a 

 specific gravity not appreciably higher than that of distilled 

 water, and aftbrds out of 1000 parts, by weight, not more 

 than 3 parts of indigo-blue, and nearly the same quantity of 

 carbonate of lime, equivalent to about a grain and a half of 

 quicklime in 1000 of the liquid; which is the proportion in 

 common lime-water. 



If that yellow liquor be introduced into a glass globe, with a 



