1823.] Mr. Crum on Indigo. 85 



Determination of the Ultimate Constituents of Indigo. 



The apparatus which I employ for the analysis of indigo with 

 peroxide of copper, is simply a tube of green glass, seven inches 

 long, and three-eighths of an inch wide, shut at one end, and 

 connected with a mercurial trough by means of a small bent tube, 

 joined to the other by a tube of caoutchouc firmly tied. 



The indigo is first ground with a quantity of peroxide of cop- 

 per, and when that is introduced into the tube, another portion 

 of peroxide is put over it, and then some metallic copper thrown 

 down from its sulphate by a plate of zinc, and ignited in a close 

 vessel. Lastly, 1 fill up the remaining space (about a couple of 

 inches) with a piece of glass tube, shut at both ends, which 

 nearly fits the other. This keeps the materials in their place, 

 and reduces as much as possible the quantity of common air in 

 the apparatus. The tube is then heated with three spirit-lamps, 

 of which two keep the metallic copper and the pure peroxide at 

 a red heat ; while the third traverses gradually that part of the 

 tube which contains the indigo. 



When I began these experiments, I took the usual precaution 

 of heating the peroxide of copper to redness immediately 

 before I made use of it. I soon found, however, that before I 

 could get the vegetable matter ground with it, and introduced 

 into the tube, it had attracted a quantity of water from the air, 

 which varied according to the hygrometric state of the atmo- 

 sphere, and of course, the length of time employed. It then 

 occurred to me that all error from this source would be obviated, 

 if I allowed the peroxide to be saturated with moisture, by leav- 

 ing it freely exposed to the air, and at every analysis (at least 

 when the state of the atmosphere changed), press 150 grains of 

 it into a small platina crucible which just holds that quantity, 

 heat it dull-red within another crucible, and ascertain its loss of 

 weight without removing the cover.* 



I have in this way made several analyses of indigo, which 

 differed very little from each other. I choose the following one 

 as being nearly a mean of the rest : 



Analt/sis. — One grain of sublimed indigo was ground with 90 

 grains of peroxide of copper ; 25 grains more were put over it in 

 the tube ; and above that, 30 grains of metallic copper. These 

 substances contained 0T1 grain of water. 



• Berzelius has pointed out the instantaneous attraction of water by the oxide of cop- 

 per ( Ann. de Chimie, xvii. 27 ), and cautioned chemists against the fallacy arising from 

 the neglect of it; and M. de Saussure, in a note to his analysis of hog's lard (Ann. de 

 Chimie, xi. 305), makes a general remark upon those analyses where peroxide of cop- 

 per is employed. " Ses resultats" (those of M. Berard), " ainsi qu'un grand nombre 

 de ceux obtenus par ce precede, m'ont paru pecher par execs d'hydrogene." Tliik 

 excess is owing in all probability to the neglect of a small portion of water attracted by 

 the peroxide. I have found 100 grains when newly prepared take as much as .'J-IOths 

 of a grain of water for saturation in damp weather. When it has been several times 

 used, it does not take bo much. 



