ANALYSIS OF SUBLIMED AND PRECIPITATED INDIGO. 
191 
viewed through a microscope, they appear as 
Song, flat, ocicular crystals, appearing red by 
reflected, and blue by transmitted light ; 
they are not, however, always so, sometimes, 
particularly at the commencement of their for- 
mation, assuming the form of very thin plates, 
appearing almost opaque ; indeed, when lying 
in a mass, they always have a brown colour. 
Sublimed indigo may be analysed by heat- 
ing it with peroxide of copper in green glass 
tubes. Mr. Crum gives its ultimate consti- 
tuents thus : — 
Carbon 73*22 
Azote 11*26 
Oxygen 12 60 
Hydrogen **•* .... 2.92 
100 . 
These numbers correspond very nearly to, 
1 atom of azote I*75orl0*77 
2 atoms of oxygen 2*00 or l2 31 
4 atoms of hydrogen .... 050 or 3o8 
l6 atoms of carbon 12*00 or 73*84 
16*25 100.. 
I am, however, disposed to consider the 
quantity of carbon to be greater, and the quan- 
tity of oxygen to be smaller. I have repeat- 
edly analysed both sublimed and precipitated 
indigo over peroxide of copper and protochlo- 
ride of mercury (calomel), and have obtain- 
ed 84 as the mean proportion of carbon in 100 
parts ; and it was only when calomel was 
employed that I obtained satisfactory proof of 
the presence of hydrogen. But since Mr. 
Crum’s analysis is generally considered pretty 
correct, 1 do not at piesent placemuch reliance 
on my own discordant results. Organic ana- 
lysis is a very delicate operation, and requires 
much experience and a peculiar apparatus, 
neither of which have I the advantage of. 
In the year 1327, Berzelius published an 
excellent memoir on indigo. He found in it 
four peculiar substances, which constitute its 
chief ingredients, viz. 1st, a substance close- 
ly resembling gluten ; 2d a brown matter ; 3d, 
a red matter (the resin of Bergman and Chev- 
reul j) and 4th, the proper colouring principle. 
From a sample of good East India indigo 
I extracted the gluten by first boiling it with 
diluted sulphuric acid, then filtering and neu- 
tralising the acid by carbonate of lime, after 
which it was evaporated to dryness and alcohol 
'boiled on the residue ; this extracted a sub- 
stance resembling gluten, and particularly 
characterised by its smell, which was very 
similar to broth. Gluten is itself a substance 
possessing properties in common with both 
the animal and vegetable kingdoms, hence it 
has been called a vegeto-animat substance. 
The brown matter I separated from the re- 
sidue left by the acid by gently heating it with 
a weak solution of potash, and from the re- 
sidue again alcohol extracted the red matter. 
The alcoholic solution being evaporated to 
dryness, left a ruby-coloured powder, which 
was dissolved by nitric acid, forming a fine 
port-wine coloured liquor, which colour it 
did not long retain, but soon, in consequence 
of decomposition, turned yellow. 
After these operations have been performed 
on it the indigo is not left in a state of parity ; 
it contains, besides insoluble impurities, a 
ortion of the green, red, and brown matter 
ut by acting on it by the protosulphate of 
iron and lime, and pouring the yellow solution 
of deoxidised indigo, thereby obtained into 
diluted muriatic acid, a copious blue matter 
falls down, which, after washing, may be re- 
garded as tolerably pure. 
By acting on indigo by means of protosul- 
phate of iron and lime, Liebeg produced a 
substance which he considered to be pure 
deoxidised indigo. The proportions I used in 
repeating his experiment were, 1,000 grains of 
the during, 1,500 of copperass, and 1,600 of 
lime ; these were put into a stone jar, and 3 
quarts of water poured on them ,* the whole 
was then heated to l30® Fahr., and so kept for 
18 hours, guarded as much as possible from 
atmospheric air ; the clear yellow solution 
was then drawn off by a syphon, previously 
filled with hydrogen gas and mixed with dilu- 
ted muriatic acid, holding in solution a little 
sulphate of ammonia ; a thick precipitate fell 
down, which was washed with water that had 
been boiled, and dried at the temperature of 
212® ; when quite dry it retained its white 
colour even when exposed to the air, but when 
moist it speedily became blue. To this sub- 
stance Liebeg gave the name of indigogen, and 
he ascertained that in passing into the blue indi- 
go it absorbs 11*5 per cent, of oxygen. The pre- 
paration of this substance, owing to its power- 
ful affinity for oxygen, is extremly difficult, 
and it was only after repeated trials that 1 suc- 
ceeded in producing it. It is absolutely ne- 
cessary that all the vessels em ployed should be 
previously filled with either nitrogen or hydro- 
gen, and the water employed be deprived of 
air by long boiling. 
The action of some of the acids on indigo is 
extremely interesting. With the nitric acid, it 
forms two distinct substances, according to the 
strength of the acid and the manner in which 
it is applied. When a part of indigo is mix- 
ed with 8 or 9 parts of moderately strong 
nitric acid, and boiled as long as nitrous fumes 
are evolved, carhnzolic acid is formed. When 
the indigo is added to diluted nitric acid kept 
boiling, as long as effervescence continues, 
hot water being occasionally added to supply 
the loss by evaporation, indigotic acid is 
formed. 
The particulars of the preparation of each 
are as follows : 
To form carbazotic acid, I boiled some 
small fragments of the best East India indigo 
in ten times their weight of nitric acid; the 
mass frothed and swelled, giving out a large 
quantity of nitrous gas, mixed with carbonic 
prussic acids. It is recommended by some 
chemists to add successive portions of nitric 
acid whilst boiling ; but there is nothing, I 
believe, gained by this, I have tried repeated- 
ly both plans. The solution is bright yel- 
low, and contains, besides carbazotic acid, 
artificial tannin, resinous matter (which 
forms a film on the surface), and indigotic 
acid— on cooling, carbazotic acid is freely de- 
posited, but not in a pure state, mixed pro- 
bably with a considerable quantity of indi- 
