190 
IMPORTANT EXPERIMENT ON INDIGO. 
moving chain, rope, or hawser, placed beneath 
the surface of the water of a navigable river, 
lake, or canal for the purpose of tracking or 
towingboats or any othervessels as hereinbefore 
described at figs. 1, 2, 3, 4, and 5; and I also 
claim, as of my invention, that peculiar con- 
struction of chain, as set forth and described 
at fig. 6, for the purpose of tracking or towing 
boats and any other vessels ; and these my 
improvements being to the best of my know- 
ledge and belief new and never before used, 
I deliver this as my true and faithful spe- 
cification of the same. — In witness whereof, 
ike. 
Enrolled October 12, 1835. — Repertory of 
Arts. 
EXPERIMENTS ON INDIGO. 
By Nath Ogle, Eso. 
1 have lately been engaged in a set of 
experiments on the indigo ; and as that sub- 
stance is now so universally known as a per- 
manent and beautiful blue dye, it may not be 
altogether uninierestiiig to your readers to give 
a sketch of its chemical characters, which are 
very striking and rather complicated. 
Indigo of commerce is by no means a pure 
colouring principle. It contains a variety of 
foreign matter, part of which it may liave de- 
rived from the plant from which itwas extract- 
ed, and part may have been added to it 
through carelessness in its preparation ; of 
100 parts, a good specimen will not afford 
more than 50 of real blue. 
It is a matter of considerable importance 
to devise some simple, and, at the same time 
economical plan of analysing this drug, not 
only for the purpose of ascertaining the exact 
quantity of colouring matter a given speci- 
men contains, but also what is the nature of 
its impurities, which 1 have found to vary con- 
siderably in different sorts. In order to find 
the value ofa sample with respect to its pro- 
portion of blue, Mr. Dalton proposes to dis- 
solve one grain in sulphuric acid, transfer the 
solution into a tall cylindrical glass jar con- 
taining water, and then to destroy the colour 
by chloride of calcium, the value of the indi- 
go being in proportion to the quantity of the 
chloride necessary to destroy its colour. I 
consider this to be, at best, a troublesome me- 
thod, and not entirely to be depended upon. 
1 made several experiments on two samples, 
one an excellent East India, and the other a 
very inferior Guatamala ; but the quantities 
of chloride of calcium required to destroy the 
colours were so nearly the same, that the su- 
periority of the East India was not manifested. 
Chevreul gives us a very good method of 
analysing indigo in the rough manner. He 
directs that it be first digested with water, 
which will take up 12 or 14 per cent , but the 
quantities varies much in different samples. 
The water acquires sometimes a yellow, but 
usually, especially with Guaiamala’s a dark 
brown colour ; this solution by exposure to the 
air precipitates flocks, having a green colour, 
which appear to be partly composed of indigo, 
becoming blue v^'hen left in the air ; the 
greater part continues green; is soluble in al« 
cohol and solution of potash, but does not ever 
turn blue. I have found that this green mat- 
ter, which is very slowly thrown down by the 
action of the air, is immediately and plentiful- 
ly precipitated by dropping muriatic acid into 
a concentrated liquor ; and in the specimens 
on which my experiments were made, the pre- 
cipitate from the Guatamala was much more 
abundant than that from the East India ; the 
liquor from the former was much darker than 
that from the latter, and it was remarked that 
the Guatamala was very inferior as a dye to 
the East India, yet the quantity of real indigo 
in each did not appear to vary much. I con- 
clude, therefore, that the difference in quality 
was owing to a more than usual quantity of 
gluten and brown matter, and that these sub- 
stances are more injurious than is generally 
supposed, tending to destroy the peculiar bril- 
liancy of the indigo. 
After water has extracted all that is soluble 
in that menstruum, Chevreul directs that the 
residue be treated with alcohol in successive 
portions, by which a further quantity of green 
matter is taken up, but so mixed with another 
red substance that it assumes a dark, ruby 
colour, Chevreul states, that 30 grains out of 
100 are taken up by alcohol, which is rather 
more than I found. Lastly, muriatic acid 
takes up a further portion of red matter, to- 
gether with alumina, lime, and oxide of iron ; 
and pure indigo, amounting to 45 or 50 per- 
cent., remains, usually mixed with a small 
quantity of silex. 
When indigo is exposed to a temperature 
about equivalent to that of melting lead, it 
rises in the form of a beautiful purple smoke. 
This was known long before any attempt was 
made to obtain it in a crystalline form by sub- 
limation. If, however, a proper apparatus is 
employed, and precautions adopted, it may 
be thus produced, and then assumes a very 
beautiful appearance. The best indigo for the 
purpose is that precipitated by agitating in 
contact witli air the yellow solution of deoxi- 
dised indigo, which forms the dyer’s blue vat; 
but where that cannot readily be obtained, 
common indigo may be used. In the latter 
case, 30 or40 grains in coarse powder must be 
placed in a shallow metallic saucer, and a 
spirit-lamp applied to the bottom till the sur- 
face becomes covered with a copper-coloured, 
mossy-looking substance, taking care to re- 
move the source of heat the moment purple va- 
pours appear. When the saucer is cool the 
crystal must be brushed off with a feather, and 
placed in another similar saucer furnished with 
a cover, so applied that the internal surfaces 
may not be more than half an inch apart. A 
second application of heat will cause the pure 
crystals to rise and plant themselves on the 
upper vessel, the impure substance remaining 
behind ofa coaly appearance. 
The crystals thus produced bear a very small 
proportion to the quantity of indigo employed. 
As an average of four experiments from 10 
grains of impure indigo, obtained by sublima- 
tion half a’^grain of crystals, and the residue 
vyeighed6§ grains, showing 3 grains of vola- 
tile matter to have escaped, d’he crystals 
volatised leave no residue. When they are 
