GLEANINGS IN SCIENCE. 
13 
Original (Eoinmuni ration#. 
I . — Experiments oh Indigo. 
Chemists in Europe, who have engaged in the examination of Indigo, have ge« 
nerally had to deal with the prepared colouring matter as manufactured for the mar- 
ket ; and have therefore limited themselves to the separation and measurement of the 
foreign ingredients with which it was contaminated, — to the properties of the pure 
colouring matter itself, — and to the analysis of its chemical composition. The 
rationale of what passes in the process of the manufacture may be, and has been, 
deduced with tolerable certainty from the discoveries thus made ; but where we have 
the whole fermentation carried on anioug us on an immense scale, — when we have 
the Indigo in its nascent state, and in its colourless soluble state too, capable of 
being submitted to tests and processes, it becomes a matter of curious interest to 
follow the changes of this singular substance, and compare them with the theories 
formed in the laboratories at home. 
It is however more tbuii.a mere matter oi curiosity to set on a sound basis the 
causes of the different manipulations, ami to examine the real effect of processes 
in which different manufacturers have a diversity of practice ; — although it must 
be confessed that, setting aside the difference of quality in the plant, from season, 
care in its cultivation, soil, and other causes, the business of the vats is so simple 
as to allow of little deviation in practice or result. 
The most convenient manner of bringing before the reader the various subjects 
of a short series of experiments which I made this season, in conjunction with 
an intelligent friend engaged in an Indigo establishment, will be to incorporate 
them with a relation of the general process of manufacture ; but I think it will 
save a good deal of repetition and explanation, to give in the first place an epi- 
tome of what has already been written on the subject ; that is, on the qualities 
and properties of Indigo itself, which have been elaborately examined by Bergman, 
Berthollet, Chevreul, Thompson, Crum, and latterly by Berzelius. Their notices 
extend to minute details of the action of every neutral salt, acid, and alkali 
of the chemical calendar ; but the general results which are likely to be useful to 
the manufacturer may be condensed into a very small space. 
Indigo is a definite vegetable product which appears to exist in greater or less 
quantity in a variety of plants, or rather the elements of which it is composed 
are found in these plants ; for it is not until the juices of the vegetable begin to 
act upon one another in fermentation, that Indigo is developed. It might at 
first be supposed that the green colour of plants was connected with the presence 
of indigo ; but this is not the case, as after the leaves have been steeped ill the 
vats, they retain entirely their original colour. 
When first dissolved from the plant, the Indigo is in a colourless state, and is 
readily soluble in water ; but it becomes blue on absorbing oxygen from the air, 
and appears then to have assumed the nature of a peroxide, for it is very unal- 
terable, quite insoluble in water, alcohol, ether, saline infusions, alkalies, and 
dilute acids. Concentrated sulphuric acid alone acts as a solvent, without chang- 
ing its nature. Nitric acid converts it into a yellow bitter principle. Acetic acid 
by degrees deoxydizes it. Chlorine also destroys its colour immediately. 
In saying that concentrated sulphuric acid effects no change upon Indigo, I am 
not strictly correct. It does not destroy the colouring matter, or cause a decom- 
position, as would be the case with most other vegetable compounds ; but the 
researches of Crum and Berzelius have rendered it probable, that three different 
modifications of Indigo may be brought about by the agency of this acid, differ- 
ing from one another in the quality of oxygen or of water in their composition. 
The three modificatons are thus distinguished : — 
1. The Purr Indian, which is obtained by sublimation from the crude cakes at 
a temperature of 550°. This is crystallized in long flat prismatic needles ; lias a 
Copper colour by reflected, and a fine blue by transmitted light. It sublimes 
entirely at the temperature stated, without residue : its specific gravity is 1.35. 
2. Cerulin , or Savon Blue , is obtained by digesting the former substance in sul- 
phuric acid : it is supposed by Crum to have lost its water of composition thereby. 
It is soluble in water, but is precipitated by most of the neutral salts. 
