163 
On Colouring Matter. 
Vth*. This proposition and the next, being at variance with received opinions, 
will perhaps at first seem a little startling ; I think, however, that upon strict exa- 
mination they will be found to rest upon sufficient and satisfactory evidence. In 
this country, it is well known that a steeping vat at an indigo factory is when in use 
filled with indigo plant and water ; that fermentation speedily takes place, and a so- 
lution of indigo is the consequence : the fermented liquor of this vat is what, in fu- 
ture, I design when I speak of indigo solution. I made the following experiment. I 
took two phials, one empty, and the other containing a little pure potass. I dipped 
them into the vat, and when they were nearly filled with indigo solution I closed their 
mouths with my thumbs and withdrew them; a little time and agitation rendered 
the indigo solution with potass of a dark blue colour, and by rest a dark blue preci- 
pitate was deposited ; whereas no time or agitation effected any change in the pare 
indigo solution. 
Alkalis employed in the manufacturing of indigo always deepen the colour, and 
carry it towards a black. 
It+ is well known that alkalis act powerfully in precipitating indigo. If a solution 
of pure alkali be sprinkled over indigo solution, it immediately produces upon those 
places where it happens to fall, a beautiful blue colour. Indeed its power of regene- 
rating indigo was so efficient, that I employed it as a test to inform me of the state 
of the water (technically called dirty water) which remains above the indigo after 
its precipitation or subsidence. 
By mixing a little caustic ley with some of this water, and then dipping into it 
a bit of white cotton, it was easy to discover whether it contained indigo, and 
in what proportion. Alkalis are almost, if not quite, indispensable in dying with 
indigo ; I could not obtain}: a colour of any body from indigo solution, but with 
caustic leys it dyed very speedily and effectually. I conceive that the following 
is the explication of the modus operand! of alkalis in promoting the. solution 
of indigo and other dyes. Alkalis promote the oxygenation of the disoxygenating 
agents, by which their action on the indigo or other dye is increased. If the alkali be 
present in too small a proportion, the indigo is not deoxydated ; and if in too large a 
proportion, the agents may get saturated, and thus lose their effect. Or the equilibrium 
between them and the indigo may be destroyed, and a tendency be excited, in the 
primary principles of both, to enter'into neweom bi nations ; thus producing decompo- 
sition. In fine, the general effect of alkalis is to deepen colonr, by, as I believe, oc- 
casioning an additional portion of oxygen to enter into the composition of the co- 
louring matter. And although this property may be modified bythe facts stated in the 
vnth proposition, yet I believe it will be found, that the effects ordinarilyprodneed 
by alkalies on colouring matter, are in nature the same with those which a combina- 
tion of oxygen with colouring matter produces, in whatever way that combination 
may be effected. Of course reference will always he had to any other specific pro- 
perties possessed by the agents employed. The reverse of what I have said of alkalis 
will very exactly apply to acids, . 
Vlth. Acids mixed with indigo solution render it incapable of dying; they prevent 
the oxygenation of indigO§. And thus 1 conclude that in the manufacture of mdigo, the- 
use of beating is to expel the carbonic acid gas ; since the indigo cannot combine 
with oxygen whilst this is present. Acidulous salts possess in a less degree this dis- 
oxygenating property of acids. 
* Disoxygenating and oxygenating are merely relative terms. Alkalis, under the 
circumstances adduced in this article, appear to be oxygenating ; and as, " lien em- 
ployed in bleaching, they restore colour, I incline to believe that they favour the 
combination of oxygen with colouring matter ; they are perhaps, however, subject 
to the qualification elicited by the examination of the Vllth proposition. 
t Some of the effects here mentioned, are in part attributable, when caustic 
alkalis are employed, to the neutralizing of the carbonic acid, but the deepening 
of the colour would seem to proceed from an addition of oxygen. 
} Perhaps heat may have dissipated the carbonic acid, and promoted the combi- 
nation of oxygen with the indigo, and thus supplied the place of au alkali. I did 
not try it. 
§ It would be a wonderful improvement in the construction of indigo factories, 
in my opinion, especially where the manufacturing is protracted to a late period, to 
build the vats ou arches, and to make flues through all the walls ; for the purpose, 
of course, of making fires and assisting the manufacturing process by means of ar- 
tificial heat. This would enable the manufacturer to ferment his plant, and obtain 
from it all the indigo in the coldest weather ; it would also, I think, improve the 
quality of the indigo. 
