164 
On Colouring Matter , 
As no evidence is SO good as that which is extorted by truth from those with 
whose opinions it is at variance, X will here adduce an experiment and observation 
of Doctor Bancroft. The Doctor wished to remove the alkali from the blue employ- 
ed in calico printing. He first tried the muriatic acid; hut, he says, he afterwards 
found that neither the sulphuric nor the nitric regenerated, as he expected they 
would do, the indigo : he states, as the cause of failure in effecting the object which he 
had in view, the speedy precipitation of the indigo I however suppose that the neu- 
tralisation of .alkalis must have operated prejudicially, by preventing the manifes- 
tation of a blue colour*. Doctor Bancroft is of opinion, that sulphuric acid, in the 
process for making Saxon blue, acts first by disoxygenating the indigo, and then 
dissolves it ; the fact seems to me to be indisputable, and I have further been dis- 
posed to think, that the difference between the precipitate obtained from Saxon blue 
and indigo, may rather be owing to a combination of a sulpliuret than to any de- 
struction of the essence of the colouring matter. A solution of tin is said to throw 
indigo down from its solution in the form of a light blue precipitate ; it is probable 
that this precipitate recently prepared as regards oxygenation may resemble the pre- 
cipitate from Saxon blue, and like it be soluble in acids+. With regard to other co- 
louring matter, the disoxygenating property of acids is equally marked and incontro- 
vertible. It increases their solubility}, reddens blues, yellows reds, and bleaches 
yellows. 
The vilthl proposition rests upon a number of facts which, no doubt, observation 
will continually increase. Experiment as well as reason led me to doubt the accu- 
racy of Doctor Bancroft’s experiments respecting the effects || on the scarlet dye, of 
the nitric and sulphuric acids, when combined together in whatsoever proportion in 
solutions of tin. 
In order therefore to satisfy' myself on this point, I instituted a set of experiments, 
the result of which was, that no specific effect in the scarlet dye was produced by the 
union of these acids in tin solution ; but that highly oxydated solutions, however 
produced, carried lac colouring matter towards a yellow or salmon colour ; and I 
found that disoxygenating substances (I used sugar’ , by' reducing the oxyeenisation 
ot tin solution, whether added to the solution, or to them, not only brought the lac 
colouring matter to a good scarlet, but that this was effected when nitric and 
sulphuric acids were combined in the solution, in a very large proportion. 
In this instance the disoxygenating property of tin solution seems to he increased 
by an addition of oxygen and diminished by an abstraction of it. Should bran, 
hour, linseed meal, or other disoxygenating substances be added in excess to the 
SCa T I-LT bath > 2r e dls °*ygenaUng property of the tin solution will be destroyed, 
and little or no effect be produced on the colouring matter : this may serve as a key 
to unlock an experiment of Doctor Bancroft. 
After stating that sulphuretted hydrogen destroyed the colour of a decoction of 
Brazilwood, he adds, that this cannot be by disoxygenation, because potass restores 
According to the doctrine already laid down, alkalis although oxygenating, yet 
exerting this property oil the agent or agents employed for disoxygenating the in- 
digo, they promote its disoxygenation. In this view of the matter their neutralisa- 
t L° n “ a ![ ^generate and precipitate the indigo; if this effect were not produced, 1 
should think the absence ol an alkali would act as predicated, i. e, occasion the 
colour to be famt, 
, In * ,octo :.' Bancroft s work “Philosopy of permanent Colours,” it is astonishing 
the number ot inconsistencies, and the perpetual warfare between theory and fact, 
which anse from acids being considered as oxygenating. 
+ This effect is produced, as is well known, by sulphuretted hydrogen. Sulphur- 
ous acid gas extinguishes combustion, whence it has been recommended, w hen chine 
nies are on fire, to burn sulphur in the grate or fire place. This property I imagine 
■ rom tbe S aa taking oxygen from the combustible matter, andwouldob- 
evtir,!?,,; i'a C i 0mb “ SU t e ,* ullstances havi »g an e< i uai affinity with oxygen. Chlorine. 
gui.-hes burning bodies, a collateral proof of the mode in which it acts in colour, 
to he a°c anomalies, regarding the elective attractions, are only 
Cinitates silvi d r*’ tb , B P. nnca P ,es laid downin this proposition. Thus ironpre- 
nroiiertv-1,3. f ™ m US , so)ut, °n ; when silver is highly oxvdised, iron loses this 
P pe ty, brass pins, under certain circumstances, precipitate tin and thev an - 
A hundred Othe ^ th ® tiu be hig f lly ox > rdated ’ tbe braas fiina canno’t precipitate it. 
A lire i other instances imist be familiar to every chemist P 
scarlet. pi ' oduci,,g a s “ lmou colour > witU colouring matter employed to dye 
