1830.] 
On Indigo . 
209 
its evaporation, the red principle in the form of a blackish -brown varnish' 
5. Phosphate of lime. . i have found the bone phosphate in notable quantity in 
some line indigo, constituting 1 another feature of resemblance between this vege- 
table and animal products. Hence, also, the charcoal of indigo is most difficult 
of incineration, and requires, for perfect combustion, in some cases, the defla«ra- 
tory powers of nitric acid. ° 
Pure indigo-blue is most easily obtained from the blue vat of the indigo-dyer ; 
the yellow liquid of which being acidulated faintly with muriatic acid, and exposed’ 
with occasional agitation, in a shallow basin, soon deposits the blue precipitate, 
mixed, however, with a considerable quantity (more or less according to the 
quantity of indigo used,) of indigo-red. This must be removed from the dried blue 
powder, by the solvent action of boiling alcohol applied in successive quantities. 
In my paper on the Ultimate stnuly sis of f' egetable and minimal Substances , which 
the Royal Society did me the honour to read at their meeting in June 1822, and 
to publish in the volume of their Transactions for that year, I gave an analysis of 
indigo-blue, to which I appended the following remarks “ I had intended to pursue, 
at considerable detail, my researches on this azotized product of vegetation, but 
the subject having been lately taken up by my pupil and friend Mr. Waller Crum, 
I was induced to leave it in his hands.” I then thought it likely that some slight 
modification might require to be made in the weight of the constituents given by 
me, for “ I did not (then) resume the subject of indigo, after I had become most 
familiar with the manipulations. I have found since that my mode of analysis 
was not in fault, but the revived indigo blue, which I employed, had not been 
entirely purged of the red principle, by sufficient ebullitions with alcohol ; for it 
adheres very tenaciously. Hence that resinous matter introduced a little oxygen 
and hydrogen, more than absolute indigo-blue contains. But the error will ap- 
pear inconsiderable, if we compare the result with the analysis previously pub- 
lished by Dr. Thomson*. The following is a view of the ultimate constituents 
of indigo-blue, as given by different chemists: 
Thomson. Ure. Crum 2 . 
Carbon, 
Oxygen, 
Azote, 
Hydrogen, 
40.384 
46.154 
13.462 
0-000 
71.37 
14.25 
10.00 
4.38 
73.22 
12.60 
11.26 
2.92 
Roger and Dumas. 
71-71 
12.18 
13.45 
2.66 
That pure indigo contains hydrogen, I have recently placed beyond a doubt, by 
heating a mixture of it and calomel in a green glass tube, the open end of which 
terminated in an inverted tube, filled with nitrate of silver. Copious fumes of 
muriatic acid were evolved, and chloride of silver was precipitated in its charac- 
teristic curd. 
The liquor of the dyer’s vat (for calico printing) contains indigo deoxydised by 
protoxide of iron, and dissolved in lime water. This solution, in its average state 
l Dr. Ure, we conclude, means that, on comparing his and Dr, Thomson’s 
analysis with the other two, his will be found to differ less than Dr. T.’s By calling 
this difference error, Dr. Tire must mean, that he considers Mr. Crum’s and MM. 
Roger and Dumas’ analysis to be correct. Our opinion, we are aware, will be thought 
worth little; yet, we will venture to say, that any one who runs his eye over the most 
recent analyses of organic substances, must be struck with difficulties and anomalies 
which can only he explained by supposing Analytical Chemistry to be not the perfect 
art which Dr. Ure would wish us to suppose. Even in the analyses of inorganic sub- 
stances we see discrepancies, which are sufficient to unsettle our faith in chemistry as 
one of the exact sciences. When we find Sir Humphrey Davy, and many of the first 
chemists in Europe, so completely mistaking the nature of Wavellite, as determined by 
more recent analyses; when we observe Berzelius, after despoiling Thorinaof its 
title to be considered one of the earths, (a title originally conferred by himself, and 
which it had quietly possessed for so many years,) coming forward with a new 
Thorina, which he insists is the real Simon Pure ; we cannot help a certain degree 
of incredulity stealing on us in regard to the dogma of the infallibility of analytical 
chemistry. What, if after all, Dr. Ure, Mr. Crum, and MM. Roger aud Dumas, 
were to be found in error, and Dr. Thomson right ? or what if all four analyses 
were to be found erroneous ? then what would the comparison with Dr. Thomson’s 
result prove? That such a supposition does not involve any impossibility, Dr. Ure 
will not venture to assert. Tne attempt to regulate prices by the analyses of the 
chemist, when we see such discrepancies as appear in the list given at the end of 
the present paper, is visionary ; at least till the subject is better understood than it 
appears to be at present. — E. Ol. 
2 Dr. Ure omits to notice Crum’s analysis of Cerulin and Phenicin, in which there 
appears to be 29 and 21 per cent, of oxygen ; neither does he affirm whether the 
“ pure indigo” was in the state of sublimed crystals or precipitated vat-fecula. — E.G1. 
