332 
Pithalines.—The discovery of this class of bodies dates back 
to 1871, and was the result of the investigation of Baeyer. He 
found that phenols unite with a number of polybasic acids and 
with aldehydes, with separation of water when the mixture is 
heated alone, or with glycerol and sulphuric acid, the compounds 
formed not being ethers. Those produced when phthaiic anhyd- 
ride is employed and which embrace those of practical value, are 
called phthalines. The first of these discovered by Baeyer was 
gallein (Ber. iv. 457), produced by heating pyrogallol with 
phthalic anhydride ; its formula is C.9H,,O;; by reduction it 
loses the elements of water and with hydrogen forming 
cerulein. These colouring matters, which for a long time remained 
unnoticed, are now being extensively used. 
Later, in 1871, Baeyer discovered resophthalin, or fluorescein 
(Ber. iv. 555). This substance, which is remarkable for its 
yellowish green, fluorescence, dyes silk and wool yellow, but 
does not combine with mordants, and is not a very useful dyeing 
agent. But it was discovered by Caro in 1874, the subject being 
afterwards worked out jointly with Baeyer, that fluorescein when 
brominated yielded that beautiful dyestuff now called cosine; this 
was introduced into the market in July, 1874. Other substitution 
products were then studied, and the iodine product was found 
to give bluer shades of red than the bromine. One of the most 
beautiful colours of this series is the dichlortetraiodofluorescein, 
in which dichlorphthalic anhydride is used in its preparation. 
It is called phloxine. The methylic ether of eosine and its 
nitro derivative also have become commercial articles. These 
bodies are now manufactured in a practically firm condition. 
Their structure has been made out by research to be 
as follows :— 
ee oa! 
Ce a 
| DS O 
(oreye) X 
C,H,0H 
Eosinetetrabromofluorescein (Potassium Salt). 
C,H. Br,(OK) 
CoH, | 
l ee O 
coo x 
‘C,H. Br,(OK) 
Tetraiodofluorescein (Potassium Salt). 
/SeHISOK) 
CoH Ya | 
1S: O 
coo” ™~ 
Fluorescein 
\c,HI,(OK) 
Dichlortetraiodofluorescein (Potassium Salt). 
ae I,(OK) 
<r {8) 
yO 
coo % NC, HI.(OK) 
Gallién CH, _/CsH,(OH)s 
| C : 
! 
GOO” © 
| 
C HOH)» 
The introduction of these colouring matters had a great in- 
fluence on the manufacture of phthalic acid. This acid, it will 
be remembered, was proposed a good many years since for the 
production of benzoic acid, which was largely in demand for the 
manufacture of aniline blues, phthalic acid when carefully treated 
with lime yielding calcium benzoate. But as phthalic acid was 
required to be produced in an extensive way, new experiments 
had to be made on the subject. The difficulties connected with 
this were surmounted by the Badische Aniline Fabrik, who are 
now the chief manufacturers of this body and its anhydride, 
which is the substance required ; when freshly prepared it is one 
of the most beautiful products one can see. 
Dichlorophthalic acid is now also manufactured for the pre- 
paration of the bluish shades of fluorescein derivatives already 
referred to. But this is not all; it was not only necessary to 
produce this anhydride in quantity, but it was necessary also to 
produce vesorceno/. This substance was originally prepared from 
galbanum by fusing it with potash, or by distilling brazilin, 
&c., both technically impractical processes. It was afterwards 
produced by fusing various halogen derivatives of phenol and 
benzene sulphonic acid with alkali; these also were not practical 
processes. It was, however, eventually found that it could be 
NATURE 
[August 6, 1885 
produced by fusing metabenzenedisulphonic acid with potash, 
the original observation being made by Carrick; and by this 
process this product, which was a rare compound, is now manu- 
factured and has become a common one, being produced in very 
large quantities. 
Indigo Sertes,—Indigo is too wellknown a substance for me 
to make any remarks in reference to its history as a colouring 
matter, and with reference to the chemical side of the question 
I suppose few substances have had more work bestowed upon 
them than this body, so that I must confine any few remarks to 
its artificial formation. There is one point of interest, however, 
connected with indigo, and that is that it was the original source 
of aniline, this base being discovered in the products of its 
destructive distillation by Unverdorben, in 1826, as already 
referred to. 
Notwithstanding the large amount of work which has been 
bestowed upon this colouring matter, its constitution has only 
been lately arrived at, and for this, and the methods of its 
artificial formation, we are indebted tothe beautiful and labo- 
rious researches of Baeyer. The first process for its artificial 
production was patented by Baeyer in March, 1880. The 
process consists in preparing orthonitropropiolic acid and acting 
upon it in presence of an alkali, with a reducing agent, such as 
grape sugar, xanthate of sodium, &c. 
2[C,H,(NO,)O,] + Hy=2CO,+C,sHy)N,0. + 2H,0. 
This process renders the application of artificial alizarin very 
easy to calico printing, because the products can be applied to 
the fabric and the reaction then co npleted, and thus the indigo 
is formed and fixed in the fibre, and this process is in use in some 
of the printworks of Mulhouse, where there is a continued though 
small demand for nitroorthopropiolic acid. Other processes have 
been discovered by Baeyer for the formation of Indigo ; he has 
found that it can easily be formed from orthonitrobenzaldehyde 
by condensation with bodies containing CH,CO group, such as 
acetone. 
Hitherto this artificial formation of indigo has not met with 
much practical success. This does not arise from difficulties in 
its manufacture, but in its cost compared with natural indigo, 
which is a very cheap dyestuff. 
So far as it has been manufactured, however, the possibility of 
this has been entirely dependant upon scientific research discon- 
nected with its study. To prepare nitropropiolic acid it is 
necessary to begin with cinnamic acid as a raw material. This 
acid, until 1877, was only obtained from certain balsams, and 
was a very costly material. It was then discovered that it could 
be produced with comparative ease by the action of acetic anhy- 
dride and an acetate on benzaldehyde (Horn. Chem. Soc. xxxi. 
428). Caro afterwards found that this process might be simplified 
by heating a mixture of benzylidene dichloride with sodiam 
acetate, and it is by this process that it is now prepared. 
The constitution of indigo Baeyer represents as follows :— 
CHG .CO—G.H, 
| | 
NH—C =—=C—NH 
Several derivatives have been made which are interesting dyes, 
such as methyl indigo, tetrachlor indigo, etc. 
so compounds.—The commencement of the history of the 
azo colours in an industrial sense has little to do with the theo- 
retical side of the question, the early products being the offspring 
of empirical observations, and in no way connected with the 
theory of the diazo compounds, a condition of things very differ- 
ent from that now existing. Time will not allow me to enter 
into the beautiful work of Griess, much of which will be found 
in the Philosophical Transactions for 1864. 
The first definite compound of this class, shown to possess 
dyeing powers, was a substance discovered by Prof. Church and 
myself, known first as nitrosonaphthalene, then as azodinaphthyl- 
diamine, but now called amidoazonaphthalene. This substance, 
however, was of no practical value, because its salts, which are 
violet, cannot exist except in the presence of a certain amount of 
free acid. This substance has since been found of value in the 
preparation of the Magdala red. 
The first substance of this class sent into the market was the 
phenylic analogen of amidoazonaphthalene by amidoazobenzene, 
which was discovered by Mene. It was introduced by Nicholson, 
who prepared it by a proeess which has not been published. It 
was afterwards patented by Dale and Care, in 1863. This was 
' a yellow dye, but did not demand success, because of its vola- 
« 
