32 
discovery of the sea passage to India that it became gene- 
rally known in Europe ; but its use as a dye was greatly 
retarded by the opposition it met with from the large vested 
interests of the cultivators of woad, isatis tinctoria, the Euro- 
pean indigo -plant. The English, French, and several Ger- 
man governments were induced by the growers of woad to 
promulgate severe enactments against it. Thus Henry IY. 
of France issued an edict condemning to death anyone who 
used that pernicious drug called “ devil’s food.” The em- 
ployment of woad was however gradually superseded by 
that of indigo, and as soon as organic chemistry had ad- 
vanced far enough, chemists began to examine this import- 
ant colouring matter, which was first obtained in the pure 
state by O’Brien, who states in his treatise “ On Calico 
Printing,” 1789, that on heating indigo the colouring matter 
volatilises, forming a purple vapour, which condenses as a 
blue powder, whilst the impurities of the commercial pro- 
duct are left behind. Indigo-blue, or indigo tin as the pure 
compound is called, was afterwards analysed by several 
chemists, who found that its most simple formula is C 8 H 6 NO, 
which was subsequently doubled for several reasons. 
The literature of the chemistry of indigo is very large; 
of the numerous researches, I can here mention only those 
bearing directly on my subject. 
In 1840, Fritzsche found that on distilling indigo with 
potash a basic oil is produced, which he called aniline 
CeHg.NHa from anil, 'by which name the Portuguese intro- 
duced indigo first into Europe. The word is Arabic, and 
means simply, the blue.’ In the following year he obtained 
by boiling indigo with caustic soda solution and manganese 
peroxide a compound which he called anthranilic acid, 
and which, now known as orthamidobenzoic acid C 6 H 4 (NH 2 ) 
C0 2 H. He also observed that by heat it is resolved into 
aniline and carbon dioxide. 
At about the same time Erdmann and Laurent inde- 
