102 OeMmrath Professor Otto N. Witt [March 21, 



search for new dye-stuffs was thus opened, a field which has occupied 

 hundreds of bnsy workers for many years, many of whom carried 

 home a rich reward. 



But whilst this field bore its rich harvest, others were by no 

 means neglected. The search for dye-stuffs, which will dye cotton 

 without a mordant, could not make us forget that just those colour- 

 ing-matters which imperatively demand the use of mordants are 

 those which from times immemorial have been used in preference 

 for the production of fast and lasting shades. The brilliant synthesis 

 of alizarine by Graebe and Liebermann, which made the world ring 

 with admiration early in the seventies, had given us ample proof 

 that the old and to this day not wholly forgotten axiom, that there 

 are two kinds of dyes: natural ones, which are fast, and artificial 

 ones, which are fugitive, was a preconceived idea, totally devoid of 

 any scientific foundation. The enormous financial success of the 

 alizarine industry formed a tempting invitation to search for other 

 dye-stuffs, which, similar to alizarine, would be endowed with the 

 power of forming almost indestructible lakes with mordants of a 

 sesquioxydic nature. Here too, like everywhere in science, we have 

 marched for some time on the paths of empiricism, but here too 

 logical deduction has come to our aid in disclosing the laws which 

 govern the formation of lakes. In this case it is not (as in the sub- 

 stantive azo-dyes) the carbonic nucleus which determines the physical 

 properties (viz. the ratio of solubilities) of the dye-stuff, but it is 

 the peculiar position of the auxochromic groups contained in the 

 molecule, which governs its chemical properties. We know now, 

 that a dye-stuff must contain, in order to be able to form lakes with 

 sesquioxydic mordants, two hydroxyl groups in juxtaposition. If 

 this condition be fulfilled, the dye-stuff will dye in the same way and 

 with equal fastness as alizarine, even if it be no derivative of anthra- 

 cene, like the early alizarine dyes ; and if these two hydroxyl groups 

 or a suitable equivalent for them be missing it will lack all power 

 of dyeing mordants, even though derived from anthracene. With 

 this law once established the synthesis of mordant-dyestuffs became 

 a very easy matter, and to-day there is hardly a group of colouring- 

 matters in which there are not some members possessed of this 

 peculiarity and owing it to the same uniform cause. Still the group 

 of the oxyketones, to which alizarine itself belongs, remains the true 

 home of mordant-dyes, but this group has grown to-day into a very 

 numerous and varied one. Mordant-dyes of every shade are to be 

 found in it, and cotton is no longer the only fibre to which such dyes 

 are applied. It is a fact worthy of notice, that amongst the many 

 dyes of this class which we now possess and the constitution of which 

 is fully established there is not a small number, the molecule of 

 which contains three, four, five or even six hydroxyl groups. Yet 

 this increase of auxochromic groups does not influence in the least 

 the behaviour of these dyes to mordants, this is only governed by 

 the two hydroxyl groups in ortho-position, and any other such group 



