98 Oeheimrath Professor Otto N. Witt [March 21, 



According to my theory, the process of dyeing, considered so 

 problematical by many experts in this ancient and useful art, is 

 strictly analogous with this wandering of molecules governed by the 

 laws of solution, which we can so easily observe and control in 

 operating with two non-miscible liquid solvents. 



In my next experiment (Exp. III.)* we see that a dye-stuff wanders 

 from the bath on to the fibre in much the same way as it wandered 

 from water into ether. And if the fibre be previously dyed with a 

 colouring matter little soluble in its substance, then this may be ex- 

 pelled and replaced by another of greater solubility. (Exp. IV.) * 



We see now that, in order to become a dye-stuff, a substance 

 must not only be so intensely coloured that it can communicate its 

 own shade to colourless substances holding it in solution ; it must 

 not only be soluble in water or any other liquid suitable for pre- 

 paring a dye-bath ; but it must also be soluble, and even much more 

 soluble than in water, in the colloid, which forms the substance of the 

 textile fibre. The finished dyed fabric is nothing more nor less than 

 a solid solution of the dye-stuff in the substance of the fibre, unless 

 there are secondary chemical influences, such as that of the mordants, 

 at work, which change the solution into a suspension by precipitating 

 the dye-stuff after its immigration into the fibre. 



This peculiar combination of solubilities is very rarely met with 

 amongst the coloured substances of an ai) organic nature. In the vast 

 domain of organic compounds of the aliphatic series we meet with 

 very few dye-stuffs, because its members are mostly colourless, or but 

 very faintly coloured. In the aromatic series, on the contrary, the 

 power of selective absorption of light is so very frequent, that it 

 would be very curious indeed if just that combination of solubilities, 

 which is the making of the dye-stuff, were not of common occur- 

 rence. Taking as a basis the universally admitted axiom, that the 

 physical properties of every compound are direct functions of its 

 molecular constitution, we may easily believe that that peculiar 

 combination of solubilities which I have shown to be the character- 

 istic feature of the dye-stuff, would be the result of certain general 

 conditions fulfilled in the constitution of many members of the 

 aromatic group. My theory, proposed five-and-twenty years ago, was 

 nothing else than an attempt to ascertain these general conditions 

 by investigating the constitutional peculiarities of all those dye- 

 stuffs the constitution of which had been fully established in those 

 days. 



I have no intention to tax your patience by explaining in detail 

 the results of that old investigation. It will be sufficient to sum- 

 marise them by saying that in the molecule of every colouring 

 matter, the constitution of which has been ascertained to this day 



• In Exp. III. wool was dyed with erythrosine in the ordinary way, whilst 

 in Exp. IV. a coltoa cloth, previously dyed with patent blue, was treated in a 

 bath of Congo red. 



