324 SCIENCE PROGRESS 



extenl of its surface, exercises a catalytic action. Zacharias 

 thus distinguishes explicitly between dyestuff and dye on the 

 fibre. In the case of acid and basic dyestuffs, which are true- 

 salts, the dyes formed are the free colloidal colour-acids or 

 bases. With the direct cotton dyestuffs of high molecular 

 weights or pronounced colloidal character, the dyes on the fibre 

 do not differ in chemical constitution from the dyestuff in 

 solution. These dyes are not fast to washing, since they are 

 readily transformed back to the soluble condition. Dyes which 

 are fast to washing are always produced by some chemical 

 alteration of the dyestuff. 



On this theory the various dyestuffs can be divided into 

 crystalline and colloidal, but the division between these two 

 groups is not sharp. Picric acid for example is crystalloidal, 

 but has weak colloidal properties ; it dyes wool quickly, but does 

 not give a fast colour and it is incapable of dyeing vegetable 

 fibres. Magenta is colloidal, but not sufficiently so to be capable 

 of dyeing cotton direct. Every substance which assists the 

 colloidal separation of the dye is a useful addition to the 

 dye-bath. For this purpose acids are often added and dyeing 

 seldom takes place in an alkaline bath, since alkalies tend to 

 bring colloids into solution. Such addition to the dye-bath will 

 of course affect both K and v in the distribution equation, but 

 will have no effect on the nature of the process. 



This theory of Zacharias has much to recommend it, and it 

 seems to be a distinct advance towards a real solution of the 

 problem of dyeing. At the same time it is the logical outcome 

 or extension of the solid solution theory of Witt : it merely 

 assumes that the absorption is always accompanied by 

 secondary chemical changes. Many cases have been investi- 

 gated that support this, although some cases are known in which 

 the ratio of the concentrations in the liquid and in the fibre is 

 constant, i.e. 



p — Jv, 



and is independent of concentration within the limits examined. 

 This may be attributed purely to simple absorption without 

 any accompanying polymerisation or any chemical change, i.e. 

 the results in these particular cases clearly support Witt's 

 theory. Nevertheless such a theory cannot hold good in general 

 for the non-reversibility of the process in most cases speaks 



