238 DYEING 



difference in rate of penetration. Many dyes are dispersed in 

 particles that are larger than single anions. Estimates of the size of 

 the particles can be obtained by measurement of osmotic pressure 

 or electrical conductivity or by ultra-centrifuging or ultrafiltra- 

 tion through cellophane or collodion film.*^^''^^ It is clear that 

 the anions of a dye often aggregate together. Sometimes the par- 

 ticles are formed by the aggregation of several anions only, some- 

 times by the aggregation of several anions with a smaller number of 

 sodium or other cations, the remaining cations being free. The 

 resultant charge on the dye particles of an acid dye therefore varies 

 in different cases, though it is always negative. It is probable that 

 ion-aggregates break up below the temperatures at which textile 

 dyeing takes place, but they are present at room temperature and 

 play an important part in microtechnique. The dyes used for 

 collagen are the ones that form large aggregates, while the typical 

 background or levelling dyes are dispersed as single ions. 



The size of the spaces or pores in the constituent parts of fixed 

 microscopical preparations is not known, but the structure of wool 

 may give us some impression of what to expect. The protein is 

 here in the form of long, nearly parallel chains. Here and there the 

 chains become quite parallel and closely bound together, so that 

 a submicroscopic crystal or micelle is formed. The chains emerging 

 from the end of a micelle wander loosely for a bit before they enter 

 and form part of several other micelles. Thus there are minute 

 crystalline and non-crystalline regions in the fibre, the former too 

 compact to be entered by any dye, the latter loose and containing 

 spaces between the somewhat irregularly arranged threads. The 

 available evidence suggests that the diameter of these spaces is 

 about 3 J or 4 m/x.^^^ Dyes must enter these if they are to permeate 

 and colour the fibre. The cellulose fibres of cotton are arranged in 

 a very similar way, the spaces being about 2 to 10 m/x in diameter. 



It is probable that the various constituents of a fixed micro- 

 scopical preparation vary greatly in the size of the spaces within 

 them.*^^'*^^ Collagen is an example of a substance of very loose 

 texture, readily entered by any dye; cytoplasm has a tighter con- 

 sistency and is more selective towards dyes; the contractile sub- 

 stance of muscle is somewhat tighter still; while the red blood- 

 corpuscles of mammals are among the least pervious of all tissue- 

 constituents. The dyes that colour red blood-corpuscles are those 

 that diffuse particularly easily through a fine collodion membrane. 

 With three selected acid dyes one can colour collagen, ordinary 



