THE DIRECT ATTACHMENT OF DYES TO TISSUES I97 



water. (IVIetachrome yellow, an acid dye, is exceptional in being 

 nearly insoluble in water, so that it could not easily escape even if 

 the bond with the tissue were loosened.) Acids and alkalis release 

 respectively basic and acid dyes, and once again the iso-electric 

 point of a particular tissue-constituent will determine whether 

 or how quickly it will release a dye. These facts form the basis of 

 regressive dyeing. Instead of allowing the dye to colour the tissues 

 progressively until the desired effect is obtained, one may over- 

 stain and then extract the excess of dye differentially from the 

 various tissue-constituents. The hydronium and hydroxide ions 

 diffuse through the tissue much more rapidly than any dye-ion 

 and therefore give more even results, especially if the piece of 

 tissue be thick. 



Although the basic dyes show more resistance to washing out 

 by distilled water than acid ones, many of them are very quickly 

 removed by the alcohols used in dehydration, which remove the 

 acid dyes more slowly. Indeed, the dehydrating alcohols act like 

 very weak acids. *^^ Their powders of extracting basic dyes diminish 

 in the series methanol, ethanol, propanol, butanol, pentanol.^^® 

 There appears not to have been any full investigation of the various 

 isomers of the three last-mentioned alcohols, but tertiarv butanol 

 is said not to remove any toluidine blue that has combined with 

 nucleic acids. ^^" It might be thought that basic substances, such as 

 aniline and pyridine, would help to hold basic dyes in the tissue, 

 but in fact they appear to compete with the dye for the acidic 

 components of the tissues. '^^^ 



A simple experiment, described in the Appendix (p. 324), 

 shows that acidified alcohol rapidly removes all colouring of 

 tissues by a basic dye, but leaves intact the colouring by an acid 

 dye. 



Mollendorff ^^^ considered that a sharp distinction was to be 

 drawn between the action of acid dyes and certain basic ones on 

 one hand, and of other basic dyes on the other. The latter, in his 

 view, were especially liable to flocculation and tended to be 

 precipitated on the surface of the objects for which they had an 

 affinity. This process he called precipitation-dyeing [Nieder- 

 schlagsfdrbiing). Other dyes, not subject to flocculation, penetrated 

 into the interstices of objects and dyed them uniformly throughout. 

 He called this permeation- dyeing (Durchtrdnkungsfdrbung). This 

 distinction is perhaps valid. One sometimes has the impression 

 that certain basic dyes and dye-lakes (p. 207) have a tendency to 



