Synl//('/ic Dyes 109 



come colorless.) Because the nuclei hold considerably more dye tli;in 

 the cytoplasm, the dye is lost more completely from the latter, while 

 some still remains in the former. At the proper point of extraction from 

 the cytoplasm and when correct intensity is left in the nuclei, the slides 

 are taken out of the mordant and thoroughly washed, usually in run- 

 ning water, to remove excess mordant. Remaining traces can cause the 

 stain to fade in time. 



Method 2: By Use of Acids 



Acids are effective differentiators for some dyes, but a completely 

 adequate explanation for their action is not available. 



Method 3: By Use of Oxidizers 



Oxidizers furnish a third method of regressive staining, and by this 

 method the dye can be oxidized to a colorless condition. Oxidizers are 

 slow in action; the parts of the cell which hold only a small amount of 

 dye will be bleached before those possessing greater quantities of it. A 

 complete explanation is not available in this case. Picric acid, a com- 

 monly used chemical in this category, has both a moderate oxidizing 

 and a Aveak acidic action. 



Not to be confused with mordants are accentuators and accelerators. 

 Accentuators are substances which, contrary to the action of inordants, 

 do not becoine a part of the dye complex or lake. Instead they increase 

 the selectivity or stainability of the dye (example: phenol in carbol- 

 fuchsin). Accelerators, as their name implies, accelerate the action — 

 usually of importance here in silver iinpregnation (chloral hydrate). 



Synthetic Dyes 



Natural dyes had no competition until the middle of the nineteenth 

 century Avhen William Perkin worked out the processes for making 

 aniline or coal-tar dyes. 



Synthetic dyes, like natural ones, can be used either progressively or 

 regressively. An acid solution often is used to remove excess basic dye; 

 an alkaline solution is used to remove excess acidic dye. In some cases, 

 alcohol can act as a differentiator, particularly for basic dyes; but in 

 general a sharper differentiation is achieved by using an acid. 



The real importance of synthetic dyes lies in their use for double 



