134 METALLIC STAINS (IMPREGNATION METHODS). 



CHAPTER XII. 



METALLIC STAINS (IMPREGNATION METHODS). 



210. The Characters of Impregnation-Stains. By impregna- 

 tion is understood a mode of coloration in which a colouring 

 matter is deposited in certain elements of tissues in the form 

 of a more or less finely granular or sometimes even flocculent 

 precipitate the impregnated elements becoming in conse- 

 quence opaque. By staining, on the other hand, is understood 

 a mode of coloration in which the colouring matter is retained 

 by the tissues in a state of solution, showing no visible solid 

 particles under the microscope, the stained elements remain- 

 ing in consequence transparent. But it should be understood 

 that it is not correct to draw a hard and fast line between 

 the two kinds of coloration. Some of the metallic salts 

 treated of in this chapter give, besides an impregnation, in 

 some cases a true stain. And some of the dyes that have 

 been treated of in preceding chapters give, besides a stain, 

 a true impregnation. Methyl en blue, for instance, will give 

 in one and the same preparation an impregation and a stain ; 

 and critical examination of most fairly successful gold-chlo- 

 ride preparations will show that the coloration is in places of 

 the nature of a finely divided solid deposit, in others a per- 

 fectly transparent stain. 



Thus is justified the alternative title that this chapter has 

 always borne Metallic Stains, or Impregnation Methods. 

 Impregnations are distinguished as negative and positive. 

 In a negative impregnation, intercellular substances alone are 

 coloured of a deep black or brown or violet, according to the 

 method employed ; the cells themselves remaining colourless 

 or very lightly tinted. In a positive impregnation the cells 

 are stained and the intercellular spaces are unstained. (This 

 explanation is the more needful as a directly contrary state- 

 ment is made in a recent Lehrbuch.) 



