CHAPTER XVIII. 



METALLIC STAINS (IMPREGNATION METHODS). 



333. 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 under- 

 stood a mode of coloration in which the colouring matter is 

 retained by the tissues as if in a state of solution, showing 

 110 visible solid particles under the microscope, the stained 

 elements remaining 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. Methylen blue, for 

 instance, will give in one and the same preparation an im- 

 pregnation and a stain; and critical examination of most 

 fairly successful gold chloride preparations will show that 

 the coloration is in places of the nature of a finely divided 

 solid deposit, in others a perfectly transparent stain. 



Thus is justified the alternative title that this chapter has 

 always borne Metallic Stains, or Impregnation Methods. 



334. Negative and Positive Impregnations. Impregnations 

 are distinguished as negative and positive. In a negative 

 impregnation intercellular substances alone are coloured, 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 



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