CHAPTER XVII. 



METALLIC STAINS (IMPREGNATION METHODS). 



347. The Characters of Impregnation Stains. — By impregna- 

 tion is understood a mode of coloration in which a colouring 

 matter is deposited in tissues in the form of a precipitate — ■ 

 the impregnated elements becoming in consequence opaque. 

 By staining, on the other hand, is understood a mode of 

 coloration in which the colouring matter is retained by the 

 tissues as if in a state of solution, showing no visible solid 

 particles under the microscope, the stained elements remaining 

 in consequence transparent. But it is not right 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 the preceding chapters 

 give, besides a stain, a ti-ue impregnation. Methylen blue, 

 for instance, will give in one and the same preparation an 

 impregnation and a stain ; and in most gold chloride prepara- 

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

 divided solid deposit, in others a perfectly transparent stain. 



348. Negative and Positive Impregnations. — 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 inter- 

 cellular spaces are unstained. (A directly contrary statement, 

 made in a recent Lehrbuch, is erroneous.) 



Negative impregnation is generally held to he primary because brought 

 about by the direct reduction of a metal in the intercellular spaces ; posi- 

 tive impregnation to be secondary (in the case of silver nitrate at least) 

 because it is brought about by the solution in the liquids of the tissues 

 of the metallic deposit formed by a primary impregnation, and the con- 



