CHAPTER XIX 

 METALLIC STAINS (IMPREGNATION METHODS) 



387. The Characters of Impregnation Stains. By impregnation 

 is understood a mode of coloration in which a metal 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 

 solntion, 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 true impregnation, Methylen 

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

 impregnation and a stain ; and in most chloride preparations the 

 coloration is in places of the nature of a finely divided solid deposit, 

 in others a perfectly transparent stain. 



Negative and Positive Impregnations. In a negative impregna- 

 tion intercellular substances alone are coloured, the cells them- 

 selves remaining colourless or very lightly tinted. In a positive 

 impregnation the cells are stained and the intercellular spaces 

 are unstained. 



Negative impregnation is generally held to be primary because brought 

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

 positive impregnation to be secondary (in the ease of silver nitrate at 

 least) because it is brought about by the solution in the liquids of the 

 tissues of the metaUic deposit formed by a primary impregnation, and 

 the consequent staining of the cells by the new solution of metallic salt 

 thus formed. These secondary impregnations take place when the 

 reduction of the metal in the primary impregnation is not sufficiently 

 energetic (see on these points His, Schweizer Zeit. Heilk., ii, Heft 1, p. 1 ; 

 Gierke, Zeit. zviss. Mik., i, p. 393 ; Ranvier, Traite, p. 107). 



As to the nature of the black or brown deposit or stain formed in 

 the intercellular spaces in cases of primary impregnation see Schwalbe, 

 Arch. mik. Anat., vi, 1870, p. 5 ; Gierke's Farberei zu mikroskopischen 

 Zzvecken, in vols, i and ii of Zeit. zviss. Mik. ; Joseph, Sitzb. Akad. Wiss. 

 Berlin, 1888 ; Zeit. zviss. Mik., xi, 1, 1894, pp. 42 et seq. It evidently 

 cannot consist of metallic silver, as it is soluble in hyposulphite of soda. 

 See also Macallum, Proc. Roy. Soc, Ixxvi, 1905, p. 217, and Achard 

 and Reynaud, C.R. Soc. Biol., Ixi, 1906, p. 43. 



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