CHAPTER XVII. 



METALLIC STAINS (iMPKEGNATION METHODS). 



347. The Characters of Impregnation Stains. By impregnation 

 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 impregna- 

 tion, 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. 



348. Negative and Positive Impregnations. In a negative impreg- 

 nation 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. (A directly contrary statement, made in a recent 

 Lehrbuch, is erroneous.) 



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 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 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. wiss. 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 Fdrberei zu mikroskopischen 

 Zwecken, in vols. i and ii of Zeit. wiss. Mik. ; JOSEPH, Sitzb. Akad. Wiss. 



