THE CHARACTERS OP IMPREGNATION-STAINS. 105 



CHAPTER XII. 



METALLIC STAINS (IMPREGNATION METHODS). 



197. The Characters of Impregnation-stains. Impregna- 

 tions 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.) 



Negative impregnation is primary because it is brought 

 about by the direct reduction of a metal in the inter- 

 cellular spaces. Positive impregnation is secondary (in the 

 case of silver nitrate), because it is brought about by the 

 solution in the liquids of the tissues of the metallic deposit 

 formed by a primary or negative impregnation, and the con- 

 sequent 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. f. Heilk., ii, Hft. i, p. 1. G-IERKE, Zeit. f. iviss. 

 Mik., i, p 393. RANVIEB, Traite, p. 107). 



There still exists considerable obscurity as to the nature of 

 the black or brown deposit formed in the intercellular spaces in 

 cases of primary impregnation with a silver salt ; v. Reckling- 

 hausen held that the silver salt combined with a hypothetical 

 intercellular cement-substance (Kittsubstanz), forming a com- 

 pound that blackens under the influence of light. Other 

 authors refuse to believe in the intercellular cement, and hold, 

 either that the coloured lines represent stained cell-membranes, 



