ACTION OP LIGHT ON SOLUTIONS OF METALLIC SALTS. 135 



Negative impregnation is primary because it is brought 

 about by the direct reduction of a metal in the intercellular 

 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 consequent stain- 

 ing 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, Heft 1, p. 1 ; GIEKKE, Zeit. f. wiss. Mik., i, 

 p. 393; RANVIER, 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. Reck- 

 linghausen held that the silver salt combined with a hypo- 

 thetical intercellular cement-substance (Kittsubstanz) , forming 

 a compound 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-mem- 

 branes, or that the metallic salt combines with the albuminous 

 and saline liquids that surround the cells, and is precipitated 

 in simple intercellular spaces. SCHWALBE (Arch. f. mik. Anat., 

 vi, 1870, p. 5) thinks that two cases should be distinguished; 

 the black lines that are obtained by the action of very weak 

 solutions for a very short time being due to a true precipitate 

 formed by reduction of metal in the intercellular liquids, the 

 brown lines that are obtained by exposing tissues for a longer 

 period to the action of more concentrated solutions being due 

 to the formation of a compound of metal and cement-sub- 

 stance that becomes brown on exposure to light. (For the 

 history of these questions see G-IERKE'S Farberei zu mikro- 

 skopischen Zwecken.) 



211. Action of Light on Solutions of Metallic Salts. Stock 

 solutions of metallic salts are generally kept in the dark, or 

 at least in coloured bottles, under the belief that exposure 

 to light spoils them by precipitating the metal in a state of 

 reduction. It has been pointed out above ( 25) that in the 

 case of osmic acid not light, but dust is the reducing agent ; 

 and that solutions may be exposed to light with impunity if 



