242 CHAPTER XVIII. 



more needful as a directly contrary statement is made in a 

 recent Jjeh/rbuch.} 



Negative impregnation is generally held to be primary 

 because it is brought about by the direct reduction of a 

 metal in the intercellular spaces. Positive impregnation is 

 held 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 or 

 negative impregnation, and the consequent staining of the 

 cells by the new solution of metallic salt thus formed. 

 These secondary impregnations take place when the reduc- 

 tion of the metal in the primary impregnation is not suffi- 

 ciently energetic (see on these points His, Schweizer Zeit. f. 

 Heilk., ii, Heft I, p. 1 ; CTIKRKK, Zeit. /. wins. Mik., i, p. 393 ; 

 RANVIKR, Tra-iti', p. 107). 



335. Nature of the Metallic Deposit. As to the nature of the black 

 or brown deposit or stain formed in the intercellular spaces in cases of 

 primary impregnation see previous editions, or SCHWALBE, Arch. f. mik. 

 Anat., vi, 1870, p. 5; GIERKE'S Fiirberei zu mikrosJeopischen Zwecken, in 

 vols. i and ii of Zeit. f. wiss. Mik. ; JOSEPH, Sitzb. d. k. preuss. AJcad. 

 d. Wiss. Berlin, 1888 ; Zeit. f. wiss. Mik., xi, 1, 1894, p. 42, et seq. 



336. 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 in 33 that in the case 

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

 that solutions may be exposed to light Avith impunity if dust 

 be absolutely denied access to them. I have now good 

 evidence to the effect that the same is the case with other 

 metallic solutions ; and the interesting point is raised whether 

 such solutions are not positively improved for impregnation 

 purposes by exposure to light ! Dr. LINDSAY JOHNSON has 

 been examining this question both from a histological and 

 from a photographic point of view, and writes me as follows : 



" One may (I find by experiment) state as a rule without 

 exception that all the solutions of the chlorides and nitrates 

 of the metals will keep indefinitely in clean white stoppered 

 bottles in the sunlight ; and as far as osmium, uranium, gold 

 <ind silver, and platinum are concerned, actually improve or 



