MKTALIJC STAINS (IMPREGNATION METHODS). 213 



sequent staining of the cells by the new solution of metallic salt thus 

 formed. These secondary impregnations takes place when the reduction 

 of the m.etal in the primary impregnation is not sufficiently energetic 

 (see on these points His, Schweizer Zeit. Heillc, ii, Heft 1, p. 1 ; Gierke, 

 Zeit. wiss. Mik., i, p. 393 ; Ranvibe, 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 Schwalbb, 

 Arch. mile. Anat, vi, 1870, p. 5 ; Gibeke's Fiirberei zu onikroskopischen 

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

 Berlin, 1888 ; Zeit. wiss. Mik., xi, 1, 1894, p. 42 et seq. It evidently 

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

 See also Macallum, Proc. Boy. Soc, Ixxvi, 1905, p. 217, and Achakd 

 and Eeynaud, C. B. Soc. Biol, Ixi, 1906, p. 43 



349. 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 reduces them. It has been pointed out in § 85 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 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 point is raised 

 whether such solutions are not positively improved for im- 

 pregnation purposes by exposure to light ! Dr. Lindsay 

 Johnson 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 

 and silver, and platinum are concerned, actually improve or 

 ripen by a good sunning. All photographers tell me their 

 papers salt more evenly by old well-sunned silver nitrate 

 than by a fresh solution kept in the dark ; and I go so far 

 as to say that this is one of the reasons why gold stains are 

 so unsatisfactory." 



Apathy {Milt. Zool. Stat Neapel, xii, 1897, p. 722) leaves 

 his gold solutions exposed to light, so long as there are no 

 tissues in them. 



350. State tf the Tissues to he Impregnated. — The majority of 

 stains given by dye-i are only obtained with tissues that have 

 been changed in their composition by the action of fixing and 



