198 METALLIC STAINS (IMPREGNATION METHODS). 



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 KEYNAUD, C.R. 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 ]east in coloured 

 bottles, under the belief that exposure to light reduces them. It 

 has been pointed out in 35 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 impregnation 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 (Mitt. 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 of the Tissues to be Impregnated.- The majority of 

 stains given by dyes are only obtained with tissues that have been 

 changed in their composition by the action of fixing and preservative 

 reagents. With metallic impregnations the case is different ; 

 perfectly fresh tissues that is, such as are either living, or at all 

 events have not been treated by any reagent whatever will also 

 impregnate with the greatest ease and precision. Indeed, some 

 impregnations will not succeed at all with tissues that are not fresh 

 in the sense above explained. 



Silver. 



351. Silver Nitrate : Generalities.- -The principles of its employ- 

 ment are given by RANVIER (Traite, p. 105) as follows : 



Silver nitrate may be employed either in solution or in the solid 

 state. The latter method is useful for the study of the cornea and 

 of fibrous tissues, but is not suitable for epithelia. For the cornea, 



