METALLIC STAINS 201 



388. 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 § 40 that in the ease 

 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. We 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 purj^oses by exposure to light ! Dr. 

 Lindsay Johnson wrote to Lee 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. 



389. 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 pre- 

 cision. Indeed, some impregnations will not succeed at all with 

 tissues that are not fresh in the sense above explained. 



SILVER 



390. 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, for instance, proceed as follows : The eye having 

 been removed, a piece of silver nitrate is quickly rubbed over the 

 anterior surface of the cornea, which is then detached and placed 

 in distilled water ; it is then brushed with a camel's hair brush in 

 order to remove the epithelium. The cornea is then exposed to 

 the action of light. It will be found that the nitrate has traversed 

 the epithelium and soaked into the fibrous tissue, on the surface 

 of which it is reduced by the light. The cells of the tissues will 

 be found unstained. 



It is generally employed in solution, in the following manner : 

 In the case of a membrane, such as the epiploon, the membrane 



