136 METALLIC STAINS (IMPREGNATION METHODS). 



dust be absolutely denied access to them. I Lave 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 im- 

 pregnation purposes by exposure to light ! My able friend 

 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 

 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 un- 

 satisfactory/' I have not been able myself to perform any 

 experiments for the purpose of confirming the hypothesis of 

 ripening by sunning, but I must say that it appears to me 

 very plausible and deserving of further inquiry. 



Silver. 



212. Silver Nitrate : Generalities. This is the most commonly 

 used salt of silver. The general principles of its employment 

 are so well stated by RANVIEE (Traite, p. 105) that I cannot 

 do better than abstract his account. 



Silver nitrate may be employed either in solution or in the 

 solid state. The latter method is the less frequently em- 

 ployed, but is easy and gives good results. It is useful for 

 the study of the cornea and of fibrous tissue, but is not suit- 

 able for epithelia. For the cornea, for instance, proceed as fol - 

 lows : 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. 

 On subsequent examination it will be found that the silver 

 nitrate, which was dissolved by the liquid that bathes the 

 surface of the cornea, has traversed the epithelium and soaked 



