MKTALLIC STAINS (IMPREGNATION METHODS). 243 



ripen by a good sunning. All photographers tell me their 

 papers salt more evenly by old well-sunned silver nitrate 

 rhan 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 (Mitth. Zool. Stat. Neapel, xii, 1897, p. 722) 

 leaves his gold solutions exposed to light, so long as there 

 are no tissues in them. 



337. State of the Tissues to be impregnated. It has been 

 pointed out in earlier chapters that the majority of histo- 

 logicnl stains are not obtained with fresh tissues, but with 

 tissues that have been more or less charged with metallic 

 salts, or otherwise changed in their composition by the action 

 of fixing and preservative reagents. With metallic impreg- 

 nations 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 sorts of im- 

 pregnations will not succeed at all with tissues that are not 

 fresh in the sense above explained. 



Silver. 



338. Silver Nitrate: Generalities. This is the most com- 

 monly used salt of silver. The general principles of its 

 employment are so well stated by RANVIER (Trait*', 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 

 employed, but is easy and gives good results. It is useful 

 for the study of the cornea and of fibrous tissue, 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. 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 



