106 METALLIC STAINS. 



or that the metallic salt combines with the albuminous and 

 saline liquids that surround the cells and is precipitated in 

 simple intercellular spaces. SCHWALBE (Arch. f. mik. Anat., vi, 

 1870, p. 5) thinks that two cases should be distinguished; 

 the black lines that are obtained by the action of very weak 

 solutions for a very short time being due to a true precipitate 

 formed by reduction of metal in the inter-cellular liquids ; the 

 brown lines that are obtained by exposing tissues for a longer 

 period to the action of more concentrated solutions being due 

 to the formation of a compound of metal and cement-substance 

 that becomes brown on exposure to light. (For the history 

 of these questions, see G-IERKE'S Fdrberei zu mikroskopischen 

 Zivecken.) 



Silver. 



198. Silver Nitrate. This is the most commonly used salt 

 of silver. The general principles of its employment are so 

 well stated by RANVIER (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 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 epi- 

 thelium. 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 

 into the fibrous tissue, on the surface of which it is reduced 

 by the action of light. The cells of the tissues will be found 

 unstained. 



Silver nitrate is generally employed in solution in the fol- 

 lowing manner : A 1 per cent, solution is taken, to which 2, 

 3, or 4 volumes of water are added according to circumstances. 

 The mode of employment varies in its details according to 

 circumstances, a point which it is very important to observe. 



