METALLIC STAINS (IMPREGNATION METHODS). 249 



monia be avoided, as this would dissolve out the silver. 

 These stains will only succeed, however, with successful 

 negative impregnations, as nuclei that have been impreg- 

 nated will not take the second stain. 



Impregnation with silver may be followed by impregna- 

 tion with gold. In this case the gold generally substitutes 

 itself for the silver in the tissues, and though the results are 

 sharp and precise, the effect of a double stain is not pro- 

 duced. See hereoii GEROTA, loc. cit. 341. 



Gold. 



346. The Characters of Gold Impregnations. Gold chloride 

 differs from nitrate of silver in that it generally gives positive 

 ( 334) impregnations only. It only gives negative images, 

 so far as I know, when caused to act on tissues that have 

 first received a negative impregnation with silver, the gold 

 substituting itself for the silver. In order to obtain these 

 images you first impregnate very lightly with silver ; reduce ; 

 treat for a few minutes with a 0*5 per cent, solution of gold 

 chloride, and reduce in acidulated distilled water. 



This process, however, is in but little use, and except for 

 the staining of cytoplasm for cytological researches and for 

 certain special studies on the cornea, and on connective 

 tissue, the almost exclusive function of gold chloride is the 

 impregnation of nervous tissue. For this tissue, gold chloride 

 exhibits a remarkable selectivity, in virtue of which it justly 

 ranks as a most valuable reagent for the study of nerve end- 

 organs and the distribution of nerves. 



For all the objects above named gold chloride is capable 

 of furnishing preparations that for beauty and clearness 

 cannot be surpassed, if even they can be equalled by any 

 other means. But not every gold preparation is successful. 

 Further, gold chloride is very uncertain in its action. 



It is now acknowledged that the very best gold preparations 

 give images that are only worthy of credence as to what 

 they show, and furnish absolutely no 1 evidence whatever as 

 to the non-existence of anything that they do not show ; for 

 you can never be sure that the imbibition of the salt has 

 not capriciously failed, or its reduction capriciously stopped 



