112 METALLIC STAINS. 



Impregnation with silver may be followed by impregnation 

 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 produced. 



Gold. 



204. The Characters of Gold Impregnations. Gold chloride 

 differs from nitrate of silver in that it generally gives positive 

 ( 197) 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 sub- 

 stituting 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 O5 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 impregna- 

 tion 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. 

 A successful gold preparation shows at a glance, with diagram- 

 matic clearness, a wealth of minute detail which perhaps can 

 only be painfully glimpsed by other means. But not every 

 gold preparation is successful. I think there is no use in 

 blinking the fact that very few are successful (one of the 

 most experienced authorities in the matter told me lately that, 

 as to nerve-end-organs at all events, one preparation in ten 

 thousand is successful). I took up in the first edition of this 

 work the doubtless unpopular position that " with all possible 

 precautions gold chloride is uncertain in its action, and that 

 the results obtained by means of it need to be controlled by 

 the employment of other methods," and illustrated that 

 position at considerable length. Time has only confirmed in 

 me the opinion there expressed. It appears to me super- 



