METALr,rC STAINS (IMPREGNATION METHODS). 219 



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 

 certain special studies on the cornea and on connective 

 tissue, the almost exclusive function of gold chloride is the 

 impregnation of nervous tissue, for which it exhibits a 

 remarkable selectivity. 



360. Pre-impregnation and Post-impregnation. — Gold methods 

 may be divided into two groups : viz. pre -impregnation 

 methods, characterised by employing perfectly fresh tissues, 

 and post-impregnation methods, characterised by the employ- 

 ment oi fixed and hardened tissues. Both are chiefly used 

 for nervous tissue. They give in some respects opposite 

 results. Pre-impregnation gives nuclei unstained, cytoplasm 

 rather strongly stained, axis-cylinders reddish-violet. Post- 

 impregnation gives nuclei sharply stained, cytoplasm pale, 

 axis-cylinders black, and (when successful) showing their 

 neurofibrils sharply distinguished from the interfibrillar 

 substance. 



In Apathy's view {Mitth. Zool. Stat. Neapel, xii, 1897, 

 p. 718) successful gold preparations should show a true stain, 

 not an impregnation (§ 347), the stain being brought about 

 by the formation of gold oxide (AuO) which combines with 

 the tissue elements. He advises in consequence that prepara- 

 tions should not he moved alout more than can be helped in 

 the reducing bath, so that the colouring oxide may not be 

 washed away from the tissues before the stain has taken 

 eifect. 



361. As to the Commercial Salts of Gold.— Squiee's Methods 

 and Formula}, etc. (p. 43), says : " Commercial chloride of 

 gold is not the pure chloride, AuClg, but the crystallised 

 double chloride of gold and sodium, containing 50 per cent, of 

 metallic gold. 



" Commercial chloride of gold and sodium is the above 

 crystallised double chloride mixed with an equal weight of 

 chloride of sodium, and contains 25 per cent, of metallic 

 gold." 



This, however, appears not to be the case in Germanj'. 



