CHAPTER XVII. 203 



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 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. 



). 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-impregna- 

 tion methods, characterised by the employment of 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 im- 

 pregnation ( 347), the stain being brought about by the formation 

 of gold oxide (AuO) which combines with the tissue elements. He 

 advises in consequence that preparations should not be moved about 

 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 effect. 



361. As to the Commercial Salts of Gold. SQUIRE'S Methods and 

 Formulas, etc. (p. 43), says : " Commercial chloride of gold is not 

 the pure chloride, AuCl 3 , 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 Germany. Dr. 

 GRUBLER, writing to MAYER (see the Grundziige, LEE und MAYER, 

 p. 215), says : " Aurum chloratum fuscum contains about 53 per 

 cent. Au, the fiavum about 48 per cent. ; in both of them there 

 should be only water and hydrochloric acid besides the gold, no 

 sodium chloride. Pure Auronatrium chloratum contains 14-7 per 

 cent, of sodium chloride, though samples are found in commerce 

 with much more," 



