METALLIC STAINS 205 



precise, the effect of a double stain is not produced. See hereon 

 Gerota, he. cif., § 394. 



397. Impregnation of Nerve Tissue. For this subject, which 

 includes the important hichromate-and-silver method of Golgi, 

 and the neurofibril methods o/Bielschowsky and Ramon y Cajal, 

 see Part II. These give important results, not only -with Nervous 

 tissue, but zvith various forms of Connective tissue, mitochondrial 

 formations, etc. 



GOLD 



398. The Characters of Gold Impregnations. Gold chloride 

 differs from nitrate of silver in that it generally gives positive 

 (§ 387) impregnations only. It generally gives negative images 

 only with such tissues as have first received a negative impreg- 

 nation 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 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. 



399. 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 employment 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 success- 

 ful) 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 (§ 387), 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. 



400. As to the Commercial Salts of Gold. Squire's Methods 

 and Formulce, etc. (p. 43), says : " Conunercial chloride of gold is 

 not the pure chloride, AuCl.j, 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 crystal- 



