250 CHAPTER XVIII. 



at any point. Thafc the images frequently do stop caprici- 

 ously short in the representation of reality there is abundant 

 evidence. 



Few of the methods about to be described give perfectly 

 permanent preparations. Most of them will not retain all 

 their beauty for more than a few weeks. Still, the greater 

 the care taken in preparation, and particularly the greater 

 the care taken to ensure thorough reduction of the gold, the 

 longer will be the life of the preparations. 



Careful attention to the devices to this end detailed in 

 the following paragraphs will do much ; and possibly LINDSAY 

 JOHNSON'S suggestion (supra, 336) of the utility of " sun- 

 ning " the solutions before use may prove an unexpected 

 help. 



347. As to the Commercial Salts of Gold. SQUIRE'S Method* 

 <uid Formulte, etc. (p. 43), an excellent authority on the 

 chemistry of histological reagents, 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. GRUBLEU, writing to MAYER (seethe Grundziic/e, LEE und 

 MAYER, p. 215) says : " Aurtim chloratum fusaum contains 

 about 53 per cent. Au, the fiaonm 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." 



APATHY (Mitth. ZooL Stat. Neapel, xii, 1897, p. 722) 

 formerly employed the aiirum chloratum Jtavum, but now 

 prefers the fuscunt. 



348. Foregilding and Aftergilding. Gold methods may be 

 divided into two groups ; the one, chiefly concerned with the 

 study of peripheral nerves or nerve end-organs, is charac- 

 terised by employing either perfectly fresh tissues or tissues- 



