144 METALLIC STAINS (IMPREGNATION METHODS). 



claim for them that they give permanent preparations. 1 

 warn the reader against indulging in the hope that, with all 

 possible precautions, his preparations will retain all their 

 beauty for more than a few weeks. A successful gold pre- 

 paration is certainly a thing of beauty, but it is exactly the 

 opposite of a joy for ever. The able histologist whose ex- 

 perience 1 have taken the liberty of quoting above tells me 

 that " as to permanence, they are 



" ' Like the snowfall on the river.' " 



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 pre- 

 parations. 



Careful attention to the devices to this end detailed in 

 the following paragraphs will do much; and possibly LINDSAY 

 JOHNSON'S suggestion (supra, 211) of the utility of " sun- 

 ning " the solutions before use may prove an unexpected help. 



221. As to the Commercial Salts of Gold. It is necessary to 

 remind the histologist that " all is not gold that glitters." 

 Many things are not what they seem, and gold chloride is 

 one of them, as will appear from the following quotation from 

 SQUIKE'S Methods and Formula*, &c. (p. 43), an excellent 

 authority on the chemistry of histological reagents : 



" 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." 



222. The Two Types of Method. 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 

 that have been subjected to a special treatment by organic 

 acids ; the other, concerned with the study of nerve-centres, 

 is characterised by the employment of tissues hardened in the 

 usual way. 



The hitherto classical rule, that for researches on nerve- 



