150 METALLIC STAINS (IMPREGNATION METHODS). 



soda ; wash in water ; reduce until the sections turn crimson 

 (about an hour) in 1 per cent, formic acid kept " fairly hot " 

 on a water-bath, in the dark ; wash for half an hour in cold 

 water; dry the sections (sic) and mount them in glycerin 

 jelly (" specimens mounted in balsam always go wrong"). 



Dr. LINDSAY JOHNSON writes to me that besides the " sun- 

 ning" of the impregnating solution recommended above 

 ( 211), the following precautions should be taken : " The 

 tissue must be well washed in distilled water, and the gold 

 carefully acidulated with a neutral acetate or forrniate, or 

 acetic or formic acid, at least twenty-four hours before using ; 

 and then afterwards the tissue must be washed until no re- 

 action occurs to test-paper." 



For the details of the application of the methods of which 

 the principles have been set forth above, and for those of the 

 important processes of impregnation of central nerve organs, 

 the reader is referred to those chapters of Part II. which treat 

 of nerve-tissues and organs. 



229. Ulterior Treatment of Impregnated Preparations, Pre- 

 parations may be mounted either in balsam or in acidulated 

 glycerin (1 per cent, formic acid). 



Theoretically they ought to be permanent if the reduction 

 of the metal has been completed effected. 



In practice, all are doomed to destruction in course of time 

 by after-blackening, and few will be found to survive more 

 than a few months. Ranvier states that this can be avoided 

 by putting the preparations for a few days into alcohol, which 

 possesses the property of stopping the reduction of the gold. 

 But this must be taken to mean that by this means the period 

 of usefulness of the preparations may be prolonged for some 

 time, not indefinitely. 



Blackened preparations may be bleached with cyanide or 

 ferrocyanide of potassium. REDDING employs a weak solution 

 of ferrocyanide ; CYBULSKY a 0'5 per cent, solution of cyanide. 

 But the results are far from being perfectly satisfactory. 



Preparations may be double-stained with the usual stains 

 (safranin and methyl green being very much to be recom- 

 mended), but nuclei will only take the second stain in the 

 case of negative impregnation. 



230. Impregnation of Marine Animals. For some reason that 



