208 METALLIC STAINS (IMPREGNATION METHODS). 



water for from two to six hours, or are rinsed in water, treated for 

 one minute with 1 per cent, formic acid, and again well washed 

 with water. 



They are then put for twenty-four hours, or at least overnight, 

 into the gold-bath, which is preferably 1 per cent, gold chloride 

 (see 361), but may be weaker, down to 0-1 per cent., after which 

 they are just rinsed with water or superficially dried with blotting- 

 paper. The slides are then set up on end in a sloping position, the 

 sections looking downwards, so that precipitates may not fall on 

 them, in glass tubes filled with 1 per cent, formic acid. The tubes 

 are then exposed to light until the gold is reduced, as directed in 



I seem to have found it advantageous to reduce in weak solution 

 of formaldehyde, either with or without formic acid. 



SztiTZ (Zeit. wiss. Mik., xxix, 1912, p. 292) reduces as APATHY 

 for one day, then rinses and puts back for the night into the gold, 

 then for the next day again into the formic acid. 



372. Impregnation of Marine Animals. For some reason the 

 tissues of marine animals do not readily impregnate with gold in 

 the fresh state. It is said by FOL that impregnation succeeds 

 better with spirit specimens. 



373. Preservation of Impregnated Preparations. Preparations 

 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 completely effected, but they are very liable to go 

 wrong through after-blackening. KANVIER states that this can be 

 avoided by putting them for a few days into alcohol, which he says 

 possesses the property of stopping the reduction of the gold. 



Blackened preparations may be bleached with cyanide or ferri-' 

 cyanide of potassium. REDDING employs a weak solution of ferri- 

 cyanide, CYBULSKY a 0-5 per cent, solution of cyanide. 



Preparations may be double-stained with the usual stains (saf ranin 

 being very much to be recommended), but nuclei will only take the 

 second stain in the case of negative impregnation. 



Other Metallic Stains. 



374 Osmic Acid and Pyrogallol. This method was first published 

 by me in 1887 (La Cellule, iv, p. 110). It consists in putting tissues 

 that have been treated with osmic acid into a weak solution of 



