METALLIC STAINS ([MPREGNATION METHODS). 225 



celloidin. Sections are made and fixed on slides, and after 

 the ^ usual treatment with iodine, etc., are either put into 

 distilled 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 over- 

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

 gold chloride (see § 361), but may be weaker, down to O'l 

 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 uiitil the gold is reduced, as directed 

 ill § 368 sub. fin. 



I seem to have found it advantageous to reduce in weak 

 solution of formaldehyde, either with or without formic acid. 



SziJTZ {Zeit. wiss. Mil:, 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 spii-it 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. Ranvier 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 

 ferricyanide of potassium. Redding employs a weak solution 

 of ferricyanide, Gybulsky a 0'5 per cent, solution of cyanide. 



Preparations may be double-stained with the usual stains 

 (safranin being very much to be recommended), but nuclei 



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