PRUSSIAN BLUE. 119 



of usefulness of the preparations may be prolonged for some 

 time, not indefinitely. 



Blackened preparations may be bleached with cyanide or 

 ferrocyanide of potassium. BEDDING 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, methyl green, and iodine green being very much to 

 be recommended), but nuclei will only take the second stain 

 in the case of negative impregnation. 



213. Impregnation of Marine Animals. For some reason that 

 I am unable to explain, the tissues of marine animals do not 

 readily impregnate with gold in the fresh state. It is stated 

 (FoL) that impregnation succeeds better with spirit specimens. 



Other Metallic 8tains. 



214. Perchloride of Iron. This reagent, introduced by POLAILLON 

 (Journ. de VAnat., iii, 1866, p. 43) sometimes gives most useful results, 

 especially in the study of peripheral nerve ganglia, in which it stains the 

 nervous tissue alone, the connective tissue remaining colourless. The 

 method consists in impregnating in perchloride of iron, and reducing in 

 tannic, gallic, or pyrogallic acid. 



The HOGGANS, who have done very good work with this reagent, proceed 

 as follows (Journ. Quekett Club, 1876; Journ. Roy. Mic. Soc., ii, 1879, 

 p. 358) : The tissue (having been first fixed with silver nitrate, which is 

 somewhat reduced by a short exposure to diffused light) is dehydrated in 

 alcohol, and treated for a few minutes with 2 per cent, solution of per- 

 chloride of iron in spirit. It is then treated with a 2 per cent, solution of 

 pyrogallic acid in spirit, and in a few minutes more, according to the depth 

 of tint required, may be washed in water and mounted in glycerin. 



FOL (see ante, 50) fixes in perchloride solution, and treats the prepara- 

 tions for twenty-four hours with alcohol containing a trace of gallic acid. 



POLAILLON (1. c.) reduces in tannic acid. 



This method is not applicable to chromic objects. 



I should add that in my own experience I have found it very useful in 

 certain special cases. 



215. Pyrogallate of Iron (ROOSEVELT, Med. Rec., ii, 1887, p. 84; 

 Journ. Roy. Mic. Soc., 1888, p. 157). A stain composed of 20 drops of 

 saturated solution of iron sulphate, 30 grm. water, and 15 to 20 drops pyro- 

 gallic acid. 



216. Palladium Chloride (F. E. SCHULTZE, see ante, 48 and 84). 



217. Prussian Blue (sec LEBER, Arch. f. Ophthalm , xiv, p. 300 ; 

 KANVIER, Traite, p. 108). 



