OTHEH METALLIC STAINS. 151 



I am unable to explain, 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. 



Other Metallic Stains. 



231. 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 perchloride 

 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, 52) 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. 



232. 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 grms. water, and 15 to 20 drops pyro- 

 gallic acid. 



233. Osmic Acid. Everybody knows that osmic acid stains tissues. 

 Most people, I should think, would be heartily glad if it did not. Meanwhile, 

 to make the best of this willy-nilly stain, you may sometimes find it useful 

 to treat the tissues with weak pyrogallic acid, which will very quickly turn 

 them of a fine greenish black, sometimes giving useful differentiations. 



This method was first published by me in 1887 (La Cellule, t. iv, fasc. 1, 

 p. 110), and I have since republished it twice, with qualified recommendation. 

 I have found it occasionally very useful, but have hesitated to recommend it 

 for general work because I have usually found the stain too energetic and 

 too little discriminating. It frequently happens that everything in a pre- 

 paration comes out blacker than ink. The process has lately been re-invented 

 by KOLOSSOW (Zeit.f. wiss. Mik., ix, 1892, p. 38) in a somewhat complicated 

 form. He finds that the reaction obtained as I have described is not energetic 

 enough, and recommends treating first with the osmium mixture given in 

 28, and then with an elaborate " developing " mixture composed of water, 

 alcohol, glycerin, tannin, and pyrogallic acid. For the reasons above stated, 

 I fail to see the utility of this modification of the method (Kolossow himself 



