160 CHAPTER XIII. 



the acidity of the staining bath, it is a common practice to 

 treat them with weak ammonia, in the belief that the blue 

 colour is restored by neutralisation of the acid that is the 

 cause of the redness. According to Mayee, the ammonia 

 acts, not by neutralising the acid, but by precipitating the 

 alumina, which carries down the hsematein with it (if no 

 alumina were present the colour would be purple, not blue).* 

 The same result can generally be obtained by merely washing 

 out with common tap-water, which is usually sufficiently 

 alkaline, and can be obtained with certainty by treatment 

 with bicarbonate of soda or acetate of soda or potash. And 

 this is the preferable course, as ammonia is certainly a 

 dangerous thing to treat delicate tissues with. Of course 

 this is a different question from that of neutralising with an 

 alkali tissues that have been treated with an acid to correct 

 over-staining. Here the neutralisation may be indicated in 

 the interest of the preservation of the stain. 



Squiiie {Methods, p. 22) finds that sections can be blued in 

 a few seconds by treatment with a 1 : 1000 solution of bicar- 

 bonate of soda in distilled water. Mayer holds that acetate 

 of potash is the most inoffensive reagent to take ; a strength 

 of 0'5 to 1 per cent, may be taken. 



Several of these solutions have a great tendency to over- 

 stain. Over-stains may be corrected by washing out with 

 weak acids (e. g. O'l to 0"2 or even 0"5 per cent, of hydro- 

 chloric acid, or with oxalic or tartaric acid), but this is not 

 favourable to the permanence of the stain. Caenoy {La 

 Celhile, xii, 2, 1897> p. 215) recommends iodised water. If 

 acids be used, it is Vi^ell to neutralise afterwards with ammonia 

 or bicarbonate of soda (O'l per cent.). 



Bicarbonate of soda may be used for neutralisation with 

 70 per cent, alcohol as the vehicle (von Wistinqhausen, 

 Mitth. Zool. Stat. Neapel, x, 1891, p. 41). 



Over-staining may be avoided by staining very slowly in 

 dilute solutions. The purest chromatin stains are obtained 

 by staining for a short time (sublimate sections half an hour, 

 say) in solutions of medium strength, such as hfemalum 

 diluted ten to twenty-fold with water. The stain obtained 



* PisCHEB, in his Fixirung, Fcirbung u. Bau des Protoplasmus, pp. 

 156, 157, does not admit this explanation. He proposes another one of 

 a highly speculative nature. 



