50 CHAPTER IV. 



prolonged. They may remain almost indefinitely exposed 

 to its action without much hurt. 



The strength of the solutions employed is from 2 to 5 per 

 cent. As with chromic acid, it is extremely important to 

 begin with weak solutions and proceed gradually to stronger 

 ones. About three weeks will be necessary for hardening a 

 sheep's eye in solutions gradually raised from 2 to 4 per cent. 

 Spinal cord requires from three to six weeks ; a brain, at 

 least as many months. 



After hardening, the objects should be well soaked out in 

 water before being put into alcohol. They had better be 

 kept in the dark when in alcohol (see above, 41) (BoHM 

 and OPPEL [Taxchenlrnch, 3 Auf., 1896, p. 22] fix in the 

 dark). If you iris/i to liarc a good stain irltli carmine you 

 should not, put the objects into alcohol at all, even for a 

 second, until they have been stained. 



You may stain either with carmine or ha3matoxylin. 



Bichromate objects have an ugly yellow colour which cannot be removed 

 by soaking in water. It is said that it can be removed by washing for a 

 few minutes in a 1 per cent, solution of chloral hydrate. Gierke, however, 

 says that this treatment is prejudicial to the preservation of the tissues. 



Prof. GILSON writes me that alcoholic solution of sulphurous anhydride 

 (SO.,) is very convenient for the rapid decoloration of bichromate objects. 

 A few drops suffice. Sec also 40 and 41, and " Bleaching." 



To facilitate staining with hsematoxylin, WOLFF (Zeit.f. wiss. Mik., xv, 

 3, 1899, p. 311) first stains in Boehrner's hreniatoxylin for twenty-four 

 hours, and then for a few minutes in the same hsernatoxylin to which has 

 been added 1 drop per watch-glassful of 5 per cent, solution of oxalic acid. 



The simple aqueous solution of bichromate is hardly to be 

 recommended as a Jixi.ni/ agent, because not only does it not 

 preserve nuclei (though it preserves cytoplasm) as explained 

 in the last section, but also because it penetrates very slowly. 

 The first of these defects may be overcome entirely, the 

 second to some extent by addition of acetic acid ; whence 

 i he liquid of TKLLYESNICZKY, next . 



56. Acetic Bichromate (TELLYESNICZKY, Arch. f. mill. Aunt., 

 Hi, 2, 1899, p. 242). After a comparative study of all the 

 usual fixing agents, Tellyesniczky concludes that the two 

 I test preservatives of cytoplasm are osmic acid and bichromate 

 of potash ; they are indeed the <ndv :i gents that fix in such a 

 manner as to eiiMire the subsequent insolubility of the fixed 



